Bradforth Research Group
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Excited State Dynamics of Biomolecules
Evgeniia Butaeva

My scientific curiosity is driven by the desire to implement a unique combination of theoretical simulations with experimental time-resolved spectroscopy in order to investigate the ultrafast dynamics caused by UV/Vis irradiation of a large variety of molecular systems. The structures I am interested in range from small model molecules, on which you can effectively study bond-making and bond-breaking mechanisms, to DNA bases with their complex photochemistry.

Using transient absorption spectroscopy, we can accurately establish ultrafast relaxation pathways and disentangle competing decay channels by varying excitation wavelength of the pump beam as well as the pulse duration. Currently my experimental effort is focused on photochemistry of tyrosine upon 266- and 200-nm excitations. Tyrosine is one of the most versatile and ubiquitous redox centers among the aromatic amino acids. It is essential for biochemical reactions such as DNA damage repair in eukaryotic cells and oxygen evolution in photosynthesis (Kok Cycle) in the reaction center of Photosystem II. A number of these key redox reactions involving tyrosine are found to be photodriven, therefore the photophysical/photochemical properties of its light absorbing chromophore, phenol, are of immense interest.
Phenol has been studied quite thoroughly, as evidenced by the abundance of literature on phenol and its electrochemical properties alone. However, phenol is still proving to be a conundrum in that there is little knowledge of certain aspects such as the mechanism of phenoxyl radical formation, excited state proton transfer (ESPT), proton-coupled electron transfer (PCET), and the high yield of triplet states.

Alternatively, DNA damage mechanisms can be studied through the examination of rates of electron (hole) transport between two nucleobases or between a nucleobase and an aromatic amino acid, approximated by Marcus theory. To predict such rates, accurate knowledge of two fundamental energetic parameters is required: the energy change for the electron to travel between biomolecules and the total reorganization energy connected with this process. Although there is a plethora of literature on traditional electrochemical measurements reporting the half-cell redox potentials of biomolecules in water, frequently these values lead to uncertain if not erroneous conclusions. A new approach that connects steady state photoelectron spectroscopy, using a liquid microjet, with ab initio calculations is found to accurately predict standard redox potential values. The obtained values are not influenced by follow-up reactions, such as, deprotonation and subsequent radical reactions, which render electrochemical measurements irreversible. Therefore, liquid jet photoelectron spectroscopy not only establishes a new spectroscopic technique to explore molecular electronic structure in liquids, but also offers a new tool to obtain equilibrium thermodynamic parameters which are inaccessible via conventional analytical methods.