Direct Evidence of Photoinduced Protonation-Site Switching in Flavins

Flavins frequently operate as visible-light photoreceptor in both biological and artificial systems. Previous studies (mostly computational) suggest photoinduced protonation-site switching as the fundamental origin of the flavin photoreactivity. In particular, photoexcitation is predicted to change the preferred protonation site to the opposite side of the tricyclic aromatic flavin ring. However, direct experimental identification of such protonation-site switching by photoexcitation has remained an outstanding challenge. Herein, we directly prove the photoinduced protonation-site switching by combining cryogenic ion-trap infrared spectroscopy with controlled microsolvation by water molecules. Using protonated lumiflavin as prototypical model flavin, we show that S₁ ← S₀ photoexcitation triggers proton transfer catalyzed by a water bridge across the aromatic ring. These results provide a general design principle of flavin photochemistry, with implications for developing biological/artificial photoactive molecules.