Photolysis of methylcobalamin in aqueous solution: A kinetic study.

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]