Effect of temperature, pressure and lipid acyl chain length on the structure and phase behaviour of phospholipid–gramicidin bilayers

Abstract

We investigated the effect of incorporation of gramicidin D (GD) on the structure and phase behaviour of aqueous dispersions of fully hydrated neutral phospholipid bilayers of different chain lengths 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC), 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) and 1,2-distearoyl-sn-glycero-3-phosphatidylcholine (DSPC). A temperature range of 0–80°C and a pressure range of 0.001–10 kbar (1 bar=10⁵ Pa) were covered. Pressure was applied so as to be able to tune the lipid chain lengths and conformation and to select specific gel phases. Infrared spectral parameters obtained by FT-IR spectroscopy, such as frequencies, intensities and bandwidths, were used to detect structural and dynamic changes upon incorporation of GD into the lipid bilayers. Analysis of the amide I band frequencies allowed us to determine the corresponding peptide structure adopted in the lipid environment. At a concentration of 5 mol% of GD in lipid bilayers, the structure of the temperature- and pressure-dependent lipid phases is significantly altered by the insertion of the polypeptide and broad gel–fluid co-existence regions are induced. Further, the lipid matrix has the ability to modulate the conformation of the inserted polypeptide. The balance between double-helical and helical dimer structures depends significantly on the phospholipid hydrocarbon chain length and phase state. The larger the hydrophobic mismatch, the higher is the population of the double-helical structures. By formation of broad gel–fluid co-existence regions, the system has a further means of avoiding large hydrophobic mismatch. In these regions, a molecular sorting mechanism seems to be very likely. No pressure-induced unfolding of the polypeptide is observed up to pressures of 10 kbar.