Interaction of Alkali Ions with Flavins: Infrared and Optical Spectra of Metal-Riboflavin Complexes.

Flavin compounds are of great interest in biochemistry because of their diverse functions in catalytic and photochemical processes. The intrinsic optical properties of flavins depend sensitively on their environment such as complexation with metal ions. Herein, we characterize the interaction of alkali metal ions (M+) with riboflavin (RF, vitamin B2). To this end, two different experimental spectroscopic approaches are employed to determine the structural, vibrational, energetic, and optical properties of M+RF complexes by comparison with density functional theory (DFT) calculations at the PBE0/cc-pVDZ level. First, infrared multiple photon dissociation (IRMPD) spectra recorded at room temperature demonstrate that M+ binds to one of the two available nucleophilic carbonyl groups (CO2, CO4) of RF, denoted O2 and O4+ isomers, as revealed by characteristic shifts of the CO stretch modes upon metalation. Second, the optical spectrum of K+RF is recorded between 428 and 529 nm in a cryogenic ion trap held at 6 K by visible photodissociation (VISPD). Analysis of the VISPD spectrum by time-dependent DFT calculations coupled to Franck-Condon simulations demonstrates that in fact only the O2 isomer of M+RF is formed by electrospray ionization, while the spectroscopic signatures of the O4+ isomer are absent. The VISPD spectrum is attributed to the S1 ← S0 (ππ*) transition of the O2 isomer, which is calculated to be much more stable than the O4+ isomer because of additional multiple interactions of M+ with the OH groups of the ribityl (sugar) side chain attached at N10 of RF. In contrast, there is no evidence for the presence of the O4+ isomer, in which M+ forms a chelate complex, with M+ binding to both O4 and N5. A comparison between RF (ribityl at N10) and lumiflavin (LF and CH3 at N10) reveals the drastic effects of the side chain on the structural, energetic, and optical properties of the flavin interaction with metal ions. While for M+LF the O2 and O4+ isomers are close in energy and both observed experimentally, for M+RF the O2 isomer is strongly favored due to the additional interaction with the side chain. Although the S1 energies of M+RF(O2) and M+LF(O2) are quite similar, because the ππ* transition is localized on the same isoalloxazine chromophore for both flavins, the vibrational structures are strongly different because the soft bending potential for the M+···flavin interaction is strongly affected by the ribityl side chain at N10. In contrast to H+RF, which prefers protonation at N1, steric repulsion of the larger M+ ions with the ribityl side chain prevents metalation at N1, leading to the formation of the O2 global minimum.