Biaxial ordering of terminal diene groups in lipid membranes: an infrared linear dichroism study

The molecular order within the hydrophobic core of membranes of the diene l ipid di-tetradecadienoylphosphatidylcholine was studied by means of infrare d spectroscopy on multibilayer assemblies which orient macroscopically on t he surface of an attenuated total reflection crystal. The relative humidity and temperature were used as variable parameters to demonstrate that there were profound differences in the melting transition of lipids possessing p redominantly cis and trans diene groups. The cis isomer undergoes the phase transition at a vapor pressure which is increased by similar to 0.15 GPa w hen compared with that of the trans isomer. The methylene wagging band prog ression gives no indication of differences between the acyl chain conformat ion of the cis and trans forms in the gel state. The frequencies of a numbe r of absorption bands of the diene groups reveal that these moieties are pr edominantly in the s-trans conformation to accommodate a favorable packing within the bilayer, The linear dichroism of selected in-plane and out-of-pl ane vibrations of the diene groups gives indications of the biaxial orderin g of these moieties. We present the basic equations for the quantitative an alysis of IR dichroism data of lamellar structures in terms of transverse a nd longitudinal molecular order parameters. It turns out that the planes of the rigid diene groups orient preferentially in a perpendicular direction with respect to the bilayer surface and parallel to each other forming in t his way a layer of well-aligned diene groups in the bilayer center. This fi nding is confirmed by the results of X-ray measurements. We suggest that th e partial interdigitation of the diene groups of the sn-1 acyl chains promo tes the formation of the inverse H-II phase and/or enables the formation of covalent bonds between both the monolayers upon polymerization of diene li pids. (C) 1999 Elsevier Science B.V. All rights reserved.