The kinetics of photolysis of riboflavin (RF) in water (pH 7.0) and in organic solvents (acetonitrile, methanol, ethanol, 1-propanol, 1-butanol, ethyl acetate) has been studied using a multicomponent spectrometric method for the assay of RF and its major photoproducts, formylmethylflavin and lumichrome. The apparent first-order rate constants (k obs) for the reaction range from 3.19 (ethyl acetate) to 4.61 × 10(-3) min(-1) (water). The values of k obs have been found to be a linear function of solvent dielectric constant implying the participation of a dipolar intermediate along the reaction pathway. The degradation of this intermediate is promoted by the polarity of the medium. This indicates a greater stabilization of the excited-triplet states of RF with an increase in solvent polarity to facilitate its reduction. The rate constants for the reaction show a linear relation with the solvent acceptor number indicating the degree of solute-solvent interaction in different solvents. It would depend on the electron-donating capacity of RF molecule in organic solvents. The values of k obs are inversely proportional to the viscosity of the medium as a result of diffusion-controlled processes.
Keywords: dielectric constant; kinetics; photolysis; riboflavin; solvent effect; viscosity.