Photoreduction of dyes catalysed by organic and bio-molecules

Summary Photoreduction of methylene blue in the presence of various organic and biomolecules has been studied spectrophotometrically and potentiometrically. During the photoreduction, the emf of the system measured against calomel electrode increases and attains a plateau long before the complete reduction of the dye. This indicates that the emf developed in this system is not that of the dye/leucodye in which case the plateau of the emf value would be expected at the complete reduction of the dye. The emf has been interpreted to be mainly that of dye/dye free radical, although dye/leucodye and dye free radical/leucodye may contribute to the emf value of the system. The free radical may be formed in the system immediately after exposure to light or even in the dark at high pH. Photoreduction of methylene blue in the presence of EDTA is completely inhibited by excess magnesium ions. By using various compounds to act as electron donors in the photoreduction, it has been found that compounds having the general formula R₁R₂·N·CH₂COOH can cause the reduction of methylene blue where R₁ and R₂ may be–CH₂COOH or alkyl or aryl substitution. Photoreduction becomes faster with more than one–CH₂COOH group in the compounds. Glycine or N-methyl glycine fails to act as a catalyst, N-phenyl glycine or N:N-diethyl glycine catalyses the photoreduction. N-phenyl- or N-ethyl amino diacetic acid causes faster photoreduction of methylene blue. Analgin causes photoreduction even in dark. The dye capri blue is not photoreduced by EDTA though there is an increase in the emf of the system on its exposure to light; but the presence of methylene blue in the system catalyses its photoreduction.Presented at the VIth International Congress of Photobiology, held at Bochum in August 1972.