Excited-State Decay Pathways of Flavin Molecules in Five Redox Forms: The Role of Conical Intersections

Flavin molecules play an important role in light-driven biological activities. They have drawn significant interest for decades because of their rich photochemistry. In addition to the well-explored FADH^- (anionic hydroquinone), which is supposed to be the only catalytic active state to repair DNA lesions, other four flavin molecules (i.e., FAD, FAD·^-, FADH·, and FADH₂) in three redox forms combined the redox cycle of flavins. Although extensive studies have been carried out for steady-state spectroscopic properties of five redox flavins in various proteins and solutions, the photochemistry and photophysical properties of those different redox states significantly complicate the corresponding theoretical studies. In present work, we employed the ab initio wave function based CASSCF method to systematically investigate the excited state decay pathways of flavins in five redox forms through two approaches. First, the comparison of the absorption and emission spectra from both theoretical calculation and experiment allows a detailed mapping of the transition properties of different redox states in flavins. Second, we identified four kinds of conical intersections (CIs) for five different redox states as the possible deactivation mechanisms responsible for internal conversion or intersystem crossing from the initially populated excited state. The theoretical calculations provide atomic details for the photochemical and photophysical properties of flavins on photoinduced processes. Our findings highlight the indispensable effects of CIs in the excited state decay of flavin molecules and thereby provide basic theoretical information for light-driven biological activities.