Saturation-Transfer Difference (STD) NMR: A Simple and Fast Method for Ligand Screening and Characterization of Protein Binding

Saturation transfer difference (STD) NMR has emerged as one of the most popular ligand-based NMR techniques for the study of protein ligand interactions. The success of this technique is a consequence of its robustness and the fact that it is focused on the signals of the ligand, without any need of processing NMR information about the receptor and only using small quantities of nonlabeled macromolecule. Moreover, the attractiveness of this experiment is also extendable to the classroom. In the context of a practical NMR class, this experiment is ideal to illustrate some fundamental NMR concepts, such as the nuclear Overhauser effect and relaxation in a multidisciplinary context, bridging chemistry and biochemistry with a taste of medicinal chemistry. We use the readily available human serum albumin (HSA), 6-D,L-methyl-tryptophan (6-CH(3)-Trp), and 7- D,L-methyl-tryptophan (7-CH(3)-Trp) to introduce the STD-NMR experiment and to illustrate its applicability for ligand screening, mapping of binding moieties, and determination of the dissociation constant, in a context that can be explored or adapted to the student's course level and topic (chemistry or biochemistry). We also cover the most important theoretical aspects of the STD experiment, calling attention to some of its limitations and drawbacks without a complex theoretical approach.