Effect of divalent anions on photodegradation kinetics and pathways of riboflavin in aqueous solution.
The present investigation is based on a study of the effect of buffer and non-buffer divalent anions (phosphate, sulphate, tartrate, succinate, malonate) on the kinetics, product distribution and photodegradation pathways of riboflavin (RF) at pH 6.0-8.0. RF solutions (5x10(-5)M) were photodegraded in the presence of divalent anions (0.2-1.0M) using a visible light source and the photoproducts, cyclodehydroriboflavin (CDRF), formylmethylflavin (FMF), lumichrome (LC) and lumiflavin (LF) were assayed by a specific multicomponent spectrophotometric method. RF degradation in the presence of divalent anions follows parallel first-order kinetics to give CDRF and LC as the final products through photoaddition and photoreduction reactions, respectively. The divalent anion-catalysed CDRF formation is affected in the order: phosphate>sulphate>tartrate>succinate>malonate, showing maximum activity of the anions around pH 7. The divalent anions cause deviation of the photoreduction pathway in favour of the photoaddition pathway to form CDRF. The first- and second-order rate constants for the reactions involved in the photodegradation of RF have been determined and the rate-pH profiles and pathway relationships discussed. The catalytic activity of the divalent anions appears to be a function of the relative strength and chemical reactivity of the RF-divalent anion complex acting as a mediator in the photoaddition reaction.
Ahmad
Iqbal
I
Institute of Pharmaceutical Sciences, Baqai Medical University, 51, Deh Tor, Toll Plaza, Super Highway, Gadap Road, Karachi 74600, Pakistan.
Ahmed
Sofia
S
Sheraz
Muhammad Ali
MA
Vaid
Faiyaz H M
FH
Ansari
Izhar A
IA
D000838:Anions / Q000737:chemistry*; D002384:Catalysis; D005415:Flavins / Q000032:analysis; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D010782:Photolysis / Q000187:drug effects*; D012256:Riboflavin / Q000031:analogs & derivatives / Q000032:analysis / Q000737:chemistry* / Q000378:metabolism*; D012996:Solutions / Q000737:chemistry*; D013053:Spectrophotometry / Q000379:methods*
C115829:10-formylmethylflavin; D000838:Anions; D005415:Flavins; D012996:Solutions; C509070:cyclodehydroriboflavin; C001559:7,8-dimethylalloxazine; C001922:lumiflavin; D012256:Riboflavin