Investigation of phase transitions in bilayer membranes

Abstract

This article described three techniques used to study phase transitions in phospholipid bilayers. The complementarity of the three techniques in characterizing the thermotropic and structural properties of phospholipid bilayers has been demonstrated by describing their use to characterize a series of mixed-chain-length PCs. It has been shown that an understanding of the energetics that govern the packing of phospholipid chains in the gel phase can be used to construct a model to interpret thermodynamic data of the PCs. This model, in turn, provided a framework for designing and interpreting the Raman spectroscopic and X-ray diffraction experiments on this series of phospholipids. The result was a complete description of the phase transitions and gel phase packing properties of the mixed-chain-length PCs. The phase diagram of Fig. 5B has been expanded to include the mixed-chain-length PC series C18C18PC through C18C0PC. Furthermore, the phase diagram and the chain inequivalence parameter have been shown to describe the behavior of any mixed-chain-length PC, irrespective of the lengths of the hydrocarbon chains or the position of the chains on the glycerol backbone. This is demonstrated by the additional mixed-chain-length PCs plotted in Fig. 5B. With minor modifications, the phase diagram also accurately describes the behavior of mixed-chain-length phosphatidylethanolamines, sphingomyelins, and unsaturated PCs. Finally, it has been demonstrated that a correlation exists between the thermodynamic and the Raman spectroscopic parameters determined for the phase transition of phospholipid bilayers. This correlation is based on the common chain energetics being measured by these two techniques.