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.