Induced circular dichroism as a tool to investigate the binding of drugs to carrier proteins: Classic approaches and new trends

Induced circular dichroism (ICD) is a spectroscopic phenomenon that provides versatile and useful methods for characterizing the structural and dynamic properties of the binding of drugs to target proteins. The understanding of biorecognition processes at the molecular level is essential to discover and validate new pharmacological targets, and to design and develop new potent and selective drugs. The present article reviews the main applications of ICD to drug binding studies on serum carrier proteins, going from the classic approaches for the derivation of drug binding parameters and the identification of binding sites, to an overview of the emerging trends for the characterization of binding modes by means of quantum chemical (QC) techniques. The advantages and limits of the ICD methods for the determination of binding parameters are critically reviewed; the capability to investigate the binding interactions of drugs and metabolites to their target proteins is also underlined, as well as the possibility of characterizing the binding sites to obtain a complete picture of the binding mechanism and dynamics. The new applications of ICD methods to identify stereoselective binding modes of drug/protein complexes are then reviewed with relevant examples. The combined application of experimental ICD spectroscopy and QC calculations is shown to identify qualitatively the bound conformations of ligands to target proteins even in the absence of a detailed structure of the binding sites, either obtained from experimental X-ray crystallography and NMR measurements or from computational models of the complex.