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

Abstract

The photochemical properties of flavins depend sensitively on their environment and are strongly modified by coordination with metal ions. Herein, the electronic spectra of cold complexes of the smallest flavin molecule (lumichrome, LC, C₁₂N₄O₂H₁₀) with alkali ions (M⁺LC, M = Li–Cs) are measured by photodissociation in the visible range (VISPD) in a cryogenic ion trap coupled to a tandem mass spectrometer and an electrospray ionization source. The observed vibronic spectra of all ions are assigned to the optically bright S₁ ← S₀ (ππ*) transition of the most stable O4 isomer of M⁺LC by comparison with quantum chemical calculations at the PBE0/cc-pVDZ level coupled to multidimensional Franck–Condon simulations. The rich vibronic spectra indicate substantial geometry changes upon S₁ excitation. Large red shifts of the S₁ origins upon metal complexation and progressions in the intermolecular in-plane metal stretch and bend modes demonstrate that the strength of the metal–flavin interaction in M⁺LC(O4) strongly increases by S₁ excitation. The stronger M⁺⋯LC bond in the S₁ state of M⁺LC(O4) is rationalized by the charge reorganization upon ππ* excitation of the LC chromophore. The computations confirm that the optical properties of LC can be strongly modulated by metalation via both the type and binding site of the metal ion.