COMPUTER MODELLILNG AND OTHER THERORETICAL STUDIES OF BIOPOLYMERS

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The most significant result of the laboratory involved the development of a new class of algorithms for the simulation of interfacial systems. The algorithms consistently treat the surface tension as a thermodynamic variable, and thereby allow the surface area of the simulated system to fluctuate (much as isotropic "constant pressure" algorithms allow volume fluctuations). The method was refined on the water/octane interface, where calculated surface tensions were found to be in good agreement with experiment. Surface tensions have also been calculated for lipid bilayers, and shown to be a very sensitive function of surface area. Ongoing simulations of lipid monolayers will provide the pressure/area isotherm for these systems, and comparison with the bilayer results will lend insight to the elasticity and other mechanical properties of cell membranes. In the course of the research it was also discovered that the commonly used "weak coupling" constant pressure algorithm of Berendensen and coworkers contains a fundamental error which leads to unphysical results (e.g., large temperature differentials) when simulating inhomogeneous systems. A method for characterizing transition concertedness from polymer dynamics computer simulations was developed and used to analyze "kink" and other concerted dynamics in lipid bilayers. It was proposed that the diffusion of small solute molecules in membranes is better facilitated by nonconcerted transitions, which are associated with relatively large displacements of the chains, than by concerted transitions, which do little to change the chain shape.