Influence of crystal modification on the photoinduced color change in riboflavin.

Two different qualities of riboflavin (RF) i.e., synthetic (RFs) and biosynthetic riboflavin (RFbs) have been investigated with respect to photoinduced color change in the solid state. Several methods (XRD, FT-IR, VIS-, NIR- and fluorescence spectroscopy) were employed to elucidate the properties of the crystalline structure of RFs and RFbs and the influence of irradiation on the color and structural changes of the samples in the solid state. It was shown that RFs an RFbs represent two different crystal modifications of riboflavin and that RFbs can easily be transformed into a dihydrate upon exposure to humidity. Based on the observed irreversible color change and reduction in fluorescence intensity upon irradiation, an irreversible photoreduction of the molecule was assumed in case of RFs. A more pronounced, reversible color change and reversible reduction in fluorescence intensity indicated a reversible photoreduction process in the case of RFbs. The mechanism of these processes was further investigated by means of NIR and FT-IR spectrophotometry. It is apparent from the current study that the crystal modification of RF can strongly influence the solid state photochemistry of this molecule.