Molecular dynamics simulation of methyl group relaxation in water

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Abstract
Recent theoretical studies of stochastic classical and quantum dynamics suggest that quantum effects may be significant in the nuclear magnetic resonance (NMR) relaxation of a methyl group attached to intermediate and large sized molecules at normal temperatures. The magnitude of the effects depends on the reorientational correlation time of the molecule, τ0, the barrier to internal rotation, V, and the correlation time for the collision induced randomization of the internal angular momentum, τω. This note reports the results of molecular dynamics simulations used to estimate τω for a methyl group in water. The simulations indicate that extended diffusion, rather than Fokker–Planck–Langevin, dynamics are appropriate for this system. Using τω determined in the simulation and V=13 KJ/mol (3 Kcal/mol), predictions of the classical and quantum models differ by 40% for the 13C relaxation of a methyl on an intermediate sized molecule, such as a disaccharide. In larger molecules, the differences are smaller due to smaller contributions from the internal dynamics to the NMR relaxation process, unless τω and/or V are larger. Experimental evidence suggests that τω is an order of magnitude or more larger in a hydrocarbon environment than in water. With the larger τω, a plausible example is given where the models’ predictions differ by 80%. More work is needed, however, to better define τω in hydrophobic regions of macromolecules.