California NanoSystems Institute

For almost two decades our efforts have been broadly focused on "complex fluids", from both analytical and computational points of view. Examples of such systems include: micellized surfactant solutions and microemulsions; lipid bilayers; colloidal suspensions; polymer solutions, including polyelectrolytes; interfacial and surface thin films; and liquid crystals. In all cases the systems involve a mesoscopic length scale -- nanometers, typically -- intermediate between the microscopic and macroscopic. Correspondingly, it is almost always inappropriate to describe them via use of a molecular, atomistic, hamiltonian. Instead, the various energies and entropy terms are best treated phenomenologically, in order to address the basic questions involving structural evolution and phase transitions on mesoscopic scales. Our work up through 1996, along with a critical perspective of the field, is reviewed in the Centennial issue (volume 100) of the Journal of Physical Chemistry: W. M. Gelbart and A. Ben-Shaul, "The 'New' Science of Complex Fluids", pp. 13169-89. More recent investigations, featuring spontaneous pattern formation in nanoparticle dispersions, are described in W. M. Gelbart, R. P. Sear, J. R. Heath and S. Chaney, "Array Formation in Nano-colloids: Theory and Experiment in 2D", Faraday Discussions 112, 299-307 (1999).

Life cycles of viruses: DNA condensation, complexation, and encapsidation

As a natural outgrowth of our longstanding interest in the statistical mechanics of complex fluids, we have begun to concentrate recently on a wide range of problems involving biological consequences of the mesoscopic properties of DNA. Even as it is the basic repository of genetic information, DNA is also "just" a linear polyelectrolyte and 1D elastic object whose consequent physical properties are of fundamental importance to all living systems. DNA in most bacterial viruses, for example, is strongly condensed, i.e., its density is comparable to that of crystalline DNA; furthermore, it is confined in protein capsids whose dimensions are orders of magnitude smaller than its length (see Fig. 1). We are interested in how DNA is packaged in these viral capsids (Fig. 2) and how its state of stress determines the way it is injected into a bacterial cell to begin the infection process. In the case of most animal viruses, like polio, flu and HIV, on the other hand, infection is initiated by passage of the entire viral particle -- protein capsid and all -- through the plasma membrane into the cell cytoplasm. We study this process -- endocytosis -- from a physical point of view, i.e., a generic colloidal particle passing through a molecular bilayer; phenomenological connections to specific biological systems are developed by treating, for example, the effect of receptor proteins on membrane elasticity and particle/membrane adhesion energy, etc. We also investigate the structure and dynamics of nucleosomes, the basic building blocks of chromosomes in which DNA is complexed with globular aggregates of cationic proteins (the histones). In another set of problems we are attempting to understand how -- and why -- icosahedral symmetry plays such a fundamental role in the self-assembly of viral protein capsids (Fig. 3).