QUANTUM-CLASSICAL MOLECULAR-DYNAMICS SIMULATIONS OF PROTON-TRANSFER PROCESSES IN MOLECULAR-COMPLEXES AND IN ENZYMES

A quantum-classical molecular dynamics model (QCMD) designed for simul ations of proton or electron transfer processes in molecular systems i s described and applied to several model problems. The primary goal of this work is the elucidation of enzymatic reactions. For example, usi ng the QCMD model, the dynamics of key protons in an enzyme's active s ite might be described by the time-dependent Schroedinger equation whi le the dynamics of the remaining atoms are described using MD. The cou pling between the quantum proton(s) and the classical atoms is accompl ished via extended Hellmann-Feynman forces, as well as the time depend ence of the potential energy function in the Schroedinger equation. Th e potential energy function is either parametrized prior to the simula tions or can be computed using a parametrized valence bond (VB) method (QCMD/VB model). The QCMD method was used to simulate proton transfer in a proton bound ammonia-ammonia dimer as well as to simulate dissoc iation of a Xe-HI complex in its electronic excited state. The simulat ion results are compared with data obtained using a quantum-classical time-dependent self-consistent field method (Q/C TDSCF) and with resul ts of fully quantum-dynamical simulations. Finally QCMD/VB simulations of a hydrolytic process catalyzed by phospholipase Az, including quan tum-dynamical dissociation of a water molecule in the active site, are reported. To the best of our knowledge, these are the first simulatio ns that explicitly use the time-dependent Schroedinger equation to des cribe enzyme catalytic activity.