Cooperative effects in water-biomolecule crystal systems.

Abstract

Monte Carlo computer simulation techniques have been used to model non-pair-additive (cooperative) effects in the water organization around several-biomolecules. Although most models for water assume pair-additive potentials, both quantum mechanical calculations and experimental data indicate that cooperative effects are not negligible in hydrogen-bonded systems such as water. The many-body polarizable electropole (PE) model for water is used to examine the extent and the consequences of this cooperative behavior in-several biomolecule hydrate crystals. Increases in the dipole moments ofwater molecules are predicted in all systems studied so far and can be as much as 50% more than the monomer value of 1.855 debyes. The average value of the individual dipole moments for any one system differs from that of another system and, therefore, should be considered a property of the system and not of the water molecule itself. When this previously calculated average value of the dipole moment for water molecules in a given system is used as a fixed parameter in the simulation, we find differences between this fixed calculation and the original unfixed simulation. An alternative procedure, which allows for a spread in dipole moments and is not dependent on a predetermined average value, has been developed to make simulations of large water-protein systems, including cooperative effects, computationally feasible.Most interatomic potential functions used in energy calculations are of a pairwise form in which it is assumed that many-body effects are either negligible or that they can be accounted for by an effective pairwise function (1). However, it has been found that many-body effects in water are not negligible (2). Both Hankins et al. (3) and Del Bene and Pople (4) found there were non-pair-additive contributions in hydrogen bond formation in the water trimer compared with the water dimer. Campbell and *Mezei (5) have developed a quantum mechanical non-pair-additive model with which they-find large deviations from pairwise additivity in their investigations of various forms of ice. More recently, Clementi et at (6), using sophisticated basis sets, have studied the nonadditivity ofinteractions in the water trimer and have found that the nonadditive contribution is smaller than that calculated previously by Del Bene and Pople (4).Non-pair-additivity is a consequence of many-body interactions at the molecular level and depends on both the properties of the molecule and its environment. The polarizability of the electronic charge is one molecular property that is important in the interactions between water molecules and gives a large contribution to the.high value for the macroscopic property of the static dielectric constant of bulk water. One result of the polarization of water molecules in bulk water is that the dipole moment of each molecule is increased above the value found in the monomer. For example, in hexagonal ice I, the dipole moment was found experimentally to be between 2.6 and 3.0 debyes (D) (7), whic...

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