In this Letter we propose a development of a NMR methodology to study ligand-macromolecular receptor interactions, based on the analysis of proton selective spin-lattice relaxation rate enhancements of the ligand, to calculate an affinity index, related to strength of the interaction process. In this work we modified this index 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. This approach also allowed the calculation of the relaxation rate of the bound ligand as well as the equilibrium constant of the interaction process. (C) 2007 Elsevier B.V. All rights reserved.