Sharon Hammes-Schiffer of the Pennsylvania State University is supported by an award from the Theory, Models and Computational Methods program in the Chemistry division to carry out research on theoretical studies of proton-coupled electron transfer (PCET) reactions. The PI and her research group are pursuing four related projects: 1) developing both grid and nuclear-electronic orbital(NEO) methods for analyzing the electron-proton nonadiabatic effects to elucidate the mechanisms of PCET reactions. The degree of electronic nonadiabaticity for the proton transfer reaction provides a quantitative diagnostic for differentiating between hydrogen atom transfer (HAT) and PCET, where HAT is electronically adiabatic and PCET is electronically nonadiabatic. 2) developing methods to calculate electron-proton vibronic couplings to enable the prediction of trends in rate constants and kinetic isotope effects for PCET reactions. 3) developing methods for studying the dynamics of photoinduced PCET reactions. The solvent and solute nuclei are propagated on adiabatic electron-proton vibronic surfaces using a surface hopping method to incorporate nonadiabatic transitions among the adiabatic states 4) modeling the ultrafast dynamics of photo-excited PCET in DNA.
Proton-coupled electron transfer (PCET) reactions play a critical role in a wide range of chemical and biological processes, including electrochemistry, photosynthesis, respiration, and enzyme reactions. Software resulting from this project is made widely available to the broader community by being included in the public version of the GAMESS computational chemistry software package. The advances from this research are provided to the public through a website devoted to PCET that contains tutorials, code and interactive Java servletes (https://webpcet.chem.psu.edu).
A wide range of biological and chemical processes rely on the coupling between electrons and protons. These processes include photosynthesis, respiration, and energy production in solar cells. This project centered on the development of theoretical and computational methods to elucidate the fundamental principles underlying the coupling between electrons and protons. Understanding these principles enhances the ability of scientists to control electrons and protons in biologically and chemically significant processes. Moreover, computational methods that enable the simulation of these processes assist in the design of more effective drugs and energy conversion devices. The first outcome of this project was the development of computational methods that accurately describe the quantum mechanical motions of electrons and protons within molecules in a computationally tractable manner. The electrons and protons must be treated quantum mechanically because they are so much lighter than the other nuclei. This project produced a computer program that enables these types of calculations with varying levels of accuracy, depending on the property that is being probed. The second outcome of this project was the development of theoretical methods to characterize the mechanisms of proton-coupled electron transfer reactions, which involve the coupled movement of electrons and protons. These methods distinguish between sequential mechanisms, in which an electron and proton transfer in a stepwise manner, and concerted mechanisms, in which they transfer simultaneously. This theory also provides estimates of the timescales for the motions of the electrons and protons. Furthermore, these methods enable the calculation of an important parameter, denoted the vibronic coupling, for the calculation of the rate of experimentally relevant proton-coupled electron transfer reactions. The third outcome of this project was the development of computational methods for studying the ultrafast dynamics of photoinduced proton-coupled electron transfer reactions. These types of reactions are important in photosynthesis and solar cells, where sunlight excites the system to high-energy states to initiate an energy conversion process. These photoinduced reactions also lead to processes in DNA that could eventually cause skin cancer. The computational methods were applied to a model DNA duplex to better understand these processes. The broader impacts of this project include educational, computer software, and web site design components. A number of graduate students and postdoctoral researchers were trained in theoretical and computational chemistry while working on this project. The Principal Investigator presented a large number of invited talks on this research at conferences, workshops, and universities, and she wrote two reviews aimed at general chemistry audiences. Following assessment and validation, the computational methodology was incorporated into a computer program that is available to the public. In addition, the project included the maintenance and enhancement of a web site that provides useful information, tutorials, programs, and interactive demonstrations on this topic to the public.
