Abstract
The photolysis of riboflavin (RF) in aqueous solution in the presence of nicotinamide (NA) by visible light has been studied in the pH range 1.0–12.0 and the various photoproducts have been identified as known compounds. RF has been determined in degraded solutions by a specific multicomponent spectrometric method in the presence of its photoproducts and NA. The second-order rate constants (k2) for the bimolecular interaction of RF and NA range from 0.54 (pH 1.0) to 9.66 M–1 min–1 (pH 12.0). The log k2–pH profile for the photolysis reaction follows a sigmoid curve showing a gradual increase in the rate of pH due to a change in the ionization behavior of the molecule. The lower rate in the acid region is probably due to protonation of the molecule since the cationic form of RF is less susceptible to photolysis than the neutral form. Similarly, a slowing of the rate in the alkaline region is due to anion formation of the molecule. NA is involved as an electron acceptor during the sequence of reactions and thus enhances the rate of photolysis of RF. Absorption and fluorescence measurements did not provide evidence for the complex formation between the two compounds under the present conditions.
Highlights
Aqueous solutions of riboflavin (RF) degrade upon exposure to light [1,2,3] to give a number of inactive compounds under aerobic and anaerobic conditions [4,5,6,7]
The solubility of RF in aqueous solution is enhanced by nicotinamide (NA) in the concentration range of 10–2 M and above, due apparently to a molecular association/complex formation between the two vitamins [14,15,16,17,18]
NA is known to interact with RF and enhance its solubility at a high concentration (~10–2M) in aqueous solutions
Summary
Aqueous solutions of riboflavin (RF) degrade upon exposure to light [1,2,3] to give a number of inactive compounds under aerobic and anaerobic conditions [4,5,6,7]. The solubility of RF in aqueous solution is enhanced by nicotinamide (NA) in the concentration range of 10–2 M and above, due apparently to a molecular association/complex formation between the two vitamins [14,15,16,17,18]. RF and NA are both components of the vitamin B-complex and multivitamin preparations and there is a possibility of interaction between the two components upon exposure to light. It is, necessary to study the effect of NA on the photolysis of RF to understand the nature and extent of this interaction. Some studies, including the effect of NA on the photolysis of cyanocobalamin [23] and that of RF on the photolysis of folic acid [24] and cyanocobalamin [25], have been reported
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