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In addition to DNA/RNA and amino acids (proteins), flavins are an important class of biomolecules. Flavins (Fl) are derived from the 7,8-dimethyl-10-alkylisoalloxazine chromophore, which differ by their substituent R at the N10 position. The most important examples of the flavin family are lumichrome (LC, R=H), lumiflavin (LF, R=CH$_3$), riboflavin (RF, vitamin B2, R=ribityl), flavin mononucleotide (FMN, co-enzyme), and flavin adenosine dinucleotide (FAD, flavo-protein). Their diverse photochemical properties arising from the LC chromophore make them of fundamental importance for many biological systems and phenomena. Their relevance was acknowledged by the Nobel Prize in Chemistry awarded to Paul Karrer in 1937 for his work on flavins and vitamins. Flavins absorb in a wide spectral range from the optical to the UV region, and their optical and photochemical properties vary sensitively with their oxidation, protonation, metalation, and solvation state. Despite their importance, prior to our work, flavins have not been characterized in the gas phase to determine their intrinsic properties. To this end, we systematically characterized the geometric structure of protonated and metalated flavins (X+Fl, X=H, Li-Cs, Cu-Au) by IRMPD spectroscopy at room temperature in a FT-ICR mass spectrometer coupled to the IR free electron laser FELIX [1-3]. Comparison of the IRMPD spectra recorded in the fingerprint range with DFT calculations allows for establishing the preferred protonation and metalation sites, the interaction strength, and the type of bonding. In a second step, we utilized a recently commissioned cryogenic ion trap spectrometer (BerlinTrap) [4] to record first optical spectra of H+Fl and M+Fl at low temperature (T=20 K) to probe their complex electronic structure arising from the rich manifold of excited states with ππ and nπ character [4,5]. The analysis of these spectra is accomplished by TD-DFT calculations coupled to Franck-Condon simulations, providing detailed insight into the excited state properties.
[1] J. Langer, A. Günther, S. Seidenbecher, G. Berden, J. Oomens, O. Dopfer, Chem. Phys. Chem. 15, 2550 (2014).
[2] A. Günther, P. Nieto, G. Berden, J. Oomens, O. Dopfer, Phys. Chem. Chem. Phys. 16, 14161 (2014).
[3] P. Nieto, A. Günther, G. Berden, J. Oomens, O. Dopfer, J. Phys. Chem. A 120, 8297 (2016).
[4] A. Günther, P. Nieto, D. Müller, A. Sheldrick, D. Gerlich, O. Dopfer, J. Mol. Spectrosc. 332, 8 (2017).
[5] A. Sheldrick, D. Müller, A. Günther, P. Nieto, O. Dopfer, Chem. Eur. J., submitted (2018).
