Vibronic optical spectroscopy of cryogenic flavin ions: the O2+ and N1 tautomers of protonated lumiflavin

Abstract

Flavins are key compounds in many photochemical and photophysical processes used by nature, because their optical properties strongly depend on the (de-)protonation site and solvation. Herein, we present the vibronic optical spectrum of protonated lumiflavin (H⁺LF), the parent molecule of the flavin family, obtained by visible photodissociation (VISPD) spectroscopy in a cryogenic ion trap. By comparison to time-dependent density functional theory (TD-DFT) calculations at the PBE0/cc-pVDZ level coupled to multidimensional Franck–Condon simulations, the spectrum recorded in the 420–500 nm range is assigned to vibronic bands of the optically bright S₁ ← S₀(ππ*) transition of the two most stable H⁺LF tautomers protonated at the O2+ and N1 position. While the most stable O2+ protomer has been identified previously by infrared spectroscopy, the N1 protomer is identified here for the first time. The S₁ band origins of H⁺LF(O2+) and H⁺LF(N1) at 23 128 and 23 202 cm⁻¹ are shifted by 1617 and 1691 cm⁻¹ to the blue of that of bare LF measured in He droplets, indicating that the proton affinity of both tautomers is slightly reduced upon S₁ excitation. This view is consistent with the molecular orbitals involved in the assigned ππ* transition. The spectrum of both protomers is rich in vibrational structure indicating substantial geometry changes by ππ* excitation. Interestingly, while the O2+ protomer is planar in both electronic states, the N1 protomer is slightly nonplanar giving rise to large vibrational activity of low-frequency out-of-plane modes. Comparison with protonated lumichrome and metalated lumiflavin reveals the impact of functional groups and the type of the attached cation (proton or alkali ion) on the geometric and electronic structure of flavins.