Photoinduced Electron Transfer in Dry Gelatin Containing Bound Methylene Blue

Gerald J. Smith*¹, Anthony Harriman², Anthony D. Woolhouse¹, Timothy G. Haskell¹ andThomas H. Barnes³

*To whom correspondence should be addressed at: New Zealand Institute for Industrial Research, P.O. Box 31-310, Lower Hutt, New Zealand.

ABSTRACT

Excited states of methylene blue bound to dry gelatin undergo electron-transfer quenching reactions with neighboring amino acid residues to give reduced, methylene blue-free radicals. At the low loadings of methylene blue bound to gelatin used in this work, the absorption spectra indicated that the methylene blue existed principally in its monomeric form. The rates of methylene blue fluorescence quenching depended on the temperature at which the gelatin was dried. There was no detectable fluorescence characteristic of the localized excited singlet state of methylene blue when the dye is bound to gelatin dried at 15°C, where a significant proportion of the protein exists in its alpha-helical form. Instead, weak emission with a maximum at 645 nm is observed. It is inferred from this that the rate of quenching of any localized excited singlet states of the dye bound to alpha-helical gelatin is very fast thereby competing efficiently with radiative relaxation. The weak emission at 645 nm is ascribed to a fluorescent exciplex formed between bound methylene blue and a neighboring electron/proton donor residue of the gelatin. While fluorescence characteristic of the localized methylene blue singlet state was observed from dyed gelatin dried at 50°C, i.e. in its random coil form, the triplet state is subject to rapid quenching by electron/proton transfer with protein moieties.