Fig. 7. (a) and (b) Type (3) mechanism and potential energy profiles involving the S₀ → S₁ excitation of St0, intermolecular proton transfer in H-bond between prFMN^im and Glu282, ring expansion and reverse protonation, respectively. (c) The H-bond distances ( and R_{O^Glu282–H⋯O^prFMN}) and local valence charge densities on the high-energy barrier intermolecular proton transfer process (St0-[1]^Ext → St0-[1]^Ext,‡ → St1^Ext). (d) and (e) Type (4) mechanism and potential energy profiles involving the S₀ → S₁ excitation of St0 and concerted intramolecular proton transfer in prFMN^im and ring expansion. The characteristic structures are shown in detail in Table S2.† (…)^{S₀ → S₁} = S₀ → S₁ vertical excitation energy; (…)^Rel and (…)^‡ = relative and transition energies on the PES; (…)^S_1/S₀ = difference between the total energies in the S₀ and S₁ states at the S₀/S₁ intersection.