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MO: Fachverband Molekülphysik

MO 11: Biomolecules 1

MO 11.1: Vortrag

Mittwoch, 19. März 2014, 14:00–14:15, BEBEL HS213

Probing Protonation Sites of Isolated Flavin Molecules via IR Spectroscopy — •Alan Günther¹, Judith Langer¹, Giel Berden², Jos Oomens³, and Otto Dopfer¹ — ¹Institut für Optik und Atomare Physik , Hardenbergstr. 36, 10623 Berlin, Germany — ²Radboud University Nijmegen, Institute for Molecules and Materials, FELIX facility, Toernooiveld 7, 6525 ED Nijmegen — ³University of Amsterdam, Science Park 904, 1098XH Amsterdam, The Netherlands

Infrared spectra of the isolated protonated flavin molecules lumichrome, lumiflavin, riboflavin (vitamin B2), and the biologically important cofactor of blue-light receptors flavin mononucleotide are measured in the fingerprint range (600-1850 cm⁻¹) by means of IR multiple photon dissociation (IRMPD) spectroscopy. Using density functional theory calculations, the geometries, relative energies, and linear IR absorption spectra of several low energy isomers have been calculated. Comparison of the calculated IR spectra with the measured IRMPD spectra reveals that the N10 substituent at the isoalloxazine ring influences the protonation site of the flavin. Lumichrome with a hydrogen substituent is only stable as N1 hydrogenated tautomer and protonates at N5 of the pyrazine ring. The presence of the sugar unit in riboflavin leads to protonation at N1 of the pyrimidinedione moiety, and methyl substitution in lumiflavin stabilizes the tautomer which is protonated at O2. In contrast, flavin mononucleotide occurs as both the O2 and N1 protonated tautomers. The frequencies and relative intensities of the two C=O stretch vibrations in protonated flavins serve as reliable indicators for their protonation site.