Coenzyme models. 40. Spectral and reactivity studies of roseoflavin analogs: Correlation between reactivity and spectral parameters

Roseoflavin analogs with a morpholino group or a monoaza-15-crown-5 group at the 8-position [3-methyl-8-morpholino-10-ethylisoalloxazine or 3-methyl-8-(1′,4′,7′,10′-tetraoxa-13′-azacyclopentadec-13′-yl)-10-ethylisoalloxazine (NCrFl), respectively] were synthesized. We have found that the absorption spectra of these roseoflavin analogs are very sensitive to solvent effects: that is, they imparted a red color to polar solvents characteristic of the intramolecular charge transfer from the 8-amino group to the pteridine moiety while they imparted a yellow color to nonpolar solvents characteristic of the inhibition of the charge transfer. The fluorescence spectra were also sensitive to solvent effects: the emission maximum of NCrFl, for example, shifted from 564 nm (water) to 509 nm (benzene) and the fluorescence intensity increased with decreasing solvent polarity. The oxidizing ability, which was estimated by photooxidation of 1-benzyl-1,4-dihydronicotinamide, was well correlated with the absorption maxima: the shorter the maximum wavelength, the more reactive. In particular, the logarithm of the rate constants was linearly correlated with E_T. These results strongly suggest that roseoflavin, inactive in polar media, is possibly activated when it is bound to hydrophobic pockets of enzymes and that the color change can be a quantitative measure of the reactivities.