DYNAMICS OF LIPID BILAYERS FROM COMPARATIVE-ANALYSIS OF H-2 AND C-13 NUCLEAR-MAGNETIC-RESONANCE RELAXATION DATA AS A FUNCTION OF FREQUENCY AND TEMPERATURE

Analysis of the nuclear spin relaxation rates of lipid membranes provi des a powerful means of studying the dynamics of these important biolo gical representatives of soft matter. Here, temperature-and frequency- dependent H-2 and C-13 nuclear magnetic resonance (NMR) relaxation rat es for vesicles and multilamellar dispersions of 1,2-dimyristoyl-sn-gl ycero-3-phosphocholine (DMPC) in the liquid-crystalline slate have bee n fitted simultaneously to various dynamic models for different positi ons of the acyl chains. The data include H-2 R-1Z rates (Zeeman order of electric quadrupolar interaction) acquired at 12 external magnetic field strengths from 0.382 to 14.6 T, corresponding to a frequency ran ge from omega(D)/2 pi=2.50-95.3 NMz; and 2H R-1Q rates (quadrupolar or der of electric quadrupolar interaction) at 15.3, 46.1, and 76.8 MHz. Moreover, C-13 R-1Z data (Zeeman order of magnetic dipolar interaction ) for DMPC are included at six magnetic field strengths, ranging from 1.40 ro 17.6 T, thereby enabling extension of the frequency range to e ffectively (omega(C)+omega(H))/2 pi=938.7 MHz. Use of the generalized approach allows formulation of noncollective segmental and molecular d iffusion models, as well as collective director fluctuation models, wh ich were tested by fitting the H-2 R-1Z data at different frequencies and temperatures (30 degrees C and 50 degrees C). The corresponding C- 13 relaxation rates were predicted theoretically and compared to exper iment, thus allowing one to unify the C-13 and H-2 NMR data for bilaye r lipids in the fluid state. A further new aspect is that the spectral densities of motion have been explicitly calculated from the H-2 R-1Z and R-1Q data at 40 degrees C. We conclude that the relaxation in flu id membrane bilayers is governed predominantly by relatively slow moti ons. which modulate the residual coupling remaining from faster local motions (order fluctuations). Only the molecular diffusion model, incl uding an additional slow motional process, and the membrane deformatio n model describing three-dimensional collective fluctuations fit the H -2 NMR data and predict the C-13 NMR data in the MHz range. Orientatio nal correlation functions have been calculated, which emphasizes the i mportance of NMR relaxation as a unique tool for investigating the dyn amics of Lipid bilayers and biological membranes. (C) 1997 American In stitute of Physics. [S0021-9606(97)01447-5].