Theory of Mössbauer spectra of proteins fluctuating between conformational substates.

Mössbauer spectra of 57Fe in proteins fluctuating between different conformational substates are evaluated by means of a two-sided Padée approximation, which can reproduce the low and high frequency dependence of the spectral line shape I(omega) to any desired accuracy. The dynamics of the atom is modeled as Brownian motion in a multiminimum potential and described by a Fokker-Planck equation. The Mössbauer spectrum is expanded in terms of Lorentzian contributions, which can be attributed separately to fluctuations between conformational substates (potential minima) and to relaxation within the substates. In the limit of closely spaced substates, the Mössbauer spectra can be accounted for by an effective diffusion coefficient with Arrhenius-type temperature dependence. We demonstrate that the observed temperature dependence of Mössbauer spectra of proteins [Parak, F., Knapp, E.W. & Kucheida, D. (1982) J. Mol. Biol. 161, 177-194] can be accounted for by stochastic motion in a multiminimum potential.