Evaluating spin diffusion in MAS-NOESY spectra of phospholipid multibilayers.

Biological semisolids such as aqueous phospholipid dispersions are amenable to experiments that combine magic-angle spinning (MAS) to yield high-resolution 1H NMR-spectra and two-dimensional nuclear Overhauser spectroscopy (NOESY) to estimate proton-proton distances. Using several selectively deuterated dimyristoylphosphatidylcholines, a systematic investigation has been made into the origin of an anomalous NOESY crosspeak between methyl groups of the lipid headgroup and the acyl chains. Although the spin-relaxation and line-narrowing behavior in these multilamellar systems argue against efficient spin diffusion, interdigitated or chain-bendback structural models that bring these 1H nuclei into close proximity are not implicated by the MAS-NOESY results for this phospholipid system. A mechanism has been proposed for spin communication between the two types of methyl protons within a conventional lipid bilayer structure: through-space interactions link the headgroup and backbone protons on adjacent molecules, and then spin diffusion occurs along the acyl chains.