DIELECTRIC-RELAXATION IN A PROTEIN MATRIX

We report measurements of time-dependent solvation in a protein medium by the fluorescence dynamic Stokes shift method. Picosecond time-correlated single photon counting fluorescence decays on the complex formed by sperm whale apomyoglobin (apoMb) and 2'-(NN-dimethylamino)-6-naphthoyl-4-trans-cyclohexanoic acid (DANCA) at temperatures between 298 and 243 K show that the decay of the mean emission energy is extremely nonexponential with components spanning and probably exceeding the experimentally observable (20 ps to 20 ns) range. At all temperatures, the observed relaxation is approximately 700 cm-1, but the emission energy at short and long times increases with decreasing temperature. Decays on the low-energy side of the emission band show rise times clearly resolved from the instrument response and rule out a model in which the observed shift results solely from heterogeneity in the population of dye-apoMb complexes. These results suggest that either the activation energies of the rate-limiting motions in the relaxation are dependent on conformational substate, or different types of protein motions with different characteristic frequencies participate m the relaxation. The rate distributions of correlation functions constructed from the data were obtained by maximum entropy fitting and demonstrate thc distributed nature of the decay. Free energy perturbation calculations of protein energetics and simulations of electron transfer in proteins introduce a perturbation in a dynamics simulation similar to that created experimentally here. The nanosecond time scale relaxations observed indicate a possible shortcoming of simulations which typically extend at most to 100 ps. Possible relaxations on a much shorter time scale are discussed.