Publications

In the quest for fast throughput metal biosensors, it would be of interest to prepare fluorophoric ligands with surface-adhesive moieties. Biomimetic analogues to microbial siderophores possessing such ligands offer attractive model compounds and new opportunities to meet this challenge. The design, synthesis, and physicochemical characterization of biomimetic analogues of microbial siderophores from Paracoccus denitrificans and from the Vibrio genus are described. The (4S,5S)-2-(2-hydroxyphenyl)-5-methyl-4,5-dihydro-1,3-oxazole-4-carbonyl group (L(a)), noted here as an HPO unit, was selected for its potential dual properties, serving as a selective iron(III) binder and simultaneously as a fluorophore. Three tripodal symmetric analogues cis-L(b), cis-L(c), and trans-L(c), which mainly differ in the length of the spacers between the central carbon anchor and the ligating sites, were synthesized. These ferric-carriers were built from a tetrahedral carbon as an anchor, symmetrically extended by three converging iron-binding chains, each bearing a terminal HPO. The fourth chain could contain a surface-adhesive function (L(c)). A combination of absorption and emission spectrophotometry, potentiometry, electrospray mass spectrometry, and electrochemistry was used to fully characterize the corresponding ferric complexes and to determine their stability. The quenching mechanism is consistent with an intramolecular static process and is more efficient for the analogue with longer arms. Detection limits in the low nanogram per milliliter range, comparable with the best chemosensors based on natural peptide siderophores, have been determined. These results clearly demonstrate that these tris(phenol-oxazoline) ligands in a tripodal arrangement firmly bind iron(III). Due to their fluorescent properties, the coordination event can be easily monitored, while the fourth arm is available for surface-adhesive moieties. The tripodal system is therefore an ideal candidate for integr

Over the past decade, there has been remarkable progress in the development of molecular logic and arithmetic systems, which has brought chemists closer to the realization of a molecular scale calculator (a Moleculator). This paper describes a significant step in this direction. By integrating past and new approaches for molecular logic reconfiguration, we were able to load advanced arithmetic calculations onto a single molecular species. Exchanging chemical inputs, monitoring at several wavelengths simultaneously, as well as using negative logic for the transmittance mode significantly increase the input and output information channels of the processing molecule. Changing the initial state of the processor is an additional approach used for altering the logical output of the device. Finally, introducing degeneracy to the chemical inputs or, alternatively, controlling their interactions to form identical chemical states minimizes the complexity of realizing three-bits addition and subtraction at the molecular scale. Consequently, using a commercially available fluorescein molecule, acid and base chemical inputs, and a simple UV-vis measurement setup, integration of a full-adder and, for the first time, a full-subtractor is now possible within individual molecules.

A highly efficient and simple molecular arithmetic system based on a plain fluorescene dye, capable of performing a full scale of elementary addition and subtraction algebraic operations is reported. For achieving boolean functions through scid-base interactions and their control over multistate molecular switches are described. A half-adder, performing a+b algebraic operations, is obtained with two identical sodium hydroxide solutions as inputs and by monitoring the outputs at 447 and 501 nm. Subtraction is achieved by operating the half-subtractor first using acid and then using base as input numbers. This molecular arithmetic unit demonstrates a general applicable method for resetting of chemical computation system.

The biological evaluation and synthesis of lipophilic iron chelators as protective agents from oxidative stress were analyzed. A spontaneously transformed cell line of oligodendroglia origin (OLN 93) was chose as a model for neural cells. These cells were used to demonstrate remarkable sensitivity to geontoxic stress, culminating in cell death when both divalent iron and H ₂O₂ were added to cells. OLN 93 cells were seeded in 96-well polyethyleneimine precoated plates, and after 24 h attachment were further incubated for 3 h with the synthetic analog at various concentrations. The results show the proactive effect of analog 5-7 at a 200 μM.

Irradiation of plasmid DNA in the presence of Ru(II)-2, a modified tris(2,2-bipyridyl)Ru(II) complex, in which two hydroxamic acid groups are attached to one of the three bipyridyl ligands, results in total fragmentation of the DNA. The photo-chemical reaction products were analyzed by gel electrophoresis, which revealed complete fragmentation. Further evidence for the complete degradation of the DNA was obtained by imaging the pre- and post-treated plasmid DNA using atomic force microscopy (AFM). A mechanism for the reaction is proposed. It initially involves the photo-chemical generation of Ru(III) ions and superoxide radicals, as corroborated by absorbance difference spectroscopy and electron paramagnetic resonance (EPR). Consequently, Ru(III) preferentially oxidizes guanine, liberating superoxide radicals that yield OH radicals. The OH radicals were identified by observing the spectral change at 532 nm of a 5-dAdG substrate forming a colored adduct with thiobarbituric acid. These radicals are associated with the major non-specific damage exerted to DNA.

Ruthenium complexes with three bipyridyl ligands, one of which is modified by attaching one or two hydroxamic acids groups (Ru-1 and Ru-2, respectively), were synthesized. Using EPR spectroscopy, we have found that photoexcitation leads to formation of nitroxyl radicals. The nitroxyl radical concentration in Ru-2 increased dramatically in the presence of spin traps DMPO (5,5́-dimethyl-1-pyrroline-N-oxide) and PBN (N-tert-butyl-α-phenylnitrone) characterized by strong affinity to Superoxide radicals. We have attributed this behavior to the formation of a cage complex between Ru-2 and the Superoxide radical. This paper concerns the study of cages formed between ruthenium complexes and molecular oxygen and the effect of functional groups attached to modified bipyridyl ligands on cage formation. The complex between Ru-2 and O ₂ was formed in the ground state, probably with participation of the hydroxamic acid groups. The equilibrium constant of this complex was determined by EPR as K _eq ∼ 3 M ^-1. The formation of the Ru-2-O ₂ complex is supported by the temperature-dependent rate of appearance of the EPR signal in the presence of PBN. Additional evidence comes from observation of paramagnetic shifts of the peaks in the ¹H NMR spectrum of specific aromatic protons in the substituted bipyridyl ring upon exposure to O ₂. Similar shifts were observed in the spectrum of Os-2, with osmium replacing ruthenium. Model compounds with functional groups that replace the hydroxamic acid or compounds without the metal center, but with the two hydroxamic acids, were synthesized. No shifts in the ¹H NMR spectra of these derivatives were observed in the presence of O ₂. These results lead to the conclusion that both metal ions, Ru(II) or Os(II), and hydroxamic acid groups are essential components for the formation of the oxygen cage.

The multifunctional nature of proteins that have iron-heme cofactors with noncovalent histidine linkage to the protein is controlled by the heme environment. Previous studies of these active-site structures show that the primary difference is the length of the iron-proximal histidine bond, which can be controlled by the degree of H-bonding to this histidine. Great efforts to mimic these functions with synthetic analogues have been made for more than two decades. The peroxidase models resulted in several catalytic systems capable of a large range of oxidative transformations. Most of these model systems modified the porphyrin ring covalently by directly binding auxiliary elements that control and facilitate reactivity; for example, electron- donating or -withdrawing substituents. A biomimetic approach to enzyme mimicking would have taken a different route, by attempting to keep the porphyrin ring system unaltered, as close as possible to its native form, and introducing all modifications at or close to the axial coordination sites. Such a model system would be less demanding synthetically, would make it easy to study the effect of a single structural modification, and might even provide a way to probe effects resulting from porphyrin exchange. We introduce here an alternative model system based on these principles. It consists of a two component system: a bis-imidazolyl ligand and an iron-porphyrin (readily substituted by a hemin). All modifications were introduced only to the ligand that engulfs the porphyrin and binds to the iron's fifth and sixth coordination sites. We describe the design, synthesis, and characterization of nine different model compounds with increased complexity. The primary tool for characterizing the environment of each complex Fe¹¹¹ center was the Extended X-ray Absorption Fine Structure (EXAFS) measurements, supported by UV/Vis, IR, and NMR spectroscopy and by molecular modeling. Introduction of asymmetry, by attaching different imidazoles as head groups, led to the formation of two axial bonds of different length. Addition of H-bonds to one of the imidazoles in an advanced model increased this differentiation and expanded the porphyrin ring. These complexes were found to be almost identical in structure to peroxidase active sites. Similarly to the peroxidases and other synthetic models, these compounds stabilize the green, compound I-like intermediate, and catalyze the oxidation of organic substrates.

Adsorption of trivalent metalloporphyrins from nitrogen-saturated chloroform solution onto etched n-CdSe crystals causes a profound reversible quenching of the semiconductor's photoluminescence (PL). The PL responses due to the presence of M^IIIDMPPCl and M^IIITPPCl (DMPPCl = protoporphyrin IX dimethyl ester chloride; TPP = tetraphenylporphyrin; M = Fe, Mn) exhibit a concentration dependence that can be fit to the Langmuir adsorption isotherm model to yield binding constants of 10^4-10⁵M^-1. The CdSe surface may be modified by adsorption from solution of specifically designed linker ligands (1-4). These ligands are able to bind to the semiconductor surface through one end and to ligate a heme analogue axially on the other end. Surfaces derivatized by each of the linkers showed concentration-dependent metalloporphyrin-induced PL changes, corresponding to roughly order-of-magnitude increases in binding constants to 10⁵ to 10⁶ M^-1. Films of linker-metalloporphyrin complexes were coated onto the semiconductor substrates and characterized by X-ray photoelectron (XPS) spectroscopy. The linker-metalloporphyrin films can be used as transducers for dioxygen detection. Relative to a nitrogen ambient, the PL of CdSe samples coated with 1-3 is reversibly quenched by exposure to oxygen (binding constants of approx. 1 - 10 atm^-1; detection limit of approx. 0.1 atm), while bare CdSe surfaces show no response to dioxygen. These coated CdSe samples were further characterized by contact potential difference (CPD) and time-resolved photoluminescence (TRPL), which suggest that oxygen-induced PL changes are due to variations in the electric field present in the semiconductor substrate.

The use of evaporated ultrathin gold films on mica to obtain transmission UV/vis spectra of monomolecular overlayers containing chromophores is demonstrated. The gold substrates (thickness, 13-100 Angstrom) were studied by scanning force microscopy. UV/vis spectroscopy, and X-ray diffraction and found to exhibit an array of {111} textured islands, whose sizes and optical properties can be controlled by the evaporation conditions and subsequent annealing. The use of gold substrates of this kind enables one to clearly resolve UV/vis absorption spectra of (sub)monolayers. This methodology is applied to the study of the binding of cobalt and iron tetraphenylporphyrins (CoTPP and FeTPPCl, respectively) and cobalt phthalocyanine (CoPc) to various N-donor ligand monolayers, self-assembled on ultrathin Sold films. Enhancement of the gold surface plasmon absorption, resulting from molecular self-assembly on the gold island film, is used to semiquantitatively follow porphyrin binding to the monolayer.

The use of molecules to control electron transport is an interesting possibility, not least because of the anticipated role of molecules in future electronic devices(1). But physical implementations using discrete molecules are neither conceptually(2,3) simple nor technically straightforward (difficulties arise in connecting the molecules to the macroscopic environment). But the use of molecules in electronic devices is not limited to single molecules, molecular wires or bulk material. Here we demonstrate that molecules can control the electrical characteristics of conventional metal-semiconductor junctions, apparently without the need for electrons to be transferred onto and through the molecules. We modify diodes by adsorbing small molecules onto single crystals of n-type GaAs semiconductor. Gold contacts were deposited onto the modified surface, using a 'soft' method to avoid damaging the molecules(4). By using a series of multifunctional molecules whose dipole is varied systematically, we produce diodes with an effective barrier height that is tuned by the molecule's dipole moment. These barrier heights correlate well with the change in work function of the GaAs surface after molecular modification. This behaviour is consistent with that of unmodified metal-semiconductor diodes, in which the barrier height can depend on the metal's work function.

Geringe Konzentrationen von NO in physiologischen Pufferlösungen (1 ppm, ca. 3 μM) lassen sich mit dem gezeigten maßgeschneiderten molekularen Zweikomponentensystem ein pinzettenartig von einem organischen Liganden gehaltenes Eisen(III)porphyrin nachweisen, das mit einer Seite an einen Detektor auf GaAsBasis gekoppelt ist. Bindet NO an das Eisen(III)porphyrin, ändert sich die Stromstärke.

Metal-organic coordination is an attractive means for constructing supramolecular systems, providing versatility, simple synthesis, and a defined geometry. The convenience of changing 'building blocks' during multilayer assembly is exploited for the fabrication of novel ion-coordinated hybrid multilayers on gold. Two bifunctional linkers are used, a tetrahydroxamate and an organic diphosphonate, while the connection between layers is accomplished through Zr(IV) coordination, to form a well-defined hybrid multilayer. The two ion binders are compatible with respect to multilayer assembly, allowing the change of linkers during construction while maintaining the film structural integrity and organization. The different chemical reactivity of the binders enables rational structural manipulation of the multilayer, by selective dissolution of the acid-sensitive hydroxamate layers while keeping the acid-resistant phosphonates (and underlying hydroxamates) intact. The process demonstrates the multilayer structural quality, where two diphosphonate monolayers are capable of effectively blocking proton penetration to underlying hydroxamate layers. This allows nanometer-scale reshaping of the molecular film according to a scheme introduced during its construction.

Perpendicularly oriented iron porphyrins are absorbed onto a gold surface when interconnected long-chain diimidazolyl groups coordinate axially to the metal center from either side of the ring plane (see schematic representation). The stacking of the rings is simultaneously prevented. The monolayers have been characterized structurally and electrochemically.

Synchrotron radiation (SR) pulses are used to eject electrons from a gold substrate covered with organized organic thin films (OOTF) in order to investigate their transmission probability through the OOTF as a function of the electron initial kinetic energy. By variation of the SR photon energy within a few eV above the Au-4f binding energy levels we controlled the initial kinetic energy of the substrate electrons. The observed oscillations in the transmission probability for porphyrin-based films as a function of the kinetic energy is argued to be due to effects of band structure above the vacuum level in the well-ordered molecular adsorbate. We also present valence photoemission spectra (PES) of different type OOTF and demonstrate how their coverage of the substrate affects the PES.

A two-dimensional experiment, termed DONUT-HYSCORE (double nuclear coherence transfer hyperfine sublevel correlation) designed to obtain correlations between nuclear frequencies belonging to the same electron spin manifold is presented. The sequence employed is π/2-τ₁-π/2-t₁-π-τ₂- π-t₂-π/2τ₁-echo, and the echo is measured as a function of t₁ and t₂ whereas τ₁ and τ₂ are held constant. It is complementary to the standard HYSCORE experiment which generates correlations between nuclear frequencies belonging to different M(s) manifolds and is particularly useful for ¹⁴N nuclei. The experiment is first demonstrated on a single crystal of copper- doped l-histidine hydrochloride monohydrate where the modulations are induced by a single ¹⁴N nucleus, the remote nitrogen in the imidazole group. HYSCORE and DONUTHY-SCORE experiments were carried out on two crystal orientations. In the first, one Cu²⁺ site contributes to the echo and all six nuclear frequencies together with the expected correlation were observed. In the second, 12 frequencies corresponding to two Cu²⁺ ions at different crystallographic sites appeared and all expected correlations were detected as well. This rather trivial example demonstrates that the DONUT-HYSCORE pulse sequence indeed generates correlations within the M(s) manifolds. The value of the DONUT-HYSCORE experiment is demonstrated on a frozen solution of a vanadyl complex with a bis-hydroxamate ion binder (VO-RL515). The modulations in this complex arise from the two InN nuclei in the hydroxamate groups, and orientation-selective three-pulse ESEEM (electron spin-echo envelope modulation) spectra showed a number of well-resolved peaks. An unambiguous assignment of all peaks and their orientation dependences could not be achieved through HYSCORE alone because at certain orientations frequencies of one of the M(s) manifolds were absent or overlapped with those of the other manifold. The application of the DONUT-HYSCORE experiment provided new correlations that led to the complete assignment of the ESEEM frequencies, thus paving the way for future systematic spectral simulations for the determination of the best-fit Hamiltonian parameters. This example shows that, in the case that the HYSCORE experiment cannot distinguish between two sets of frequencies belonging to the same M(s) manifold in different centers (or orientations) because signals from the other manifold are missing or overlapping, the DONUT-HYSCORE becomes most valuable.

We show that the chemisorption of dicarboxylic acids on GaAs (100) is described well by a two-site mechanism, in contrast to benzoic acid adsorption which fits to a one-site mechanism. We do so by using a novel electrical method for direct measurement of adsorption kinetics. In the method we measure the current through a GaAs/(Al, Ga)As-based device, where the bare surface between two contacts is used as the adsorption domain. The results, which are in agreement with FTIR absorption equilibrium data, are obtained in ambient notwithstanding the notorious instability of GaAs surfaces under such conditions. We conclude that these acids chemisorb on the GaAs surface and that binding is significantly stronger for the di-than for the monocarboxylic acids.

A novel type of bilayer on a gold surface, based upon metal-ion coordination to hydroxamate moieties, is described, Tailor-made bifunctional ligands containing hydroxamate groups (for metal coordination) and a cyclic disulfide residue (for surface attachment) have been prepared. The bishydroxamate binding site forms 2:1 ligand/metal complexes with octacoordinating metal ions such as Zr-IV Ce-IV, and Ti-IV:the cyclic disulfide moiety anchors the complex to the gold surface, Two routes to bilayer formation are demonstrated: i) a one-step process from preformed 2:1 complexes, and ii) a stepwise process including formation of the ligand monolayers followed by binding of a guest ion and a second layer of ligand molecules. The former approach allows full characterization of the complexes before bilayer assembly, whereas the latter enables construction of either symmetric (identical) or asymmetric (nonidentical) bilayers. Both types of bilayers were characterized by ellipsometry, contact angle, and XPS measurements. Symmetric bilayers obtained by the two processes have similar properties.

The interactions between adsorbed organic molecules and the electronic charge carriers in specially made GaAs structures are studied by time- and wavelength-dependent measurements of the photocurrent. The adsorption of the molecules modifies the photocurrent decay time by orders of magnitude. The effects are molecularly specific, as they depend on the electronic properties and absorption spectrum of the molecules. These observations are rationalized by assuming that new surface states are created upon adsorption of the molecules and that the character of these states is controlled by the relative electronegativity of the substrates and the adsorbed molecules. The relevance for surface passivation and for construction of semiconductor-based sensors is indicated.

Iron uptake by the phytopathogenic fungus Ustilago maydis was studied using synthetic biomimetic ferrichrome analogues and their fluorescently labelled derivatives as structural and dynamic probes, respectively. The use of structurally distinct analogues enabled determination of the structural requirements for recognition by the fungal iron-uptake system. The application of fluorescently labelled derivatives which convert from a non-fluorescent to a fluorescent state upon iron (III) release enabled monitoring of iron uptake in real time both fluorimetrically and microscopically. Different rates of ⁵⁵Fe uptake were found for two structurally distinct synthetic analogues, B9 and B5, which differ in their amino acid building blocks. B9 mediated uptake of ⁵⁵Fe at a higher rate than B5. The behaviour of the fluorescent derivatives B9-Ant (anthracene-labelled B9) and B5-Ant (anthracene-labelled B5) paralleled that of their non-labelled precursors. Exposure of fungal cells to B9-Ant led to a higher increase of fluorescence in the medium than exposure to B5-Ant, indicating a more effective iron uptake from B9-Ant. By using fluorescence microscopy it was possible to trace the label of B9-Ant. Fluorescence was localized in regularly shaped vesicles in the treated cells. The rate of fluorescence appearance within the cells lagged behind the rate of iron uptake, suggesting use of the siderophores for iron storage.

Homogeneously mixed molecular assemblies of defined stoichiometry were created by adsorption of asymmetric, trifunctional ligands on gold and CuInSe₂ (CISe). The ligands rely on cyclic disulfide groups for binding to the substrate and can in addition possess two different substituents, one polar substituent (p-cyanobenzoyl or anisoyl) and one long-chain, aliphatic residue (palmitoyl). Because the substituents are covalently connected, no phase segregation will occur upon surface binding. Adsorption of these ligands on conducting surfaces changed both the surface potential (because of the polar substituent) and hydrophobicity (because of the aliphatic residue). Larger changes of surface potential were obtained by adsorption of the symmetric, dipolar ligands than by adsorption of the asymmetric ligands, and larger changes occurred on gold than on CuInSe₂ (up to 1.2 V between extreme modifications on Au and 0.3 V on CISe). The magnitude and direction of the observed contact potential difference changes were found to depend on the extent of coverage (as derived from electrochemical and contact angle measurements) and on the orientation of the ligands (estimated from ellipsometry and FTIR data) and could also be reconstructed using a simple, electrostatic model. These findings demonstrate that the present methodology enables simultaneous grafting of two desired properties onto solid surfaces and illustrate the predictive power of a simple, electrostatic model for molecule-controlled surface engineering.

Wavelength-dependent two-photon photoemission (WD-TPPE) spectroscopy was used to investigate the surface state properties of CdTe crystals before and after the adsorption of specially designed organic molecules. One photon was used to modify the population of the surface states and a second photon to eject electrons from the substrate. We measured the dependence of the photoemission signal on the energy of the first photon and on the delay between the two light pulses. The energy of surface states, relative to the bands, was found to correlate with the relaxation time of the semiconductor surface after being photoexcited. This is explained in terms of a simple kinetic model for electron transfer. These findings demonstrate that the properties of surface states of semiconductors can be manipulated by adsorbing suitable organic molecules on the semiconductor surface and that the WD-TPPE method is a useful tool for optoelectronic characterization of semiconductor surfaces, with sensitivity exceeding that of most commonly used techniques.

Control of the work function of GaAs single crystals, under ambient conditions, was achieved by chemisorption of a series of benzoic acid derivatives with varying dipole moments. Quantitative Fourier transform infrared spectroscopy shows that the benzoic acid derivatives bind as carboxylates, via coordination to oxidized Ga or As atoms, with a surface coverage of about one layer and a binding constant of 2.1 10⁴ M^-1 for benzoic acid. Contact potential difference measurements reveal that molecules affect the work function by changing the electron affinity while band bending is not affected significantly. The direction of the electron affinity changes depends on the direction of the dipole moments, and the extent of the change increases linearly with the dipole's magnitude. Investigation of the surface composition by X-ray photoelectron spectroscopy shows that the etched surface, onto which the molecules adsorb, is covered by an oxide layer. This may prevent the molecules from affecting band bending.

Reversed siderophores (RSFs) are artificial hydroxamate-based iron chelators designed after the natural siderophore ferrichrome. The modular molecular design of RSF derivatives allowed the synthesis of various congeners with controlled iron-binding capacities and partition coefficients. These two physicochemical properties were assessed by a novel fluorescent method and were found to be the major determinants of RSF permeation across erythrocyte membranes and scavenging of compartmentalized iron. The partition coefficient apparently conferred upon RSFs two major features: (i) the ability to rapidly access iron pools of in vitro-grown Plasmodium falciparum at all developmental stages and to mobilize intracellular iron and transfer it to the medium and (ii) the ability to suppress parasite growth at all developmental stages. These features of RSFs were assessed by quantitative determination of the structure-activity relationships of the biological activities and partition coefficients spanning a wide range of values. The most effective RSF containing the aromatic group of phenylalanine (RSFm2phe) showed 50% inhibitory concentration of 0.60 ± 0.03 nmol/ml in a 48-h test and a 2-h onset of inhibition of ring development at 5 nmol/ml. The lipophilic compound RSFm2phe and the lipophilic and esterase-cleavable compound RSFm2pee inhibited parasite growth at all developmental stages whether inhibition was assessed in a continuous mode or after discontinuing drug administration. The antimalarial effects of RSFm2phe and clearable RSFm2pee were potentiated in the presence of desferrioxamine (DFO) at concentrations at which DFO alone bad no effect on parasite growth. These studies provide experimental evidence indicating that the effective and persistent antimalarial actions of RSFs are associated with drug access to infected cells and scavenging of iron from intracellular parasites. Moreover, the optimal antimalarial actions of RSFs are apparently also determined by improved accessibility to critical iron pools or by specific interactions with critical parasite targets.

Iron chelators of the hydroxamate class arrest in vitro proliferation of malaria parasites and of mammalian cells. The factors determining the biological activity of the chelators have classically been attributed to the chelators' capacity for binding iron and to their ability to traverse membranes as free chelators and as chelator-iron complexes. We show in this work that the nature of the chelatable pool of cell iron also contributes to the susceptibility of cells to iron chelators. A class of N-terminal (N(t)) derivatives of desferrioxamine (DFO), (N(t)-DFO), is shown here to differentially affect growth and replication of intraerythrocytic parasites (Plasmodium falciparum). Methyl-anthranilic DFO (MADFO), the relatively less hydrophilic member of the N(t)-DFOs series, reduced parasite proliferation (48 hour test) with an IC₅₀ of 4 ± 1 μmol/L and mammalian cell (K562 and HepG2) proliferation with an IC₅₀ > 100 μmol/L. On the other hand, the more hydrophilic N(t)-free DFO, displayed IC₅₀ values of 21 ± 5 μmol/L for parasites and 7 ± 1 μmol/L for mammalian cells. The selective antiparasitic activity of MA-DFO, as reflected in the speed of action and IC₅₀ values on cell proliferation, is attributed primarily to membrane permeation and iron(III) binding properties of the drug. In contrast, the relatively low antiproliferative activity of the more permeant MA-DFO on mammalian cells, resulted from MA-DFO's reduced capacity for scavenging intracellular iron. This is apparent from MA-DFO reduced effects on: (1) the chelatable iron(II) pool that is associated with the cell cytosol; (2) the call chelator-extractable iron, and (3) cell ferritin levels. The potent antimalarial efficacy and biological selectivity of MA-DFO relative to the parent DFO, is of importance for improved design of chemotherapeutic agents.

Molecular recognition and signaling are at the heart of biological processes as they govern intercellular communication and information transfer. Here we probe the recognition phenomena of siderophore-mediated microbial iron(III) uptake, which involves two recognition processes: recognition of iron(III) by microbial siderophores (iron(III) carriers), and recognition and binding of siderophore-iron(III) complexes by microbial membrane receptors. In addition, we introduce fluorescent siderophore analogs that respond to iron binding with changes of their fluorescence and thereby provide artificial binders that combine iron recognition with signaling.

Novel ion-binding monolayers on gold surfaces are presented where the molecular design is based upon the natural ion binder ferrichrome. The new ion binders possess hydroxamate coordinating groups arranged in C₂ symmetry (bishydroxamate binder, BHB) or C₃ symmetry (trishydroxamate binder, THB), and a separate dialkyl sulfide moiety, which serves as an anchor to the gold substrate. The separation between the ion-binding cavity and the attachment site to the gold allows each parameter to be controlled separately, namely, cavity size, its symmetry and external envelope, as well as the functional group used for immobilization. The monolayers were characterized with respect to ellipsometric thickness, wettability (advancing and receding contact angles (CAs) for water), and surface coverage; the latter is determined by metal underpotential deposition (UPD). It is shown that the introduction of hydrophobic side chains (i-butyl) improves the CAs, thickness, and surface coverage of the monolayers. A detailed analysis of the alternating-current (AC) impedance spectra is presented for THB monolayers on gold electrodes, where the impedance data are fitted to an equivalent circuit model. It is shown that the AC response in a wide frequency range can be used to probe ion binding and release in monolayer systems on electrodes.

MAGNETIC storage of information requires the ability to manipulate the magnetization of thin films with high sensitivity and spatial resolution. Non-magnetic overlayers are known to affect the characteristics of magnetic films: for example, the direction of magnetization of cobalt and iron films can be altered by deposition of a monolayer of copper and gold, respectively¹³. The magnetic properties of cobalt films seem to be particularly sensitive to copper overlayersdeposition of only sub-monolayer amounts of copper will decrease the coercive field required to invert the magnetization direction⁴. Here we show that copper coverages as small as three-hundredths of a monolayer are sufficient to rotate by 90° the magnetization of Co films up to 20 atomic layers thick. This implies that the spins of about 500 cobalt atoms switch direction for each copper atom added. Adding more copper eventually switches the magnetization back to its original direction. This fine tuning of thin-film magnetism might be useful for developing sensitive magnetic-field sensors, as well as for magnetic recording.

Hydroxamate-based chelators of iron are potent inhibitors of in vitro growth of Plasmodium falciparum. Two types of such chelators, the natural desferrioxamine and the synthetic reversed siderophore RSF(ileum 2), are prototypes of antimalarial agents whose action spectra differ in the speed of action, stage dependence, and degree of reversibility of effects. This work explores the possibility of improving the antimalarial efficacy of these agents by using them in various combinations on in vitro cultures of P. falciparum. Growth assessment was based both on total nucleic acid synthesis and on parasitemia. The results indicate that the synthetic reversed siderophore more than complements the antimalarial action of desferrioxamine when applied during either ring, trophozoite, or mixed stages. The combined drug effects were significantly higher than the additive effect of the individual drugs. Qualitatively similar results were obtained for both reversible effects and irreversible (i.e., sustained) effects. Following an 8-h window of exposure the combined drug treatment caused parasite growth arrest and prevented its recovery, even 3 days after the treatment. The fact that such a combination of iron chelators displays a wider action spectrum than either drug alone has implications for the design of chemotherapy regimens.

The binding of VO²⁺to chiral dihydroxamate binders facilitates the transport of VO²⁺through the cell membrane into the cell interior, where it was shown to simulate glucose metabolism (Shechter, Y.; Shisheva, A.; Lazar, R.; Libman, J.; Shanzer, A. Biochemistry 1992, 31, 2063). The unique structure of the binders relies on a modular dipodal topology which generates different binding cavities. The coordination of VO²⁺to two homologues of these ligands. RL261 and RL239, having different dipod arms but identical donor groups, was investigated by orientation selective electron spin echo envelope modulation (ESEEM) spectroscopy. Relatively deep modulations were observed for the ¹⁴N nuclei in the hydroxamate groups in both complexes owing to the fulfillment of the cancellation condition at ~9 GHz. The Fourier transform (FT) ESEEM spectra showed four peaks, three corresponding to the nuclear quadrupole resonance (NQR) lines, v_o, v_-, and v₊, and one to the overtone, 2v_m. In VO-RL261 the NQR and the 2v_mpeaks appear at 1.75, 2.15, 3.9, and 6.0 MHz, respectively, whereas in VO-RL239 they are at 1.75, 2.05, 3.9, and 6.2 MHz, respectively. From the positions of these peaks the ¹⁴N quadrupole coupling constant, |e²qQ/h|, the asymmetry parameter, η, and the isotropic hyperfine constant |a_iso|, were estimated to be 4.0, 0.87, and 2.5 MHz, respectively, for VO-RL261 and 3.8, 0.91, and 2.8 MHz, respectively, for VO-RL239. The unique orientation dependence of the v₊peak, which practically disappeared when the field was set to A_||(⁵¹V), indicates that the principal axis of the quadrupole tensor, z', is either parallel or perpendicular to the VO axis. In order to obtain more accurate values of the above parameters and to determine the anisotropic hyperfine component, a_⊥, as well as the orientations of the hyperfine and quadrupole tensors with respect to the VO axis, a series of simulations were carried out. The best fit parameters showed that a_⊥is rather large (0.6-0.7 MHz) and cannot be neglected and that a_isois smaller than expected, i.e., 1.6-1.8 MHz. We also obtained that z is to a good approximation parallel to the VO axis indicating that the two hydroxamate planes are perpendicular to the VO axis in both complexes. Two possible structures, one with a C₂symmetry, trans configuration, and one with a σ_xzsymmetry, cis configuration, were considered in the simulations and the latter was found to agree better with the experimental results. The slight differences in the parameters obtained for VO-RL261 and VO-RL239 are attributed to electronic effects induced by the different groups bounded to the hydroxamate carbonyl.

Fourier transform infrared spectroscopy shows that benzoic acid and its derivatives can bind chemically to the surfaces of CdSe. The dipole moment of the adsorbed benzoate correlates linearly with changes in the semiconductor work function, as determined by vibrating capacitor (Kelvin probe) measurements. Because ligand adsorption does not induce significant changes in surface photovoltage, we conclude that changes in work function are due to changes in electron affinity rather than in band bending. This means that ligand adsorption does not induce charge transfer between the surface and the semiconductor but rather adds a surface dipole. Changes in band bending do occur, however, upon conventional etching.

Controlled surface modification of CdTe single crystals and CdTe and CuInSe₂ solar cell quality thin films was achieved by chemisorption of a series of organic ligands with varying dipole moments. Contact potential difference measurements in air showed that adsorption of benzoic or hydroxamic acid derivatives on the thin films or crystals changes the semiconductors' electron affinity without significantly affecting band bending. The magnitude and direction of surface potential changes, which reach 670 mV between extreme modifications, correlate with the ligands' dipole moments. Ligand dipole moments were controlled by varying the substituents of the ligand. Quantitative Fourier transform infrared (FTIR) spectroscopy showed that benzoic acid surface coverage is about one monolayer. Finally, FTIR spectral analysis showed that the benzoic acid derivatives adsorb via coordination to Cd on CdTe and that hydroxamic acids bind to Cd on CdTe and to In on CuInSe₂. These phenomena occur in several systems (two semiconductor compounds, two types of binding groups, and two types of surface morphologies were examined) and may prove useful in band edge engineering.

Electrospray Mass Spectrometry (ESMS) has been used to analyse protein/metal ion complexes directly in solution. A synthetic siderophore analogue and two sulphur-iron proteins have been used as models for the study of protein/iron interactions. These experiments were successfully extended to a protein/cofactor complex interaction model, myohemoglobin. Our results open the door to the characterization of weak interactions between large molecules by ESMS.

Two novel families of chiral trishydroxamate binders are described. These compounds have been modeled after the natural siderophore ferrichrome in an attempt to mimic its biological properties as iron(III) carrier and growth promoter. In these analogs the hydroxamate binding sites and tripodal topology of ferrichrome are retained, but the chiral hexapeptide anchor of the natural siderophore is replaced by C₃-symmetric tricarboxylates of two homologous types (m = 1, type 1; m = 2, type 2). The resulting loss of chirality is compensated by symmetric extension of these anchors with natural amino acids (Figure 1). In this design two elements are of particular importance: (i) the amino acid bridges that induce chirality, stabilize specific conformations of the free ligands, and allow systematic modifications and (ii) the use of two homologous anchors that dictate the conformations of the metal complexes. Type 2 binders proved to either simulate the performance of the natural ferrichrome and act as growth promoters or to inhibit the action of ferrichrome and function as growth inhibitors. On the other hand, none of the type 1 binders proved active. In an attempt to establish the origin of these differences, the structures of the two types of compounds and of their octahedral metal complexes are examined by a combination of IR, NMR, UV/vis, CD, and NMR spectroscopy. Little differences were observed in the conformations of the free ligands as both types adopt propeller-like arrangements that are stabilized by intramolecular H-bonds. Both types of complexes also predominantly assume the Λ-cis configuration when L-amino acids are used. However, pronounced differences were observed in the conformations of the metal complexes: while in type 1 complexes the amides are positioned tangentially to the molecules cross section, in type 2 complexes they are oriented radially with the amide-NH pointing inward. In the latter arrangement the amide-NH groups become fit to form intramolecular H-bonds. These findings allowed us to interpret the in vivo performance of these compounds and to identify the structural requirements for obtaining growth promoters or growth inhibitors, respectively. The systematically modified ligands allowed us to examine the effect of intramolecular interactions, specifically H-bond networks and van der Waals forces, on the complexes' stoichiometry and isomeric and optical purity. The presence of either of the two intramolecular forces is sufficient to obtain monomeric complexes of 1:1 stoichiometry, while the absence of both like in the glycine derivative of type 1 ligand, 1G, causes the formation of polymeric complexes.

We designed the N-methylanthranilic-desferrioxamine (MA-DFO) as a fluorescent iron (III) chelator with improved membrane permeation properties. Upon binding of iron (III), MA-DFO fluorescence is quenched, thus allowing traceability of drug-iron (III) interactions. MA-DFO is well tolerated by mammalian cells in culture. Its antimalarial activity is pronounced: IC₅₀ values on in vitro (24-h) growth of Plasmodium falciparum were 3±1 μM for MA-DFO compared with 30±8 for DFO. The onset of growth inhibition of rings or trophozoites occurs 2-4 h after exposure to 13 μM MA-DFO. This effect is commensurate with MA-DFO permeation into infected cells. In a 24-h exposure to MA-DFO or DFO, trophozoites take up either compound to ∼ 10% of the external concentration, rings to 5%, and noninfected cells to

We present here the physicochemical and biochemical properties of NBD-DFO, the 7-nitrobenz-2-oxa-1,3-diazole (NBD) derivative of the siderophore, desferrioxamine B (DFO) (Lytton et al., Mol. Pharmacol. 40, 584, 1991). Modification of DFO at its terminal amine renders it more lipophilic, imparts to it fluorescent properties, and is conservative of the high-affinity iron(III) binding capacity. NBD-DFO partitions readily from aqueous solution into n-octanol (P_coeff = 5) and displays solvent-induced shifts in absorption and fluorescence spectra. The relative quantum yield of the probe's fluorescence increases over a 10-fold range with decreasing dielectric constant of the solvent. Fluorescence is quenched upon binding of iron(III) to the probe. We demonstrate here the application of NBD-DFO for the specific detection and monitoring of iron (III) in solutions and iron(III) mobilization from cells. Interactions between fluorescent siderophore and the ferriproteins ferritin and transferrin were monitored under physiological conditions. Iron removal from ferritin was evident by the demonstrable quenching of NBD-DFO fluorescence by scavenged iron(III). Quantitation of iron sequestered from cells by NBD-DFO or from other siderophore-iron(III) complexes was accomplished by dissociation of NBD-DFO-Fe complex by acidification and addition of excess ethylenediaminetetraacetic acid. The sensitivity of the method and the iron specificity indicate its potential for monitoring chelatable iron under conditions of iron-mediated cell damage, iron overload, and diseases of iron imbalance such as malaria.

Two families of chiral enterobactin analogs, based on 1,3,5-tris(aminomethyl)benzene (TRAM) and on tris(2-aminoethyl)amine (TREN) as anchors and amino acids linking the anchor to catechol residues, have been prepared and their structures and iron(III) binding properties examined. The TRAM catechoylamides with l-leucyl (5a) and l-alanyl (5b) were found to adopt random conformations in protic solvents, while the corresponding TREN catechoylamides with l-leucyl (8a) and l-alanyl (8b) form H-bonded structures under analogous conditions. All ligands bind Fe³⁺ in a 1:1 stoichiometry, and most of them adopt preferentially Δ-cis configurations when l-amino acids are used, similar to enterobactin. The TREN derivative 8a was shown to be the most efficient Fe³⁺ ion binder so far prepared, approaching enterobactin's binding constant within less than three orders of magnitude. The superiority of the TREN derivative 8a is discussed in the light of experimental and theoretical (EFF calculations) results. Spectroscopic data include mainly the following: (i) NMR data of the protected ligands in relation to their H-bonded conformations and (ii) CD data of the Fe³⁺ complexes in relation to their optical purity.

The objective of this study was to investigate whether a specific siderophore-mediated iron uptake (possibly preceded by a reduction step) occurs in corn roots at a specific recognition site. The activity of ferrichrome analogs and ferrioxamine B, as iron carriers into corn roots, was studied. Stereo-specific recognition of. Fe-ferrichrome analogs was observed during Fe uptake by corn roots. All ferrichrome analogs forming Fe(III) complexes of the same absolute configuration as does natural ferrichrome (jv-cis), enhance Fe uptake, while their enantiomers lack biological activity. Gallium (III) failed to be taken up under analogous conditions, indicating that synthetic ferrichrome analogs mediate the Fe uptake via a specific reduction step. Within certain limits, binding energy of the chelator did not play a role in enhancing iron uptake. It is suggested that the iron uptake mechanism in corn roots involves binding to a specific chiral receptor in conjunction with a reductive process.

Vibrational circular dichroism (VCD) spectra have been recorded in the NH and C=O stretching regions for a series of tripodal peptides and peptides containing an unsaturated residue, in order to obtain information about solution conformations.

Natural ion binding molecules, specifically iron(III) carriers, are used as guiding models for the preparation of synthetic binders. This biomimetic approach takes advantage of the evolutionary edge of natural molecules and aims at reproducing the essential characteristics of the most potent natural compounds with the simplest possible synthetic structures. The potent Enterobactin siderophore is examined as a guiding model. Its H-bonding (hydrogen-bonding) network and elements of preorganization are reproduced with all-synthetic, C₃-symmetric, chiral Triscatecholates to provide efficient and selective iron(III) binders of high optical purity. The structural principles operating in Enterobactin are then applied to the synthesis of Ferrichrome analogs, but with a shift of emphasis to conformational characteristics and biological activity. Comparison between two homologous families of Trishydroxamates demonstrates the role of H-bonding networks and van-der-Waals interactions in dictating the complexes' isomeric and optical purity, and highlights the conformational consequences of H-bonds. Some analogs of Ferrichrome reproduce the function of the natural compound as microbial iron(III) carriers by binding to both membrane receptors and transport proteins. Other derivatives inhibit the performance of the natural carriers by blocking the membrane receptors. The three strands of the C₃-symmetric ligands are extended, each carrying two hydroxamates, to become ditopic ion binders. These ligands form triple-stranded, helical metal complexes which are stabilized by H-bond networks and allow systematic extension to polynuclear complexes. Our design was governed by a few basic principles. We separated functional elements from structural elements, and used chiral elements as structural probes. To allow systematic modifications we adopted a modular synthetic strategy, and made use of variable modules as building blocks. As a common theme we applied multiple weak forces, particularly H-bond networks, to shape the individual molecules, stabilize their complexes and possibly favor their assembly to supramolecular architectures. Throughout all stages of this work physicochemical and biological testing of the compounds was joined by theoretical calculations in order to put experimental observations into a coherent structural framework.

The synthetically produced fluorescent siderophore NBD-desferrioxamine B (NBD-DFO), an analog of the natural siderophore ferrioxamine B, was used to study iron uptake by plants. Shortterm (10-hour) ⁵⁵Fe uptake rates by cotton (Gossypium spp.) and maize (Zea mays L.) plants from the modified siderophore were similar to those of the natural one. In longer-term uptake experiments (3 weeks), both siderophore treatments resulted in similar leaf chlorophyll concentration and dry matter yield. These results suggest that the synthetic derivative acts similarly to the natural siderophore. The NBD-DFO is fluorescent only when unferrated and can thus be used as a probe to follow iron removal from the siderophore. Monitoring of the fluorescence increase in a nutrient solution containing Fe³⁺-NBD-DFO showed that iron uptake by plants occurs at the cell membrane. The rate of iron uptake was significantly lower in both plant species in the presence of antibiotic agent, thus providing evidence for iron uptake by rhizosphere microbes that otherwise could have been attributed to plant uptake. Confocal fluorescence microscopy revealed that iron was taken up from the complex by cotton plants, and to a much lesser extent by maize plants. The active cotton root sites were located at the main and lateral root tips. Significant variations in the location and the intensity of the uptake were noticed under nonaxenic conditions, which suggested that rhizosphere microorganisms play an important role in NBD-DFO-mediated iron uptake.

Linear hydroxamate derivatives, possessing chiral α-amino acid moieties, were synthesized and their iron transport activities were studied in bacteria and fungi. No growth-promoting activity could be detected in the Gram-positive hydroxamate-auxotroph Aureobacterium flavescens JG9. However, Gram-negative enterobacteria, such as Escherichia coli, Pantoea agglomerans and Hafnia alvei were able to utilize iron from these analogues. Uptake of⁵⁵Fe-labeled analogues was inhibited by sodium azide, suggesting an active transport process. The receptors involved during uptake in enterobacteria were identified by using appropriate indicator organisms which are defective in the transport of either ferrioxamines (P. agglomerans FM13), coprogens (H. alvei), or both of these siderophore classes (E. coli fhuE). Our data suggest that the chiral hydroxamates are recognized by the ferrioxamine receptor (FoxA) and the coprogen receptor (FhuE) at a ratio which depends on the optical ν/δ isomer fraction and the nature of side chains. Transport was also observed in the fungus Neurospora crassa, known to take up coprogen rather than ferrioxamines, suggesting that in this fungus the synthetic analogues behave like coprogen.

Chiral ligands are introduced whose conformations may be regulated by the presence of Ca²⁺ ions. These compounds are assembled from two types of C₃-symmetric molecules as anchors (type 1 and type 2) and extended by chiral peptide residues. Their ion-binding properties are examined by a combination of experimental and theoretical tools (empirical force field (EFF) calculations). The calculations provide a conceptual framework in which to relate all the experimental observations into a coherent picture. The first type of compounds is shown to form complexes of prismatic geometry, inherently unfit to generate chiral complexes, while the second type forms complexes of octahedral coordination. Some of the ligands are found to generate Ca²⁺ complexes of optically active configurations and to thereby represent the first examples of chiral alkaline earth metal complexes. Ca²⁺ ions may thereby provide a viable exogenous means for the generation of chiral receptors from randomly arranged tripod-like molcules.

The neutral noncyclic imide and ether containing ionophore (AS701), a selective carrier for Li⁺ among alkali cations, was found to be capable of mediating the transport of NH₄⁺ and of biogenic amines (catechols and indoles) across lipid bilayer membranes also. Ionophore-mediated electrical properties of planar lipid bilayers were studied under experimental conditions where the positively-charged amine species was dominant. The ionophore was found to act as a selective carrier of the biogenic amines, mediating their electrogenic transport across the membrane, forming 2:1 carrier-amine permeant complexes, carrying a net-charge of +1. Selectively among the amines corresponding to the following sequence: tryptamine (35) > Li⁺ (1) > serotonin (0.60) > dopamine (0.19) > norepinephrine (0.13) > epinephrine (0.05) > NH₄⁺ (0.05). The molecular factors involved in determining these selectivities are assessed.

An algorithm is described for generating atomic Cartesian coordinates of conformations of macrocyclic molecules possessing exact rotational or rotationreflection symmetries. A fragment representing the asymmetric unit of the molecule is suitably oriented in space, and then a symmetry operator is applied to generate the initial coordinates of the molecule. An empirical force field of interatomic interactions is used to generate equilibrium conformations. Results of calculations performed on two cyclic polylactones and one crown ether using this approach are given. They reveal that symmetric conformations of these molecules are often preferred. Since the latter conformations are probably responsible for the specialized properties of these molecules, this method should facilitate doing theoretical studies on these kinds of compounds.

The design, preparation, and properties of a new series of lipophilic lithium ion carriers are described. The structure of these carriers is based on an acyclic system in which a hexafunctional lipophilic envelope is formed around the metal ion in an octahedral arrangement. The synthesis involves a series of condensation reactions using a cyclic tin-oxygen compound as an activated diol precursor. The carrier properties for lithium ions were demonstrated by in vitro experiments on liposomes using a fluorescence assay. The potential pharmacological applications of these ionophores are discussed.

A series of protected β-hydroxy α-amino acids have been converted stereospecifically to their dehydro derivatives by treatment with (diethylamino)sulfur trifluoride and pyridine, three isomers giving rise to the Z derivatives and erythro isomers to the E derivatives.

Keywords: Chemistry, Inorganic & Nuclear; Chemistry, Organic

The utility of covalently bonded metalloid templates is demonstrated by syntheses with (a) Bu₂SnO and (b) Me₂Si(NCO) (or Me₂Si(NCS)₂). Approach (a) leads to macrocycles from diols and diisocyanates, and approach (b) gives unsymmetrically substituted heterocycles from symmetrical reactants (Im  imidazolyl). (Figure Presented.) (Figure Presented.)

The efficient preparation of macrocyclic amino acid derivatives via the use of tin as covalent template is described.

Silicon isocyanates are introduced as versatile reagents for the mild and efficient conversion of hydrazines to urazole derivatives.