Abstract
Electronic state of flavin core of riboflavin has been studied making use of the absorption and fluorecence emission spectra of various nonaqueous solutions of riboflavin tetrabutyrate (RTB). The absorption spectra of RTB in nonpolar solvents, such as carbon tetrachloride and diethyl ether, showed two new bands or shoulders in the vicinity of 420 and 470 mμ in addition to the well-known bands (α and β maxima at about 450 and 340 mμ, respectively). While the position of the a band maximum remained unaffected, the β band shifted toward longer wave length with increasing polarity of solvent, indicating that the latter is associated with π→π* transition. Two fluorescence maxima were observed in the emission spectra of RTB in nonpolar solvents, viz. at about 500 and 520 mμ. The fluorescence maximum of the shorter wave length migrates with solvent polarity corresponding to the migration of the β absorption band maximum, and has been ascribed to monomeric form of RTB. With the increase of RTB concentration, fluorescence intensity at 520 mμ increased while that of the 500 mμ decreased, indicating that the former maximum resulted from molecular association of the solute. A vertically stacked model was proposed for the dimerized complex, taking account of the electronic map of isoalloxazine.
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