Rotation of Lipids in Membranes: MD Simulation, ³¹P Spin-Lattice Relaxation, and Rigid-Body Dynamics

¹ University of Maryland
² Boston College
³ Brandeis University
⁴ National Institutes of Health

^* To whom correspondence should be addressed. E-mail: pastorr{at}nhlbi.nih.gov.

Submitted on September 11, 2007
Revised on November 10, 2007
Accepted on 30 November 2007

	Abstract

A comparison of molecular dynamics simulations and ³¹P NMR spin lattice R₁ relaxation rates from 0.022 to 21.1 Tesla of fluid phase dipalmitoylphosphatidylcholine (DPPC) bilayers is presented. Agreement between experiment and direct prediction from simulation indicates that the dominant slow relaxation (correlation) times of the dipolar and chemical shift anisotropy (CSA) spin-lattice relaxation are 10 ns and 3 ns, respectively. Overall reorientation of the lipid body, consisting of the phosphorus, glycerol and acyl chains, is well described within a rigid body model. Wobble, with D_perp = 1-2 x 10⁸ s^-1, is the primary component of the 10 ns relaxation; this timescale is consistent with tumbling of a lipid-sized cylinder in a medium with the viscosity of liquid hexadecane. The value for D_parr, the diffusion constant for rotation about the long axis of the lipid body, is difficult to determine precisely because of averaging by fast motions and wobble; it is tentatively estimated to be 1x10⁷ s^-1. The resulting D_parr/D_perp 0.1 implies that axial rotation is strongly modulated by interactions at the lipid/water interface. Rigid body modeling and potential of mean force evaluations show that the choline group is relatively uncoupled from the rest of the lipid. This is consistent with the ratio of CSA and dipolar correlation times reported here, and the previous observations that ³¹P NMR lineshapes are axially symmetric even in the gel phase of DPPC.

Key Words: axial rotation, diffusion, dipalmitoylphosphatidylcholine, field cycling 31P NMR, potential of mean force, wobble in a cone