Spectroscopic studies of the structure and reactivity of isolated and microsolvated flavins in a cryogenic ion trap

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Similar to DNA bases and amino acids, flavins are abundant key molecular building blocks in life chemistry. As co-enzymes in flavoproteins and blue-light photoreceptors, they play a fundamental role in numerous biochemical redox reactions involving one and two electron transfer processes and other photochemical phenomena. The impressive biochemical versatility of flavins relies on their rich photochemistry, which is based on their common isoalloxazine chromophore. The most basic members of the flavin familiy are lumichrome, lumiflavine, riboflavine (vitamin B2), and flavin mononucleotide (FMN), which just differ by the substituent (side chain) of the isoalloxazine ring. A plethora of theoretical studies and experiments in solution demonstrate that the photophysical properties of flavins strongly depend on their oxidation, protonation and metalation state, the side chain, and the solvent. However, experimental studies of flavins isolated in the gas phase to derive a molecular-level understanding of the photochemical properties of the optically active species are completely lacking. To this end, this project aims at the spectroscopic and quantum chemical characterization of essential flavin ions in the gas phase, in which we can vary in a controlled fashion the oxidation, protonation and metalation state, as well as the type and degree of microsolvation. The latter will separately reveal the effects of the solvent and bridge the gap to the condensed phase properties. The experimental strategy involves state-of-the-art vibrational (infrared) and electronic (optical) laser spectroscopy of flavin ions in a novel cryogenic ion trap mass spectrometer (4-300 K) coupled to an electrospray ionization source constructed in the previous funding period. The derived spectra are analyzed by appropriate quantum chemical calculations and yield fundamental parameters, such as geometric and electronic structure, protonation and metalation sites and binding affinities, and effects of stepwise microsolvation in polar and nonpolar solvents as a function of the redox state. The ultimate goal is a description and fundamental understanding of the photochemical properties and reactivity of the flavins and their solvation effects at the molecular level. The high-precision gas-phase spectroscopic data will also serve as valuable benchmark for testing and developing theoretical approaches to describe the energetics and dynamics of flavins in their ground and excited electronic states.