Phase behavior and kinetics of pressure-jump induced phase transitions of bicellar lipid mixtures

Mixtures of the short-chain 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) and the long-chain 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) have emerged as a powerful system to study membrane-associated, biologically relevant macromolecules and assemblies. Whereas the temperature-dependent behavior of various mixtures of this system has been studied in detail, almost nothing is known about the pressure-dependent phase behavior of the system. Here we are using small-angle X-ray scattering (SAXS) and Fourier-transform infrared (FTIR) spectroscopy to study the p, T-phase diagram of the system DMPC/DHPC (at a molar ratio of 3.2). The results reveal the existence of further high-pressure phases of the system and are prerequisite for understanding of the structure and dynamics of biomolecules in bicellar environments at high-pressure conditions, such as for high-pressure NMR studies of proteins embedded in bicellar structures. By combining time-resolved SAXS (trSAXS) with the pressure-jump relaxation technique we were also able to reveal a detailed picture of the kinetic processes involved in the phase transitions between lamellar, bicellar and nematic phases. Generally, the transitions follow a two-step mechanism consisting of a fast (subsecond) component involving conformational transitions of the lipid chains, which are followed by slower relaxational processes of the lipid mesophase structures including changes in their hydration properties. Depending on the lipid phases involved, the direction of the pressure-jump and the pressure-jump amplitude, this kinetic trapping lasts for a few seconds up to a maximum of several minutes.