Search

The kinetics of photolysis of ascorbic acid in aqueous solution on UV irradiation has been studied in the pH range 1-11 and the apparent first--order rate constants for the degradation reactions have been determined. The k-pH profile for the photolysis in the acid range is represented by a sigmoid curve indicating the gradual ionization of the molecule (AH2) to ascorbyl anion (AH-). Ascorbic acid shows maximum stability around pH 5-6 due to the lowest rate of oxidation reduction of the mono--anion form. The rate of photolysis is increased up to sevenfold at pH 10.0, compared to that at pH 5.0, due to an increase in the redox potentials with pH leading to faster oxidation of the anionic species to dehydroascorbic acid in the alkaline range. The rate is very slow in the pH range 1-2 due to the existence of the non--ionized form. The apparent first--order rate constants for the photolysis of ascorbic acid at pH 1-11 range from 0.057-3.948x10-2 min-1. A scheme for the sequence of reactions involved in the photolysis of ascorbic acid is presented. [ABSTRACT FROM AUTHOR]

The photoinitiated polymerization of 2-hydroxyethyl methacrylate (HEMA) (1-3 M) using camphorquinone (CQ)/triethanolamine (TEOHA) system has been studied in the pH range 6.0-9.0. The kinetics of the reaction has been evaluated during the initial stages causing about 5% HEMA conversion to avoid the effects of any variation in the volume of the solution. The change in the concentration of HEMA during polymerization has been determined by a UV spectrometric method at 208 nm with a precision of ±3%. The apparent first-order rate constants (kobs) for the polymerization of HEMA by CQ in the presence of 0.01 M TEOHA, carried out up to 100 s, range from 3.35-7.78 (1 M), 2.72-6.78 (2 M) and 2.33-5.89x10 s-1(3 M). The second-order rate constants for the interaction of TEOHA radicals with HEMA (1-3 M) range from 2.33-7.78×10-2 M-1 s-1 at pH 6.0-9.0 indicating an increase in the reactivity of TEOHA radicals with an increase in the pH of the solution. There is a linear relation between £0bs (1-3 M HEMA) and inverse of solution viscosity suggesting the quenching of CQ excited state with an increase in the viscosity of the medium. [ABSTRACT FROM AUTHOR]

A study of the effect of ionic strength on the photodegradation reactions (photoreduction and photoaddition) of riboflavin (RF) in phosphate buffer (pH 7.0) 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 ( k obs ) for the photodegradation of riboflavin at ionic strengths of 0.1–0.5 (0.5 M 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 k obs and √ μ/1 + √ μ. A similar plot of log k/k o against √ μ yields a straight line with a value of ~+1 for Z A Z B 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. [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]

The photolysis of methylcobalamin (MC) in aqueous solution (pH 1.0–12.0) under aerobic condition has been studied on irradiation in the visible region. It leads to the formation of aquocobalamin (B 12a ) in acid solution and hydroxocobalamin (B 12b ) in alkaline solution. This is indicated by the changes in the spectra of MC with a shift towards 525 nm in the acidic and alkaline solutions. The reaction follows first-order kinetics and the values of apparent first-order rate constants ( k obs ) using a 125 W lamp range from 2.80 (pH 12.0) to 27.25 × 10 −2  min −1 (pH 5.0). The k-pH profile for the reaction represents a bell-shaped curve with the maximum rate of photolysis around pH 5.0. The rates of the reaction decrease with a decrease in pH in the acid solution due to protonation and with an increase in pH in the alkaline solution due to ionization of the molecule. MC shows highest fluorescence around pH 5.0 with a decrease in pH in the acidic region and with an increase in pH in the alkaline region. MC and B 12a / B 12b in the irradiated solutions have been determined by a two-component spectrometric assay at 461 and 525 nm with a precision of ±2%. A reaction scheme for the photolysis of MC in aqueous solution has been presented. [ABSTRACT FROM AUTHOR]

The photodegradation of ascorbic acid (AH 2 ) in the presence of nicotinamide (NA) at pH 2.0–12.0 has been studied using a 30 W UV radiation source. The reaction follows first-order kinetics and the values of apparent first-order rate constants ( k obs ) at 1 × 10 −3  M NA concentration range from 1.17 (pH 2.0) to 3.61 × 10 −3  min −1 (pH 12.0). The values of these rate constants ( k 0 ) in the absence of NA range from 0.50 (pH 2.0) to 1.75 × 10 −3  min −1 (pH 12.0), indicating that the values of k obs for the photodegradation of AH 2 in the presence of NA are about 2 fold compared to those of the AH 2 alone. The second-order rate constants ( k ' ) for the photochemical interaction of AH 2 and NA are in the range of 0.67 (pH 2.0) and 1.86 × 10 −3  M −1  min −1 (pH 12.0). The k' -pH profile shows a gradual increase in the rate as a function of pH. This is due to the ionization of AH 2 to give ascorbyl anions (AH − ) which are more susceptible to photodegradation compared to the neutral molecule (AH 2 ). NA appears to undergo photochemical interaction with AH 2 during the reaction by acting as an electron acceptor to enhance its rate of photodegradation. The concentrations of AH 2 and NA in degraded solutions have been determined by a two-component spectrometric method at 243 and 261 nm (pH 2.0) with a precision of ±2%. The method has been validated and the results are comparable to the HPLC method. [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]