Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
4754198 | Journal of Photochemistry and Photobiology A: Chemistry | 2017 | 9 Pages |
Abstract
The photolysis of cyanocobalamin (B12) and aquocobalamin (B12a)/hydroxocobalamin (B12b) in the presence of ascorbic acid (AH2) at pH 2.0-12.0, under aerobic conditions, has been studied. It follows first-order kinetics and the values of apparent first-order rate constants (kobs) at pH 2.0-12.0 range from 0.37 to 2.63Â ÃÂ 10â4Â sâ1 and 0.21-6.35Â ÃÂ 10â4Â sâ1, for B12 and B12a/B12b, respectively. The second-order rate constants (k2) for the photochemical interaction of AH2 and B12 and AH2 and B12a/B12b range from 0.20 (pH 2.0) to 1.09Â ÃÂ 10â2Â Mâ1Â sâ1 (pH 5.0) and 5.88 (pH 2.0) to 91.08Â ÃÂ 10â2Â Mâ1Â sâ1 (pH 5.0), respectively. The values of the k2 for AH2-B12a/B12b interaction are 30-80 times greater than those of AH2-B12 suggesting a greater susceptibility of B12a/B12b to photodegradation compared to that of B12 in this pH range. The k2-pH profiles for both B12 and B12a/B12b are bell-shaped curves indicating the effect of AH2 ionization on the rates of interaction. The complete discoloration of B12 and B12a/B12b solutions on prolonged photolysis indicates the formation of corrin ring cleavage (oxidation) products in acid and alkaline solutions. These oxidation products do not absorb in the visible region. Reaction schemes for the mode of photodegradation of B12 and B12a/B12b in the presence of AH2 have been presented.
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Physical Sciences and Engineering
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Authors
Iqbal Ahmad, Kiran Qadeer, Ambreen Hafeez, Saima Zahid, Muhammad Ali Sheraz, Saif ur Rehman Khattak,