X-RAY STUDIES OF PRESSURE EFFECTS IN BIOLOGICAL SYSTEMS. Sol M. Gruner, Department of Physics, Princeton University, Princeton, NJ 08544

We have used x-ray diffraction to probe pressure effects on membranes, proteins and polymers. There is little understanding of the structural basis of the many biological effects of pressure in the 0-2 kbar range. Pressure of this magnitude is known to reverse anaesthesia, alter biomembrane properties, unfold or denature protein assemblies and change the activity of enzymes. Since the molecules involved are highly incompressible, the mechanisms involved cannot be a gross reduction in molecular volume. Rather, structural effects are mediated by subtle purturbations in the carefully balanced competition of interactions which give macromolecular assemblies their 3-dimensional conformation and degree of association, such as changes in surface hydration, modifications of fluctuation magnitudes, and anisotropies in compressibility leading to exposure of specific chemical groups. As examples, x-ray results will be presented on the way in which pressure changes the structure of membrane lipid and protein assemblies. Mechanisms whereby these changes may couple to protein activity will be discussed. The discussion will conclude with open questions which may be addressed by pressure-diffraction studies.