Biological function relies on the chemical reactions of electron and proton transfer that take place in enzyme active centers and are strongly influenced by the geometry and energetics of the surrounding medium. The small masses of electrons and protons necessitates a quantum mechanical treatment and, as electrons and protons are charged species, they strongly interact with their surroundings, the many strongly polar amino acids in the surrounding proteins. It is now appreciated that a coupling of proton motion to electron transfer is basic to mechanisms of biological energy conversion. We develop theories to predict rates of such reactions. For PCET, we focus on how to formulate rate constant expressions for the concerted transfer of an electron and a proton and how the rate of this process compares with that of the consecutive transfer of an electron and a proton. Stimulated by these theories, we develop computer programs to carry out quantum molecular dynamics simulations that couple the quantum electron and/or proton to the surrounding classically treated medium. Chains of hydrogen bonded water and/or amino acid residues are essential to proton translocation, the movement of protons across membranes to build up electrochemical gradients for energy transduction. We have developed methods to treat the protons involved in translocation quantum mechanically and have coupled them to the surrounding protein in order to simulate rates of this process. For PT, we explore the control of reaction rates by the application of external fields that couple to the transfer particle's dipole. We have found that external fields are capable of drastically enhancing or suppressing the rate of charge transfer. In multi-level tunneling systems, as found in some proton transfers, we consider the control of tunneling with an external field as a method of storing information in the different quantum states of the system. In SFS (strong field spectroscopy) two optical states of a solute are coupled by their transition dipole. The strong external field drives the system beyond the linear response regime. The average power absorbed by the sample exhibits an oscillatory decaying behavior that reflects the driving field and the electronic dephasing due to the coupling to the medium. If the solute states are electron transfer states that are strongly coupled, then the same procedures can characterize mixed-valence electron transfers. We can discuss the competition between electronic coherence and dissipation that leads to oscillatory population decay, as found in ultrafast optical measurements on such systems.
I am a founding member and director of the Center for Biological Modeling (CBM). The CBM brings together faculty and students from the physical, biological and computer sciences to work on problems of biological interest. There is support for graduate students interested in pursuing interdisciplinary research in this area.
|HOW TO CONTACT ME!|
|Department of Chemistry
Michigan State University
East Lansing, MI 48824-1322
|( http://www.cem.msu.edu/~cukier )|
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