Chemical Exchange Saturation Transfer or CEST is an experiment that allows the indirect detection of low concentrated species with the sensitivity of the pool at high concentration. One application of this technique is to study dynamics in host-guest system, which is termed GEST (Guest Exchange Saturation Transfer). In the GEST experiment, an abundant pool of a free guest is used to detect a dilute pool of the bound guest, as long as the two pools have different chemical shifts in the NMR spectrum, a and they are exchanging. By applying a selective pre-saturation radiofrequency (RF) pulse at the frequency offset of the peak with the lower concentration, its magnetization (i.e., its NMR signal) is nullified. The induced depolarization can be transferred to the abundant pool by chemical exchange. Therefore, if the exchange process between the bound and free guest (k in s−1 units) is fast enough and the applied saturation pulse is sufficiently long, a significant reduction in the intensity of the NMR signal of the free guest is observed, providing the ability to indirectly detect low concentrations of the bound guest with the much greater sensitivity of the free guest (which exists at a high concentration in the solution). In this experiment, the selective pre-saturation RF pulse is applied at various frequencies (black arrows) and the signal intensity of the free guest is being monitored:

There are many advantages in applying the GEST NMR technique when studying host-guest supramolecular systems. This method significantly improves the sensitivity of the NMR measurement by signal amplification, which allows the detection of nanomolar concentration, which is 4-5 order of magnitude lower than the detectability level of conventional NMR experiments. In addition to signal amplification, by using the GEST method quantitative information regarding the dynamic of host-guest systems can be extracted. With proper analysis, both the exchange rate of the guest and the host occupancy can be easily calculated.