ELECTRON-TRANSFER IN DONOR-ACCEPTOR PAIR IN MODIFIED ALPHA-CHYMOTRYPSIN - EFFECT OF MICROVISCOSITY, MACROVISCOSITY AND LOCAL POLARITY

Electron transfer was studied within a donor-acceptor pair in spin-lab elled alpha-chymotrypsin with the methionine-192 group modified with i odacetamide-2,2,6,6-tetramethylpiperidine-1-oxyl. The effects of tempe rature (T=200-300 K), viscosity and humidity on the rate constant of p hotoreduction of the nitroxide fragment (acceptor) by the protein tryp tophan 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 fluor escence techniques respectively. The apparent polarity of the microenv ironment was also determined by ESR and fluorescence techniques. It wa s shown that the changes in the rate constant of photoreduction correl ate with the changes in the parameters of the local dynamics of the do nor-acceptor pair microenvironment (correlation time tau(c) less than or equal to 10(-7) s) recorded by ESR spectroscopy. From the data on t he local dynamics and polarity, the energetic profile of the photoredu ction was determined, The role of local dynamics in the stabilization of photoseparated charges by time-dependent polar solvation (dynamic a daptation) is discussed. (C) 1997 Elsevier Science S.A.