We report measurements of time-dependent solvation in a protein mdium by the fluorecence dynamic Stokes shift method. Picosecond time-correlated singel photon counting fluorescence decays on the complex formed by sperm whale apomyoglobin and 2'-(N,N-dimethylamino)-6-naphthoyl-4-trans-cyclohexanoic acid (DANCA) at temperatures between 298 and 243 K show that the deay of the mean emission energy is extremely nonexponential with components spanning and pobalby 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 oin the relaxation are dependent on conformational substate, or different types of protein motions with different characteristic frequencies participate in the relaxation. The rate distribtuions of correlation fucntions constructred from the data were obtained by maximum entropy fitting and demonstrate the distributed bature of functions construc ted from the data were obtained by maximum entropy fitting and demonstrate the distributed nautre of the decay. Free energy perturbation calculations of protein energetics and simulaions of electron transfer in proteins introduce a perturbation in a dynamics simulation similar to that creasted 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.
Pierce, D. W., & Boxer, S. G. (1992). Dielectric relaxation in a protein matrix. Journal of Physical Chemistry, 96(13), 5560–5566. https://doi.org/10.1021/j100192a069