Riboflavin (RF), also known as vitamin B2, belongs to the class of water-soluble vitamins and is widely present in a variety of food products. It is sensitive to light and high temperature, and therefore, needs a consideration of these factors for its stability in food products and pharmaceutical preparations. A number of other factors have also been identified that affect the stability of RF. These factors include radiation source, its intensity and wavelength, pH, presence of oxygen, buffer concentration and ionic strength, solvent polarity and viscosity, and use of stabilizers and complexing agents. A detailed review of the literature in this field has been made and all those factors that affect the photo, thermal and chemical degradation of RF have been discussed. RF undergoes degradation through several mechanisms and an understanding of the mode of photo- and thermal degradation of RF may help in the stabilization of the vitamin. A general scheme for the photodegradation of RF is presented.

https://www.beilstein-journals.org/bjoc/content/pdf/1860-5397-10-208.pdf

Photo, thermal and chemical degradation of riboflavin

Photolysis of riboflavin in aqueous solution: a kinetic study.

The photodegradation reactions of riboflavin (RF) in the presence of 0.05–2.00 M phosphate (pH 7.0) have been studied using a specific multicomponent spectrophotometric method. The reactions involve simultaneous photolysis (intramolecular photoreduction) and photoaddition (intramolecular photoaddition) leading to lumichrome (LC) and cyclodehydroriboflavin (CDRF), respectively, as major products. The contribution of each reaction in the overall photodegradation depends upon the phosphate concentration, i.e., higher the phosphate concentration higher the extent of photoaddition. The apparent first-order rate constants for the photodegradation of RF and for the formation of LC and CDRF at 0.25–2.00 M phosphate concentration range from 0.65 to 3.03 × 10 −2 , and from 0.41 to 0.99 × 10 −2 and 0.12 to 1.63 × 10 −2 min −1 , respectively. The second-order rate constants for the phosphate catalysed photodegradation of RF and for the formation of LC and CDRF are 2.12 × 10 −4 and 0.61 × 10 −4 and 1.41 × 10 −4 M −1  s −1 , respectively. Since the formation of CDRF by photoaddition is catalysed by HPO 4 2− ions [Eur. J. Biochem. 57 (1975) 35], it is suggested that H 2 PO 4 − ions may be involved in the formation of LC by photolysis. Thus, both H 2 PO 4 − and HPO 4 2− ions may catalyse the two major reaction pathways of riboflavin photodegradation, respectively.

A study of simultaneous photolysis and photoaddition reactions of riboflavin in aqueous solution.

The effect of caffeine complexation with riboflavin on the kinetics of riboflavin photolysis in the pH range 2.0-10.5 has been studied. The photolysis of riboflavin solutions (5x10(-5) M) was carried out in the presence of caffeine (0.5-2.5x10(-4) M) using a visible radiation source. A specific multicomponent spectrophotometric method was used for the determination of riboflavin and photoproducts in photolysed solutions. The apparent first-order rate constants (k) for the photolysis reactions range from 2.71x10(-4) to 4.26x10(-2) min(-1). The values of the rate constants decrease with increasing concentrations of caffeine indicating its inhibitory effect on the reactions. The second-order rate constants (k') for the caffeine inhibited reactions lie in the range of 0.13 to 5.10x10(-3) M(-1) min(-1). The log k-pH profiles for the photolysis reactions at various caffeine concentrations involve multiple steps indicating a gradual increase in the rate up to pH 10. The lower rates at pH 2.0 and 10.5 are due to the ionization of riboflavin as evident from fluorescence measurements. The k'-pH profile for the interaction of riboflavin with caffeine represents a bell-shaped curve in the pH range 3-6 followed by a sigmoid curve in the pH range 7-10. The inhibition of photolysis of riboflavin in the presence of caffeine is a result of the monomeric interaction and complex formation of caffeine with riboflavin. The photochemical interaction of riboflavin with caffeine suggests that a pH around 6 is most appropriate for the stabilization of the vitamin. At this pH the complex shows the highest stability constant.

Effect of Caffeine Complexation on the Photolysis of Riboflavin in Aqueous Solution: A Kinetic Study

A study of the effect of ionic strength on the photodegradation reactions (photoreduction and photoaddition) of riboflavin (RF) in phosphate buffer (pH7.0) has been carried out using a specific multicomponent spectrometric method. It has been found that the rates of photodegradation reactions of RF are dependent upon the ionic strength of the solutions at different buffer concentrations. The apparent first-order rate constants (kobs) for the photodegradation of riboflavin at ionic strengths of 0.1-0.5 (0.5M phosphate) lie in the range of 7.35-30.32 × 10(-3) min(-1). Under these conditions, the rate constants for the formation of the major products, lumichrome (LC) by photoreduction pathway, and cyclodehydroriboflavin (CDRF) by photoaddition pathway, are in the range of 3.80-16.03 and 1.70-6.07 × 10(-3) min(-1), respectively. A linear relationship has been observed between log kobs and √μ/1+√μ. A similar plot of log k/ko against √μ yields a straight line with a value of ~+1 for ZAZB showing the involvement of a charged species in the rate determining step. NaCl appears to promote the photodegradation reactions of RF probably by an excited state interaction. The implications of ionic strength on RF photodegradation by different pathways and flavin-protein interactions have been discussed.

Ionic strength effects on the photodegradation reactions of riboflavin in aqueous solution.

Riboflavin undergoes a characteristic biphasic photolysis with a low-intensity light source, depending on the ionic strength of the phosphate buffer used. In higher ionic strength phosphate buffer, an initial faster phase is followed by a slower second phase, and vice versa in lower ionic strength buffer. On the basis of the circular dichroism spectra, a side chain-isoalloxazine ring interaction through hydrogen bonding is considered to be responsible for the ionic strength dependence.

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Hisaaki Chaki

Papers

Biphasic photolysis of riboflavin. III. Effects of ionic strength on the photolysis.