"On the Vibrational Spectra of the Ground and the Singlet Excited ππ* State of 6,7-dimethyl-8-ribityllumazine"W. Schreier, I. Pugliesi, F. Koller, T. Schrader, W. Zinth, M. Braun, S. Kacprzak, S. Weber, W. Römisch-Margl, A. Bacher, B. Illarionov, M. Fischer
J. Phys. Chem. B. 115, 3689 - 3697 (2011)
6,7-Dimethyl-8-ribityllumazine serves as fluorophore in lumazine proteins (LumP) of luminescent bacteria. The molecule exhibits several characteristic vibrational absorption bands between 1300 and 1750 cm–1 in its electronic ground state. The IR-absorption pattern of the singlet excited ππ* state was monitored via ultrafast infrared spectroscopy after photoexcitation at 404 nm. The comparison of experimentally observed band shifts for a number of isotopologues allows for a clear assignment of several absorption bands – most importantly the two carbonyl bands. This assignment is confirmed by normal-mode calculations by means of either density functional theory (DFT) calculations for the ground state or the configuration interaction singles (CIS) method for the excited singlet state. A good agreement between experiment and calculation is obtained for models including explicitly a first solvation shell. The results provide a basis for further investigations of lumazine protein and demonstrate the necessity of proper accounting for explicit hydrogen bonding in case of strongly polar molecular systems.
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