Correlation functions for lipid membrane dynamics obtained from NMR spectroscopy

摘要

Nuclear magnetic resonance (NMR) studies of the spin relaxation of lipid membranes provide a powerful tool for investigating the dynamics of these important biological structural elements. Here spectral densities of motion for various dynamical models have been fitted to H-2 spin-lattice relaxation rates' (R(1Z)) measured for vesicles for 1,2-dimyristoyl-sn-glycero-3-phosphocholine, in the liquid-crystalline state, over a broad frequency range (2.59-95.3 MHz; total of 15 magnetic-field strengths). Moreover, the corresponding C-13 R(1Z) values predicted from the models have been compared to experiment from 15.0 to 151 MHz, thereby enabling unification of the NMR relaxation data for bilayer lipids. A molecular diffusion model or alternatively a three-dimensional collective fluctuation model describes best the 2H and C-13 R(1Z) data. To emphasize the universality of this approach, the models have also been fitted to C-13 R(1Z) data for vesicles of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (15.0-151 MHz; eight magnetic field strengths), and the H-2 R(1Z) values for the corresponding multilamellar dispersions theoretically predicted. Correlation functions have been calculated for the lipid reorientations from the analysis of NMR relaxation data. The results suggest that slower motions are predominant in the low to mid megahertz range due to noncollective molecular motions or thermal collective excitations, whereas the bilayer interior corresponds to liquid hydrocarbon. The reorientational correlation functions derived from NMR spectroscopy are compared to recent molecular-dynamics simulations of bilayer lipids in the fluid phase.