Force field solvation process

In order to obtain more information on the solvation process Yang and Cui performed a so called natural energy decomposition analysis (NEDA) on monomethyl phosphate ester (MMP) solvated in water. They used a supermolecular approach where the solute plus a number of water molecules (up to 34) were treated quantum-mechanically. A further set of water molecules was treated with a force-field model. Their results indicate that there is a substantial charge transfer between the solute and the nearest solvent molecules. The interaction energy due to this transfer was found to amount to some 70-80% of that of the electric interactions. Since MMP forms hydrogen bonds with the water molecules, all results together suggests that for such a system it is important to include the nearest solvent molecules in the quantum-mechanical treatment, whereas a continuum approximation or a force field may not be sufficiently accurate. 

The interaction between a solvated peptide or protein and a chemically modified RPC and HIC stationary phase in a fully or partially aqueous solvent environment can be discussed in terms of the interplay of weak physical forces. The main types of physical interactions that are involved in order of relevance and dominance for the establishment of the selective recognition and binding between a peptide or protein and RPC and HIC ligates are (I) hydrophobic interactions and related phenomena mediated by polarized electron donor or electron acceptor processes, (2) Lifshitz-London forces and van der Waals and associated weak dipolar interactions, (3) tt 7t and n ->dipole interactions, (4) hydrogen bond interactions, (5) electrostatic interactions, (6) metal ion coordination interactions, and (7) secondary macromolecular interactions involving force field effects. 

The QM/MM PES is a preferred choice for large molecular systems such as a fully solvated protein because the method combines both the generality of quantum mechanical methods for treating chemical processes and the computational efSciency of a force field for large systems. This is important because the dynamic fluctuations of the enzyme and aqueous system have a major impact on the polarization of the species involved in the chemical reaction which, in turn, affects the chemical reactivity. 

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