QM/MM Studies of the Photochemistry of Biochromophores in Solution and in Protein Environments

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Sunlight-absorbing biomolecules or protein moieties are known as biochromophores. Such photoactive biomolecules can be classified either as light filters, which protect more sensitive biological structures (e.g. kynurenines in the ocular lens protecting the retina from UV damage), or as biomolecular machines, which use sunlight to trigger a chemical transformation (e.g. rhodopsin in the retina). Insight into fundamental physiological and biological photoinduced molecular processes in nature can be gained through the study of the photophysics and photochemistry of biochromophores embedded in their immediate chemical environment, either a solvation shell or a protein environment. Herein, I propose to study the photophysics and photochemistry of two physiologically and biologically relevant biochromophores in their natural chemical environment using methods of computational chemistry: the dermal UV filter urocanic acid embedded in bulk water and the photoactive protein Orange Carotenoid Protein (OCP) found in the light-harvesting complexes of cyanobacteria, which features a carotenoid chromophore. To elucidate the photochemical reactivity of the molecules as well as their dynamical behavior, I envision to use both a static approach for exploring excited-state potential-energy surfaces of relevant photochemical processes as well as a dynamical approach by performing nonadiabatic molecular-dynamics simulations. These studies shall be conducted with the QM/MM methodology employing for the QM part the MS-CASPT2 method for urocanic acid and a highly efficient GPU-based implementation of SA-CASSCF in TeraChem for the carotenoid chromophore of OCP. The ab initio full-multiple-spawning methodology shall be used for the nonadiabatic dynamics simulations. I apply to conduct these studies in the group of Prof. Todd J. Martínez at Stanford University. Successful implementation of these plans will significantly advance the knowledge on two highly important biomolecules and pave the way for even larger and more complex QM/MM studies of the biological reactivity of these molecules, which is given by their interaction with various reaction partners in their natural environment.