Biomolecular materials: structure, interactions and higher order self-assembly

Abstract

We review recent experimental work in two-dimensional (2D) membrane-protein assemblies, in lamellar fluid membrane phases containing tail anchored polymer-lipids, and in binary lipid mixtures which self-assemble into equilibrium tubular vesicle phases. We outline new directions in the development of technologically useful biomolecular materials. Synchrotron X-ray scattering studies of the purple membrane comprised of the 2D self-assembly of the membrane protein bacteriorhodopsin (bR) are described. In particular, experiments are described in the low humidity regime where a remarkable, exceedingly high temperature phase of bR has been discovered. Higher order self-assemblies of the stacked 2D membranes, achieved by water removal, result in the complete suppression of the melting transition, and the absence of protein denaturation up to 140°C. Aside from the scientific interest of elucidating the intraprotein and interprotein forces responsible for retaining the folded structure at high temperatures, the findings suggest methods for the development of “heat-proof proteins”. Higher order self-assemblies of functional proteins used in biosensors (e.g. toxin detectors), in bioreactors (purifying genetically engineered proteins with high temperature sieves) and in catalytic applications should be stabilized to high temperatures. In another series of experiments, we shall describe the properties of a new class of lamellar hydrogels based on fluid membranes which contain small amounts of single-end-anchored polymer-lipids. There are striking differences between these membrane-based liquid crystalline biogels labeled L α,g and isotropic hydrogels of polymer networks; for example, mixtures with a larger water content require a smaller polymer concentration for gelation. A defining signature of these lamellar hydrogels is the presence of a highly defected underlying microstructure. “Bioactive gels” useful in tissue healing or drug delivery applications may be envisaged with activity derived from membrane-anchored peptides, proteins or other drug molecules, and mechanical stability resulting from the polymer-lipid minority component.