Influence of the LOV Domain on Low-Lying Excited States of Flavin: A Combined Quantum-Mechanics/Molecular-Mechanics Investigation

The ground and low-lying excited states of flavin mononucleotide (FMN) in the light, oxygen, and voltage sensitive (LOV) domain of the blue-light photosensor YtvA of Bacillus subtilis were studied by means of combined quantum-mechanical/molecular-mechanical (QM/MM) methods. The FMN cofactor (without the side chain) was treated with density functional theory (DFT) for the geometry optimizations and a combination of DFT and multireference configuration interaction (MRCI) for the determination of the excitation energies, while the protein environment was represented by the CHARMM force field. In addition, several important amino acid side chains, including the reactive cysteine residue, were included in the QM region in order to probe their influence on the spectral properties of the cofactor in two protein conformations. Spin−orbit coupling was taken into account employing an efficient, nonempirical spin−orbit mean-field Hamiltonian. Our results reveal that the protein environment of YtvA-LOV induces spectral shifts for the (ππ*) states that are similar to those in aqueous solution. In contrast, the blue shifts of the (nπ*) states are smaller in the protein environment, enabling a participation of these states in the decay processes of the optically bright S₁ state. Increased spin−orbit coupling between the initially populated S₁ state and the T₁ and T₂ states is found in YtvA-LOV as compared to free lumiflavine in water. The enhanced singlet−triplet coupling is brought about partially by configuration interaction with (nπ*) states at the slightly out-of-plane distorted minimum geometry. In addition, an external heavy-atom effect is observed when the sulfur atom of the nearby cysteine residue is included in the QM region, in line with experimental findings.