Effects of internal motions on the development of the two-dimensional transferred nuclear Overhauser effect

In this paper we address the influence of internal motions on the development of the transferred nuclear Overhauser effect in a ligand undergoing chemical exchange between a free and a bound state. We examine the effects of varying the effective correlation time as well as the motional order parameter for methyl group and phenyl ring rotations in the free and bound ligand conformations. The effect of decreasing the motional order for a proton pair on a methyl group or phenyl ring is to decrease the effective correlation time of the internuclear vector, and thus to decrease the cross-relaxation rate between the proton pair. This functions to dampen the effects of spin diffusion, especially in the bound ligand where cross-relaxation rates are much faster than in the free ligand. The effect of changing the effective correlation time for methyl group motions has little effect on the build-up behaviour of the transferred nuclear Overhauser effect for small values of fraction bound, but a larger effect on how fast it decays. This effect is greater for internal motions in the free peptide than it is for internal motions in the bound peptide.