Riboflavin tetrabutyrate undergoes characteristic spectral changes, in both the first and second absorption band regions, upon hydrogen bonding with trichloroacetic acid of trifluoroacetic acid. On the basis of the calculated electron densities, hydrogen bonding at the heteroatoms of the isoalloxazine nucleus is considered to occur with increasing concentrations of the proton donor, first at N(1), then at O(12), O(14), and N(3)H, and finally at N(5). The idea that the major effect of the hydrogen bonding at the N(1), N(3)H, and oxygen atoms of the flavin nucleus is to facilitate the electrophilicity of the N(5) position, which was predicted by molecular orbital calculations, was supported by the observation that the hydrogen-bonded flavin in its triplet state abstracts hydrogen from the donor N-benzyl-n,n'-dimethylethylenediamine at a faster rate than do the non-hydrogen-bonded species in CCI4. The implications of the present study in the spectroscopic and catalytic properties of flavoproteins are briefly discussed.

Electronic structures of naphthalene and anthracene have been studied by the selfconsistent field MO method employing our semiempirical parameters. The calculated electronic spectra, bond lengths, ionization potentials and electron affinities are in a good agreement with expriment. Some discussions have been given in connection with the method of “atoms in molecules” and others.

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Time-dependent density functional theory has been applied to investigate the electronic absorption spectrum of oxidized and reduced lumiflavin and its derivative, 8-NH(2)-lumiflavin. The calculations allow the authors to explain the origin of the difference in spectral features between oxidized and reduced states of lumiflavin. For the reduced lumiflavin, a reasonable assignment of the experimental spectrum has been made for the first time. Furthermore, the results obtained reveal that the NH(2) group plays a critical role in shaping the spectral features of 8-NH(2)-lumiflavin, and offer a reasonable explanation for the spectral changes upon substituting the NH(2) group for the CH(3) group of lumiflavin.

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