Interaction of quercetin and its conjugate quercetin 3-O-beta-D-glucopyranoside with albumin as determined by NMR relaxation data.

NMR methodology has been developed in order to study phytochemical-macromolecular receptor interactions. This approach is based on the analysis of proton selective spin-lattice relaxation rate enhancements of the ligand in the presence of the macromolecule, to calculate an affinity index, [ A] L (T) , related to the strength of the interaction process. This index has been modified by normalization to the relaxation rate of the free ligand, in order to take into account the effects of motional anisotropies and different proton densities. The normalized affinity index, [ A N ] L (T) , isolates the contribution due to a decrease in the ligand dynamics caused by the binding with the protein. This methodology has been applied to the interaction between two flavonoids (quercetin, 1, and quercetin 3-O-beta-D-glucopyranoside, 2) and bovine serum albumin (BSA). The calculated values of the affinity indexes and thermodynamic equilibrium constants suggested a much stronger capacity of 1 to interact with BSA when compared with its glucosylated derivative, 2.