Spectroscopic Studies of Specifically Deuterium Labeled Membrane Systems. Nuclear Magnetic Resonance Investigation of Protein-Lipid Interactions in Escherichia coli Membranes

Deuterium nuclear magnetic resonance (NMR) spectra of membrane vesicles and lipid extracts from the fatty acid auxotroph Escherichia coli L48-2, grown on hexadecanoic acid specifically deuterated in the terminal methyl position, have been obtained at 34.1 MHz by using the quadrupole-echo technique. At temperatures of 0-30 °C, the spectra of both samples exhibit two components, the broader component with a quadrupole splitting (Acq) of ~10 kHz from lipid molecules in a gellike phase and the narrower component (Δvq of ~2-4 kHz) from a liquid-crystalline-like phase. The broad component is only ~20% of the signal at 30 °C, but by 0 °C it becomes 85% for the lipid extracts and 75% for the membrane vesicles. Its width is comparable for both samples and, within experimental error, is temperature independent from 0 to 30 °C. On the other hand, the narrow component is consistently broader and has a doublet structure for the lipid extracts (Δvq ~ 3.4 kHz at 30 °C) but is a featureless line (of ~2.2-kHz width) for the membrane vesicles. Also, it is significantly broader at 0 °C than at 30 °C for both samples. These results indicate that in the region of the terminal methyl group, the protein in the membrane vesicles interacts preferentially with the lipids in the fluid state. The interaction produces a “dynamic disordering” of the lipid, in which the motions of the lipid hydrocarbon chains are more effective in averaging out the quadrupole splitting of the methyl group. Also, at 0 °C the interaction increases the fraction of lipid in the disordered phase. There is no evidence for formation of a rigid “boundary layer” of lipid around the membrane proteins.