EFFECT OF INCREASED CHAIN PACKING ON GRAMICIDIN-LIPID INTERACTIONS

To study the effect of lipid packing on the dynamics of membrane proteins, the changes in the rotational motion of gramicidin tryptophans with increased packing brought about by high hydrostatic pressure through fluorescence spectroscopy were determined. In fluid phase dimyristoylphosphatidylcholine, the rotational motion of the residues decreased slightly with increased packing, but in the gel phase a significant reversible increase was observed. The magnitude of this increase was temperature dependent and much greater at lower temperatures. Quenching studies show that the increase in rotational motion is not due to a change in the location of the peptide in the membrane under pressure. Aromatic ring stacking between residues 9 and 15 appears to be stabilized under pressure, and there is no evidence of pressure-induced changes in peptide aggregation. The increase in rotational motion could be caused by a destabilization of hydrogen bonds between the indole hydrogens and the lipid head group oxygens due to an increase in the thickness of the compressible lipid bilayer with pressure without a concomitant lengthening of the peptide. These results indicate that specific interactions between lipids and proteins may play a major role of regulating the dynamics of membrane proteins.