Metal ion mediated photolysis reactions of riboflavin: A kinetic study

- Metal ion complexation alters the redox reactivity of riboflavin (RF) on photolysis.
- Photolysis of RF complexes is enhanced by electron transfer to RF in excited state.
- Reactivity of metal ion on RF photolysis is affected by phosphate concentration.
- Metal ions influence the kinetics of photoreduction and photoaddition pathways of RF.
- The study could throw light on the redox reactivity of RF in biological systems.

The effect of metal ion complexation on the photolysis of riboflavin (RF) using various metal ions (Ag+, Ni2 +, Co2 +, Fe2 +, Ca2 +, Cd2 +, Cu2 +, Mn2 +, Pb2 +, Mg2 +, Zn2 +, Fe3 +) has been studied. Ultraviolet and visible spectral and fluorimetric evidence has been obtained to confirm the formation of metal-RF complexes. The kinetics of photolysis of RF in metal-RF complexes at pH 7.0 has been evaluated. The apparent first-order rate constant (kobs) for the photolysis of RF and the formation of lumichrome (LC) and lumiflavin (LF) (0.001 M phosphate buffer) and LC, LF and cyclodehydroriboflavin (CDRF) (0.2-0.4 M phosphate buffer) have been determined. The values of kobs indicate that the rate of photolysis of RF is promoted by divalent and trivalent metal ions. The second-order rate constants (kʹ ) for the interaction of metal ions with RF are in the order: Zn2 + > Mg2 + > Pb2 + > Mn2 + > Cu2 + > Cd2 + > Fe2 + > Ca2 + > Fe3 + > Co2 + > Ni2 + > Ag+. In phosphate buffer (0.2-0.4 M), an increase in the metal ion concentration leads to a decrease in the formation of LC compared to that of CDRF by different pathways. The photoproducts of RF have been identified and RF and the photoproducts have simultaneously been assayed by a multicomponent spectrometric method. The mode of photolysis of RF in metal-RF complexes has been discussed.

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