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The effect of metal ion complexation on the photolysis of riboflavin (RF) using various metal ions (Ag + , Ni 2+ , Co 2+ , Fe 2+ , Ca 2+ , Cd 2+ , Cu 2+ , Mn 2+ , Pb 2+ , Mg 2+ , Zn 2+ , Fe 3+ ) has been studied. Ultraviolet and visible spectral and fluorimetric evidence has been obtained to confirm the formation of metal-RF complexes. The kinetics of photolysis of RF in metal-RF complexes at pH7.0 has been evaluated. The apparent first-order rate constant (k obs ) for the photolysis of RF and the formation of lumichrome (LC) and lumiflavin (LF) (0.001M phosphate buffer) and LC, LF and cyclodehydroriboflavin (CDRF) (0.2-0.4M phosphate buffer) have been determined. The values of k obs indicate that the rate of photolysis of RF is promoted by divalent and trivalent metal ions. The second-order rate constants (k' ) for the interaction of metal ions with RF are in the order: Zn 2+ >Mg 2+ >Pb 2+ >Mn 2+ >Cu 2+ >Cd 2+ >Fe 2+ >Ca 2+ >Fe 3+ >Co 2+ >Ni 2+ >Ag + . In phosphate buffer (0.2-0.4M), an increase in the metal ion concentration leads to a decrease in the formation of LC compared to that of CDRF by different pathways. The photoproducts of RF have been identified and RF and the photoproducts have simultaneously been assayed by a multicomponent spectrometric method. The mode of photolysis of RF in metal-RF complexes has been discussed., (Copyright © 2017 Elsevier B.V. All rights reserved.)

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.

The photolysis of riboflavin (RF) in the presence of acetate buffer (pH 3.8-5.6) and carbonate buffer (pH 9.2-10.8) has been studied using a multicomponent spectrophotometric method for the simultaneous assay of RF and its photoproducts. Acetate and carbonate buffers have been found to catalyze the photolysis reaction of RF. The apparent first-order rate constants for the acetate-catalyzed reaction range from 0.20 to 2.86 × 10(-4) s(-1) and for the carbonate-catalyzed reaction from 3.33 to 15.89 × 10(-4) s(-1). The second-order rate constants for the interaction of RF with the acetate and the carbonate ions range from 2.04 to 4.33 × 10(-4) M(-1) s(-1) and from 3.71 to 11.80 × 10(-4) M(-1) s(-1), respectively. The k-pH profile for the acetate-catalyzed reaction is bell shaped and for the carbonate-catalyzed reaction a steep curve. Both HCO3(-) and CO3(2-) ions are involved in the catalysis of the photolysis reaction in alkaline solution. The rate constants for the HCO3(-) and CO3(2-) ions catalyzed reactions are 0.72 and 1.38 × 10(-3) M(-1) s(-1), respectively, indicating a major role of CO3(2-) ions in the catalysis reaction. The loss of RF fluorescence in acetate buffer suggests an interaction between RF and acetate ions to promote the photolysis reaction. The optimum stability of RF solutions is observed in the pH range 5-6, which is suitable for pharmaceutical preparations.

A photodegradation study of 5‐fluorouracil (5‐FU) has been carried out in the water (pH 7.0) and organic solvents (chloroform, ethyl acetate, acetone, 1‐butanol, 1‐propanol, ethanol, methanol, and acetonitrile) using developed and validated high‐performance liquid chromatography (HPLC) method. The test method was found to be simple, accurate (% recovery, 100.25%–101.08%), precise (%RSD < 2.00), and sensitive. The apparent first‐order rate constants are in the range of 0.85 (chloroform) to 4.25 × 10−4 min−1 (water). The values obtained for kobs were found to be linear as a function of the dielectric constant indicating the involvement of the dipolar intermediate in the reaction. The increase in the rate of degradation of this intermediate is enhanced by the polarity of the medium. Rates of photolysis for 5‐FU show a linear relationship with the solvent acceptor number (AN) which indicates the degree of solute–solvent interaction. The values of kobs were found to be linear to the inverse of the viscosity of the medium due to the diffusion‐controlled process. [ABSTRACT FROM AUTHOR]

The photolysis of thiochrome (THC), an oxidation product of thiamine (vitamin B 1) (THE), used for its fluorimetric assay, has been studied in the pH range 7.0–12.0. THC undergoes photooxidation to oxodihydrothiochrome (ODTHC) which is oxidized to a non-fluorescent compound (OP1) on UV irradiation. The kinetics of the consecutive first-order reactions: THC → k 1 ODTHC → k 2 OP 1 , has been evaluated and the values of first-order rate constants, k 1 (0.58–4.20 × 10−5, s−1) and k 2 (0.05–2.03 × 10−5, s−1), at pH 7.0–12.0 have been determined. The rates of degradation of THC and ODTHC are enhanced with pH and the second-order rate constants k 1 ʹ and k 2 ʹ for the OH– ion-catalyzed reaction are in the range of 0.002–58.3 M−1 s−1. The quantum yields of the photolysis of THC and ODTHC in the pH range 7.0–12.0 have been determined. THC, ODTHC and OP1 have been identified by chromatographic, spectrometric and fluorimetric methods. THC and ODTHC have similar fluorescence characteristics and emit at 450 and 445 nm, respectively. THC, ODTHC and OP1 with distinct absorption maxima (370, 344 and 290 nm, respectively) have been determined by a newly developed and validated multicomponent spectrometric method during the photolysis reactions. The on-line formation of THC by the photooxidation of THE may lead to the degradation of THC and give erroneous results in the fluorimetric assay of THE. A scheme for the photolysis reactions of THC in aqueous solution is presented. Unlabelled Image • Photolysis of thiochrome (THC) by UV light has been studied in the pH range 7.0–12.0. • THC is photooxidized to oxodihydrothiochrome (ODTHC) in alkaline solution. • Photolysis of THC follows consecutive first-order kinetics to form oxidation products. • THC photolysis is pH dependent and is enhanced with pH in alkaline solution. • THC and photoproducts have been assayed by a multicomponent spectrometric method. [ABSTRACT FROM AUTHOR]

Graphical abstract The formation of cyclodehydroriboflavin (CDRF) involves the interaction of riboflavin (RF) with divalent anions (e.g. CO 3 2−) to form a RF−CO 3 2− complex. This complex on excitation to singlet state 1[RF ox ] undergoes cyclization to give a dihydroflavin intermediate which on autoxidation yields CDRF. Highlights • Photolysis of riboflavin (RF) in the presence of divalent anions is highest at pH 7.0. • Carbonate, oxalate, phthalate from cyclodehydroriboflavin by RF photoaddition. • Rate of photoaddition depends on RF fluorescence quenching by divalent anions. • Divalent anions alter photoreduction pathway of RF to photoaddition pathway. • A multicomponent spectrometric method is used for the assay of RF and photoproducts. Abstract A study of the effect of some divalent anions (carbonate, oxalate, phthalate, phosphate) on the kinetics of photodegradation reactions of riboflavin (RF) and its pathways, product composition and fluorescence quenching in the pH range 6.0–8.0 has been conducted. The photodegradation of RF in the presence of 0.2–1.0 M divalent anions on visible irradiation leads to the formation of cyclodehydroriboflavin (CDRF) by photoaddition pathway and lumichrome (LC) by photoreduction pathway as the major end products. These products are formed by simultaneous first–order reactions. The divalent anions deviate the photoreduction pathway of RF in favor of photoaddition pathway to yield CDRF. The extent of this variation depends on the strength of RF–divalent anion complex as indicated by an increase in the loss of RF fluorescence due to complex formation. The first– and second–order rate constants for the photodegradation of RF have been determined and the effect of pH on the rates of these reactions evaluated. The rate of divalent anion catalyzed formation of CDRF is maximum at pH 7 and occurs in the order: carbonate > phosphate > oxalate > phthalate. The catalytic activity of the divalent anions increases with the strength of RF–divalent anion complex resulting in an increase in the rate of formation of CDRF and a decrease in the rate of formation of LC by different pathways. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A study of the formulation of liposomal preparations of riboflavin (RF) with compositional variations in the content of phosphatidylcholine (PC) and their entrapment efficiency (26–42%) have been conducted. Light transmission characteristics of the liposomal preparations have been determined to evaluate their effect on the amount of light passing through the system to initiate a photochemical reaction. Dynamic light scattering (DLS) and atomic force microscopy (AFM) have been used to study the physical characteristics of liposomes. The liposomal preparations of RF have been subjected to photolysis using visible light and the apparent first- order rate constant, k obs , for the degradation of RF have been determined. The values of k obs (1.73–2.29 × 10 − 3 min − 1 ) have been found to decrease linearly with an increase in PC concentration in the range of 12.15 to 14.85 mM. Thus, an increase in PC concentration of liposomes leads to an increase in the stability of RF. RF and its main photoproduct, lumichrome (LC), formed in liposomes have been assayed by a two-component spectrometric method at 356 and 445 nm using an irrelevant absorption correction to compensate for the interference of liposomal components. The fluorescence measurements of RF in liposomes indicate excited singlet state quenching and the formation of a charge-transfer complex between RF and PC. It results in electron transfer from PC to RF to cause photoreduction and stabilization of RF. [ABSTRACT FROM AUTHOR]