The Incorporation of Quantum Effects in the Simulation of Proton Transfer Reactions

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The Theoretical and Computational Chemistry program is supporting Professor Hammes-Schiffer at the University of Notre Dame as part of the CAREER program for young scientists. The objective of this research is to develop and apply new methods for simulating dynamical processes involving multiple proton transfer reactions. In order to simulate proton transfer reactions, methods that incorporate the quantum mechanical behavior of the hydrogen atoms being transferred must be developed. In surface-hopping methods such as molecular dynamics with quantum transitions (MDQT), trajectories split into branches that are each assigned to a particular proton quantum state, so branching processes (i.e. processes involving multiple channels or pathways) are described accurately. The direct extension of MDQT to processes involving multiple proton transfer steps, however, is not computationally feasible. The proposed new method, MC-MDQT, combines MDQT with a multiconfigurational (MC) self-consistent-field approach and will describe branching processes properly as well as allow the quantum mechanical treatment of multiple hydrogen atoms. This method will be applied to two classes of systems. The first is proton transport along linear chains of hydrogen-bonded water molecules. This is an important process because the rapid transport of protons over large distances in biologically important systems such as bacteriorhodopsin and photosynthetic reaction centers is thought to occur along such chains. The second class of systems that will be studied is hydrogen transfer in enzymes such as alcohol dehydrogenase, which plays a vital role in glycolysis. Kinetic isotope effect experiments indicate that hydrogen tunneling is significant in a variety of enzyme reactions. The goal of these applications is to elucidate the underlying physical principles of proton transfer reactions and to clarify the detailed mechanisms of these biologically important reactions. The educational objectives of this Carreer grant are to promote the wider use of computer simulations in the areas of organic chemistry and biochemistry. A course in computer simulations has already been designed. An outgrowth of this project is the creation of a computer lab workbook, as well as a textbook which will facilitate the development of such courses at other universities. A general proton transfer reaction involves the passing of a hydrogen atom nucleus from a donor to an acceptor. Experiments have shown that the hydrogen atom can tunnel through an energy barrier required for such a reaction. This research will focus on methods that incorporate quantum mechanical behavior, such as tunneling in order to simulate proton transfer reactions. The educational objectives of this Carreer grant are to promote the wider use of computer simulations in the areas of organic chemistry and biochemistry. A course in computer simulations has already been designed. The course consists of three parts: The lectures convey the fundamental concepts for computer simulation; Computer labs provide hands on experience with molecular modeling, while independent research motivates sutdents to pursue future projects. An outgrowth of this project is the creation of a computer lab workbook, as well as a textbook which will facilitate the development of such courses at other universities.