Effect of alkali ions on optical properties of flavins: vibronic spectra of cryogenic M+lumichrome ions (M = Li–Cs) in the gas phase.

In this paper the vibronic spectra of (alkali-) metalated lumichrome are recorded by VISPD spectroscopy in a cryogenic ion trap. Significantly, these vibrationally resolved electronic spectra are the first optical spectra of any metalated flavin in the gas phase and thus provide a first impression of the effects of metalation on the intrinsic electronic structure of flavins. The observed vibronic spectra of these ions are attributed to the optically bright S₁ → S₀ transition of the most stable O4 isomer of M⁺LC. The findings reveal that the vibronic spectra indicate notable geometry changes upon S₁ excitation. Furthermore, there are significant red shifts in the S₁ origins upon metal complexation. The study concludes that the optical properties of LC can be strongly modulated by metalation, influenced by both the type and binding site of the metal ion.