Electron transfer was studied within a donor-acceptor pair in spin-labelled a -chymotrypsin with the methionine-192 group modified with 4-iodacetamide-2,2,6,6-tetramethylpiperidine-1-oxyl. The effects of temperature (T=200-300 K), viscosity and humidity on the rate constant of photoreduction of the nitroxide fragment (acceptor) by the protein tryptophan residue in the excited singlet state (donor) were investigated. In parallel, the dynamics of the microenvironment in the vicinity of the nitroxide and tryptophan were studied by electron spin resonance (ESR) spectroscopy (X-band and high-resolution 2-mm waveband) and fluorescence techniques respectively. The apparent polarity of the microenvironment was also determined by ESR and fluorescence techniques. It was shown that the changes in the rate constant of photoreduction correlate with the changes in the parameters of the local dynamics of the donor-acceptor pair microenvironment (correlation time t_c £ 10^-7 sec) recorded by ESR spectroscopy. From the data on the local dynamics and polarity, the energetic profile of the photoreduction was determined. The role of local dynamics in the stabilization of photoseparated charges by time-dependent polar solvation (dynamic adaptation) is discussed