Theory of spinhyphen;lattice relaxation in lipid bilayers and biological membranes.2H and14N quadrupolar relaxation

摘要

Based on a previous, more approximate treatment lsqb;M. F. Brown, J. Magn. Reson. 35, 203 (1979)rsqb;, expressions are derived for the quadrupolar spinhyphen;lattice (T1) relaxation rates of2H and14N in lipid bilayers. Results are presented for the most general, anisotropic rotational diffusion model describing the segmental or molecular reorientation in lipid bilayers, and the analysis is extended to include relatively slow fluctuations of the local director with respect to the macroscopic bilayer normal. Numerically computed values ofT1for the diffusion model suggest that, even for extremes of ordering and motional anisotropy, such a model cannot by itself quantitatively account for the observed2HT1values of multilamellar dispersions of 1,2hyphen;dipalmitoylhyphen;snhyphen;glycerohyphen;3hyphen;phosphocholine (DPPC), in the liquid crystalline state, as a function of temperature and frequency. The contribution from relatively low frequency motions is modeled in terms either (i) a simple noncollective model in which the slow motions are described in terms of a single effective correlation time, or (ii) a collective model in which the relatively slow reorientation is described by a distribution of correlation times, corresponding to collective fluctuations of the instantaneous director. The experimentally observed dependence of the2HT1relaxation rates on the acyl chain segmental order parameterSCDand the resonance frequency ohgr;0are most consistent with a collective model for slow molecular reorientations in lipid bilayers. The2HT1data for the saturated DPPC bilayer, in the liquid crystalline state, can be quantitatively described by a relaxation law of the formTminus;11=Atgr;f+BS2CDohgr;minus;1/20as observed for simpler nematic and smectic liquid crystals. The first (A) term is suggested to correspond totransndash;gaucheisomerizations of the lipid acyl chains, while the (B) term describes collective bilayer modes which predominantly influence the frequency dependence of the relaxation. In contrast to earlier conclusions lsqb;M. F. Brownetal., J. Chem. Phys. 70, 5045 (1979)rsqb;, the dominant contribution to the2HT1relaxation rates of the saturated DPPC bilayer may arise from collective order fluctuations rather than fast local motions. The value of tgr;fsim;10minus;11s obtained by extrapolatingTminus;11to infinite frequency or zero ordering is consistent with the correlation times calculated from2H or13CT1data fornhyphen;alkanes of equivalent chain lengths, suggesting that the microviscosity of the bilayer hydrocarbon region is not appreciably different from that of paraffinic liquids.