A simulation based model of NMR T1 relaxation in lipid bilayer vesicles

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Abstract
The results of the Brownian dynamics simulation of a hydrocarbon chain in a membrane bilayer described in the preceding paper are used to analyze the 13C NMR T1 relaxation in lipid bilayer vesicles. The analysis shows that the frequency dependence of the relaxation does not arise from gauche–trans isomerization or from axial rotation of the entire lipid molecule. However, a model in which fast axial rotation (D∥≊2×1010 s−1) and slow noncollective diffusive director fluctuations (D⊥≊1–2×108 s−1) are superimposed on the internal motions quantitatively accounts for both the magnitude and frequency dependence of the T1 data. An effective viscosity for the interior of the bilayer in the range of 1 cp, and a director order parameter of 0.5–0.7 are required to fit the NMR data. Collective effects do not appear necessary for explaining the NMR T1 data in vesicles, although they may be important for multilamellar dispersions.