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

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, Cl2N4O2H10) 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 S-1 <- S-0 (pi pi*)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 S-1 excitation. The stronger M+center dot center dot center dot LC bond in the S-1 state of M+LC(O4) is rationalized by the charge reorganization upon pi pi* 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.