9870464 Sagui This project covers two lines of theoretical investigations in Condensed Matter Physics; effects of elastic fields on the process of phase separation in binary alloy and viscoelastic fluid systems; and issues related to the refinement of X-ray data for structural analysis of proteins in a solvated environment. The kinetics of growth and ordering that take place during the process of phase separation determines much of the important microstructure of materials. In turn, many of the interesting mechanical, electrical and magnetic properties of materials depend on the microstructure. Typically, when a system is rapidly quenched from the single-phase part of its phase diagram to a point inside its coexistence curve, it orders kinetically. A long-wavelength instability creates an initial droplet-like or interconnected structure, which grows to macroscopic size as time evolves. It has long been realized that the presence of elastic fields influences the process of phase separation in important ways. For example, in alloy systems shape transformations, ordering of domains, a kinetic slowing down, and possibly reverse coarsening effects have either been observed and/or predicted. However, theoretical understanding of these effects is at present incomplete, and this project aims to investigate the effects of elastic field on binary alloy systems through large-scale three-dimensional simulations of appropriate Langevin equations. Elastic effects also play an important role in viscoelastic fluids and foams. Viscoelastic fluids such as various polymer melts, are non-Newtonian fluids that are charac terized by elasticity on relatively short time scales, and fluidity on long time scales. During phase separation, these systems are strongly influenced by the presence of long-range, correlated elastic fields, whose influence on the growth kinetics are presently not understood. The problem of phase separation of viscoelastic fluids will be investigated with numerica l simulations. The second part of the research is aimed at employing state-of-the art molecular dynamics and optimization techniques as aids in the process of refining X-ray data for protein structure calculations. Proteins are complex, biologically active macromolecules exhibiting a large variety of different structural and dynamic properties. It has long been known that much of their active biological functions are determined by their three-dimensional structure, which fold so as to produce specific binding sites and/or catalytically active regions. One of the outstanding problems in the field, is the prediction of protein structure from its basic amino acid se quence. It is in this context that structural predictions based on X-ray data have proven to be of paramount importance. The project aims to move this field forward, through the introduction of newly developed optimization techniques in order to facilitate structural predictions. The PI will concentrate specifically on cases where models for the protein structure are ill or poorly defined, as in the case of protein loops in a solvated environment. %%% This is a Visiting Professorship grant made under the Professional Opportunities for Women in Research and Education (POWRE) program, and is co-funded by the MPS Office of Multidisciplinary Activities(OMA). The phase separation activities are natural extensions of the current work of the PI, while the protein structure research will enable the PI to transition into a new area of research, namely that of molecular biology.
