MULTICOPY MODELING OF THE SOLVENT DISTRIBUTION IN MACROMOLECULAR CRYSTALS. Eduardo I. Howard and J. Raul Grigera, Instituto de Fisica de Liquidos y Sistemas Biologicos (IFLYSIB), Universidad de La Plata, c.c. 565, 1900, La Plata, Argentina; Alberto D. Podjarny and Alexander Urzhumtsev, IGBMC, UPR de Biologie Structurale, B.P. 163, 67404, Illkirch, Cedex, France

Hydration models of biomacromolecular crystals, as obtained by crystallographic diffraction studies, usually position water molecules on precisely defined sites. Other experimental results, such as NMR, indicate that a large part of the water content has high mobility and is delocalized. The objective of this work is to find a hydration model that describes these mobile water molecules, while keeping the agreement with the observed diffraction amplitudes. A multicopy water model is proposed to describe the mobility. A set of water molecules, positioned by conventional methods, is used to generate several non-interacting copies. The system is set to an initial temperature (typically 400deg. K) through assigning different initial velocities to each water molecule of the different copies. Then the system is very slowly cooled (5deg. steps) until it reaches the desired temperature (300deg. K). This dynamics simulation, implemented in XPLOR, includes the usual modeling forces and an X-ray term. The free R-factor is used to monitor the validity of the process. The method was applied to X-ray diffraction data from BPTI and RNA crystals. The results show that while some water molecules are highly localized (the different copies remain clustered in specific hydration sites), the rest are more widely distributed, sometimes forming water channels. The shape of the multicopy distribution agrees precisely with the Fo-Fc difference maps; in the BPTI case, simulations and difference maps using neutron data were used to cross-check the results. The obtained models agree with the crystallographic data and are more compatible with other experimental observations than the ones with single fixed sites.

This work is supported by the CNRS through the UPR 9004, by the CONICET

through the IFLYSIB and by the EU through the collaborative project CII-CT

93 0014 (DG 12 HSMU); JRG is an invited fellow financed by the MENESRIP

(France).