Berlin 2014 – scientific programme
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MO: Fachverband Molekülphysik
MO 11: Biomolecules 1
MO 11.1: Talk
Wednesday, March 19, 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.