Dual Photochemical Reaction Pathway in Flavin-Based Photoreceptor LOV Domain: A Combined Quantum-Mechanics/Molecular-Mechanics Investigation

The primary photochemical reaction of the light, oxygen, and voltage (LOV) domain of the blue-light photosensor YtvA of Bacillus subtilis were investigated using high-level QM(DFT/MRCI)/MM methods. After blue-light excitation, the S_γ atom of the reactive cysteine forms a covalent bond with the C_4a of the flavin mononucleotide (FMN) ring. Two conformations for the side chain of reactive cysteine with occupancies of 70% (conf A) and 30% (conf B) are observed in the X-ray crystallographic structures of the YtvA-LOV ( Möglich , A. ; Moffat , K. J. Mol. Biol. 2007 , 373 , 112 - 126 ). In conf A, the thiol group is directed toward the dimethylbenzene moiety of the FMN ring whereas it is placed directly above the N₅ atom of the FMN ring in conf B. Starting from both conformations, the singlet and triplet excited pathways were evaluated. The singlet states excited from conf A decay nonradiatively to the triplet states by intersystem crossing (ISC). After the formation of a neutral biradical, the triplet states cross over to the electronic ground state by a second ISC and the adducts are efficiently formed. The singlet states excited from conf B are located near the S₁/S₀ conical intersection (CIn). A major fraction returns to the initial states through the CIn. The rest may directly reach the adduct state. Thus, the photoexcitation has a dual reaction pathway. In YtvA-LOV, it is inferred that the efficient triplet excitation from conf A was chosen by bypassing the less efficient singlet excitation from conf B.