Double-selective excitation techniques as a means of delineating dynamics of protein-bound small molecules

A method for delineation of dynamics of protein-bound ligands is presented that relies on measuring 'H NMR spin-lattice relaxation rates upon double-selective excitation of dipolarly connected ligand resonances. Dipolar interaction energies of proton pairs are obtained in the free and bound states. For protons at fixed distances such dipolar energies are interpreted in terms of motional correlation times of relaxation vectors. As an example the reorientational correlation time of the indole moiety was evaluated at 23.5 ps for L-tryptophan in water solution and at 27.0 ns for L-tryptophan bound to human serum albumin.