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. 2014 Oct;15(5):1324-33.
doi: 10.1208/s12249-014-0160-5. Epub 2014 Jun 12.

Effect of ascorbic acid on the degradation of cyanocobalamin and hydroxocobalamin in aqueous solution: a kinetic study

Iqbal Ahmad  1 , Kiran Qadeer, Saima Zahid, Muhammad Ali Sheraz, Tehmina Ismail, Waqar Hussain, Izhar Ahmad Ansari
Affiliations

Effect of ascorbic acid on the degradation of cyanocobalamin and hydroxocobalamin in aqueous solution: a kinetic study

Iqbal Ahmad et al. AAPS PharmSciTech. 2014 Oct.

Abstract

The degradation kinetics of 5 × 10(-5) M cyanocobalamin (B12) and hydroxocobalamin (B12b) in the presence of ascorbic acid (AH2) was studied in the pH range of 1.0-8.0. B12 is degraded to B12b which undergoes oxidation to corrin ring cleavage products. B12b alone is directly oxidized to the ring cleavage products. B12 and B12b in degraded solutions were simultaneously assayed by a two-component spectrometric method at 525 and 550 nm without interference from AH2. Both degrade by first-order kinetics and the values of the rate constants at pH 1.0-8.0 range from 0.08 to 1.05 × 10(-5) s(-1) and 0.22-7.62 × 10(-5) s(-1), respectively, in the presence of 0.25 × 10(-3) M AH2. The t 1/2 values of B12 and B12b range from 13.7 to 137.5 h and 2.5-87.5 h, respectively. The second-order rate constants for the interaction of AH2 with B12 and B12b are 0.05-0.28 × 10(-2) and 1.10-30.08 × 10(-2) M(-1) s(-1), respectively, indicating a greater effect of AH2 on B12b compared to that of B12. The k obs-pH profiles for both B12 and B12b show the highest rates of degradation around pH 5. The degradation of B12 and B12b by AH2 is affected by the catalytic effect of phosphate ions on the oxidation of AH2 in the pH range 6.0-8.0.

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Figures

Fig. 1
Fig. 1
Chemical structures of a cyanocobalamin (B12), b hydroxocobalamin (B12b), c Cob(II)alamin (B12r), d ascorbic acid (AH2), and e dehydroascorbic acid (A)
Fig. 2
Fig. 2
Absorption spectra of hydroxocobalamin solution (5 × 10−5 M) stored in the presence of ascorbic acid (1.5 × 10−3 M). a 0 h, red, b 1.5 h, brownish red, c 3 h, colorless
Fig. 3
Fig. 3
k 2–pH profile for the degradation of cyanocobalamin (B12) at pH 1–8
Fig. 4
Fig. 4
k 2–pH profile for the degradation of hydroxocobalamin (B12b) at pH 1–8
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