Phospholipids, which provide valuable model systems for biomembranes, display a variety of polymorphic phases, depending on their molecular structure and environmental conditions (pH, ionic strength, temperature and pressure). By use of x-ray and neutron diffraction, infrared spectroscopy and calorimetry, the temperature and pressure dependent structure and phase behaviour of one- and two-component lipid systems, differing in chain configuration and headgroup structure, was studied. Neutron scattering in combination with the H/D contrast variation technique was used to the study of the lateral organization of phase-separated binary lipid mixtures. In addition to pure lipid systems, we present data on the effect of incorporating steroids and polypeptides (e.g., gramicidin) on the structure and phase behaviour of lipid bilayers. Besides lamellar phases also non-lamellar phases (inverted hexagonal and bicontinuous cubic phases) were investigated. Hydrostatic pressure has been used as a physical parameter for studying the stability and energetics of these lyotropic mesophases, but also because high pressure is an important feature of certain natural membrane environments (e.g., marine biotopes) and because the high pressure phase behaviour of biomolecules is of biotechnological interest (e.g., "high pressure food processing"). Furthermore, we used pressure as a kinetic variable. Applying the pressure-jump relaxation technique in combination with time-resolved synchrotron x-ray diffraction, the kinetics of various lipid phase transformations was investigated. We show that the time constants for completion of the transitions depend on the direction of the transition, the symmetry and topology of the structures involved, and also on the pressure-jump amplitude. The techniques can also be applied for studying other biomolecular phase transformations, such as protein folding and refolding. We present data on the pressure-induced unfolding and refolding of small proteins, such as Snase, Rnase and ubiquitin. These studies were performed using synchrotron x-ray scattering and Fourier-transform infrared spectroscopy, which monitor changes in the tertiary and secondary structural properties of the protein upon pressurization or depressurization. The data are compared with the corresponding results obtained using other trigger mechanisms and are discussed in the light of recent theoretical approaches for protein folding.



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1. Surfactant aggregates as nanocontainers of variable size and properties: pressure tuning of enzymes
2. Patterns in the Phase and Structural Behavior of Lyotropic Lipid Mesophases, Model Biomembranes and Proteins at High Pressure