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ABSTRACT: Isotope−labeled riboflavin in DMSO was employed
in conjunction with femtosecond time−resolved infrared
vibrational spectroscopy and quantum chemical calculations to
analyze and assign the electronically excited state vibrational
modes of the isoalloxazine unit as a prototype for the cofactors
in flavin binding blue−light receptors. Using the riboflavin 13Canalogues
RF−2−13C and RF−4,10a−13C, the carbonyl vibrations, in particular, were studied. Various quantum chemical models were
applied that take into account a polarizable environment or the impact of hydrogen bonds. The CIS quantum−chemistry method was
successfully applied to describe the lowest singlet excited electronic state in riboflavin. The experimentally observed frequencies and
isotope−shifts as well as their variability in the diverse model calculations are discussed. On these grounds, a consistent assignment of
the electronic ground and excited state vibrations is presented
