An Alternative Interpretation of Nuclear Magnetic Resonance Observations in the Gel State of Lipid Bilayers

In the established interpretation of nuclear magnetic resonance (NMR) spectra of phospholipid bilayers in the gel state, the molecules are assumed to perform rotational diffusion about their long axis. Here we present an alternative model of the molecular mobility in this phase, which considers the positions of the lipid molecules in the two-dimensional bilayer lattice as fixed within the NMR timescale. Instead we assume an intramolecular two-site hopping of the hydrocarbon chains about their long axis. It is shown that deuterium NMR spectra of chain-labeled compounds are very sensitive to the precise angle of this flip-flop motion near 90°, so that the diversity of these gel-phase spectra is easily explained by slight variations of this angle. In addition, it is argued that the axial symmetry of ¹³C spectra of carbonyl-labeled phospholipids might also result from this intramolecular mobility.