Protein electrostatics tune the photophysics of LOV1 and LOV2 domains in three organisms

Flavin's spectroscopic, photophysical, and redox properties are sensitive to its interactions with neighboring polar or charged groups. Flavoproteins capitalize on this sensitivity to tune their chemical reactivity and photochemistry. A fundamental understanding of this tuning mechanism is necessary for the design of novel flavoproteins. Photoactive flavoproteins such as Light-Oxygen-Voltage (LOV) domains have served as important scaffolds to tune photophysics through sequence mutations, resulting in a series of engineered LOV-based proteins that optimize fluorescence, intersystem crossing (ISC), photoreduction, and/or adduct formation over a range of timescales. To better guide future engineering efforts, we have recently employed hybrid quantum mechanical / molecular mechanical (QM/MM) models of LOV domains to study how intradomain electrostatics exert control over flavin's photophysics. In this work, we focus on a series of LOV1 and LOV2 domains from Arabidopsis thaliana (AtLOV), Avena sativa (AsLOV), and Chlamydomonas reinhardtii (CrLOV); by simulating their spectroscopic properties, relative energetics of low-lying singlet and triplet π,π* and n,π* states, and electrostatic projection maps, we present a comparative study that sheds light on the variations in LOV domain's ISC efficiency in these three organisms. We found that for LOV1 , unlike LOV2, the triplet n,π* (TnN,π*) excited state is higher in energy relative to the first optically active singlet π,π* (S1π,π*) state, which corroborates the statement of various literature that ISC is typically less efficient in LOV1 than LOV2 domains. We also find that unfavorable triplet state energetics can explain why CrLOV2 can form the adduct directly from the S1π,π* singlet state.