Pulsed field gradient nuclear magnetic resonance study of time-dependent diffusion behavior and exchange of lipids in planar-supported lipid bilayers.

This work reports the direct experimental observation of lipid exchange between liquid-ordered domains and their liquid-disordered surroundings in 3-component planar-supported multibilayers (1,2-dioleoyl-sn-glycerol-3-phosphocholine/1,2-dipalmitoyl-sn-glycero-3-phosphocholine/cholesterol). The measurements of lipid lateral diffusion and exchange were carried out using proton pulsed field gradient (PFG) NMR spectroscopy with high field strength (17.6 T) and high gradient amplitudes (up to 30 T/m). Application of large gradients affords the use of sufficiently small diffusion times under the condition that the width of the gradient pulses is much smaller than the diffusion time. As a result, PFG NMR studies of time-dependent diffusion behavior in lipid bilayers become possible over submicrometer length scales of displacements, which are comparable with the domain size. Comparison of the PFG NMR diffusion data and the corresponding results of dynamic Monte Carlo simulations allowed for the estimation of domain boundary permeability and domain size at temperatures near the transition temperature for the studied bilayers.