Full embedding QM/MM scheme for modeling excited state proton transfer processes

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Lyudmila V. Slipchenko of Purdue University is supported by an award from the Chemical Theory, Models and Computational Methods program in the Chemistry Division to develop new computational and theoretical tools for describing the transfer of protons involving molecules in which the electrons have been excited, often by absorption of light. The systems to be studied are in solution or in other complex environments. The phenomenon known as excited state proton transfer (ESPT) is ubiquitous. It naturally occurs in photoactive proteins, which play many important roles in biology, and takes place between DNA strands as a pathway for dissipating excess UV radiation. In technology, ESPT is exploited in artificial photo-devices, fluorescent chemo-sensors, and for manipulating the acidity of local environment with light pulses. However, mechanistic understanding of ESPT pathways, which is essential for design of efficient photochemical cells or photoacids, remains incomplete. This research is expected to contribute to the understanding and eventual control of ESPT processes by elucidating the role of environment (solvent, electrolyte, protein matrix) in these events. This knowledge will benefit scientists and engineers designing new classes of photoacids, chemo-sensors and artificial photo-devices. The algorithms developed and validated in this project will be broadly distributed and serve for predictive studies of a variety of photo-induced processes in chemistry, biology, and materials. Slipchenko and her research group aim to characterize static and dynamic environment effects on complex photochemical and photophysical processes such as ESPT by (i) developing novel first-principles based methodology of QM/MM (Quantum Mechanics/Molecular Mechanics) type, (ii) validating and benchmarking this methodology relative to other established and emerging methods by creating a condensed-phase photochemistry database, and (iii) applying newly developed tools for investigating ESPT mechanisms in solvated photoacids and proteins such as GFP and PYP. The proposed method developments are based on polarizable quantum mechanics/molecular mechanics (QM/MM) approach for electronic excited states, in which the environment (the MM part) is described by the sophisticated Effective Fragment Potential (EFP) method. The proposed work enhances the accuracy and applicability of the QM/EFP methods by (i) developing a full embedding QM/EFP model in which all interactions between the quantum and classical systems are accounted for quantum-mechanically, (ii) extending this model to biological environments, and (iii) developing QM/EFP excited state molecular dynamics algorithms. The novelty of the proposed research is in creating a unique full embedding QM/MM model and applying it to excited state dynamics in various complex environments. The proposed research contributes to the research infrastructure byintegrating new computer codes in the open-source, freely available libefp library and in open-source PSI4, GAMESS and NWChem electronic structure packages and as open-source modules in Q-Chem and MOLCAS quantum chemistry software. The proposed condensed-phase photochemistry database, containing a variety of photoactive molecules in various environments, will contribute to quantum chemistry education in the community by providing benchmarks of newly developed and established methods for extended systems and by supplying online tutorials.