Influence of the molecular dynamics of a protein medium on the kinetics of intramolecular electron phototransfer

A study has been made of the mechanism of the reaction of electron phototransfer from the singlet-excited donor (1-dimethylaminonaphthaleae-5-sulphonate) to the covalently attached acceptor (nitroxyl radical) when the photo-active molecule is incorporated into the hydrophobic pocket of bovine serum albumin. Luminescent and ESR spectroscopy has been used to measure the rates of reversible and irreversible electron phototransfers over the temperature range 77-300 K. From the size of the shift of the maximum of the fluorescence spectrum of the chromophore with change in temperature information has been obtained on the dynamic state of the protein environs of the donor. The correlation times of rotary diffusion of the nitroxyl fragment have been determined from the ESR spectra as a function of temperature characterizing the state of the water-protein micro-environs of the acceptor. The kinetic scheme of the process is analysed and conclusions drawn on the essential role of the molecular dynamics of the protein medium in the electron phototransfer mechanism.