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A STUDY OF PHOTOCHEMICAL INTERACTION OF CYANOCOBALAMIN WITH THIAMINE AND PYRIDOXINE

Ahmad, Masroor (1996) A STUDY OF PHOTOCHEMICAL INTERACTION OF CYANOCOBALAMIN WITH THIAMINE AND PYRIDOXINE. PhD thesis, University of Karachi, Karachi.

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Abstract

The present investigation is based on a study of the photolysis of cyanocobalamin (vitamin B12) in the presence of thiamine HCI (vitamin B1), its hydrolytic product, 4-methyl-5-(β-hydroxyethyl) thiazole and pyridoxine HCI (vitamin B6) at pH 1-12 using a 125W visible radiation source. The light intensity of the radiation source has been measured by potassium ferrioxalate actinometry during the reactions Cyanocobalamin is photolysed to hydroxocobalamin and other unknown degradation products both in the presence and absence of thiamine HCI, thiazole and pyridoxine HCI. Hydroxocobalamin as a photoproduct in this reaction has been identified by TLC and spectral characteristics. The assay of cyanocobalamin and hydroxocobalamin in photolysed solutions in the presence and absence of thiamine HCI, thiazole and pyridoxine HCI, has been carried out by a newly reported two-component spectrophotometer method by absorbance measurements at 525 and550 nm (pH 4.0), at which there is no interference from other components present in the solution. The method is specific and has a precision of the order of±3% Cyanocobalamin photolysed in aqueous solutions in the presence of thiamine HCI, thiazole and pyridoxine HCI follows first-order kinetics and the apparent first-order rate constants (Kobs) show a gradual increase with bimolecular interaction of cyanocobalamin with thiamine HCI, thiazole and pyridoxine HCI range in the decreasing order from 5.0m-1 min-1 (pH 1) to 0.71 m-1 min-1 (pH 1), 3.5 m-1 min-1 (pH 1) to 0.48 m-1 min -1 (pH 7) and 5.0 m-1 min-1 (pH 1) to o.87 m-1 min-1 )pH 7) respectively indicating that the rate of photolysis is accelerated by the presence of these compounds and cyanocobalamin is most stable in the pH range 6-8 The log K2-pH profiles for these reactions in the acid pH range may be represented by a sigmoid curve indicating the involvement of protonated cyanocobalamin species (pka 3.3), followed by a flat portion in the pH range 6-8 and then a gradual increase in the rate in the alkaline pH range. Thus the reaction appears to involve both acid and base catalysis An increase in citro-phosphate buffer concentration from 0.05M to 0.20M in the pH range 2.5-8.0 results in an increase in the rates of photolysis indicating a catalytic effect of the buffer salts. A comparison of the second-order rate constants of anion catalysed photolysis of cynocobalamin in the presence of equimolar concentrations of thiamine HCI, thaizole and pyridoxine HCI at pH 4.0 in bhosphate (0.80X10-3 M-1 min-1, 0.60X10-3 M-1 min-1, 0.70X10-3 M-1 min-1) and citro-phosphate buffer (1.02X10-3 M-1 min-1, 0.80X10-3 M-1 min-1, 0.80X10-3 M-1 min-1) indicates that citrate ion has a prominent effect on the rate of reaction

Item Type:Thesis (PhD)
Uncontrolled Keywords:cyanocobalamin, thiamine, pyridoxine, vitamin b12, vitamin b1, vitamin b6, thiazole
Subjects:Biological & Medical Sciences (c) > Medical Sciences (c2) > Pharmacy(c2.5)
ID Code:1201
Deposited By:Mr. Muhammad Asif
Deposited On:03 Jan 2007
Last Modified:04 Oct 2007 21:05

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