[Conformational mobility and theory of the Mössbauer effect in biological systems. (Model of a Brownian oscillator strongly damped for conformational modes)].

Specific features of the Mössbauer effect on proteins are discussed. It is found that the temperature dependence of recoilless gammaquantum absorption, f'(T), cannot be explained in terms of solid effects or diffusion spectrum broadening in liquids. A sharp decrease in f'(T) at temperature above 200 degrees K that has been found is due to the specificity of the conformational mobility of proteins and due to a smaller correlation time for spacially-confined diffusion of a Mössbauer label tracer below a critical value of tau approximately 10(-7) s (the lifetime of 57Fe excitation). Protein conformational mobility is described by a Brownian, strongly damped oscillator. For this model, a new formula for the Mössbauer effect probability is derived. Calculations by the formula and corresponding experimental data are in good accord. By the formula for f'(T), a theoretical temperature dependence of medium microviscosity (with an activation energy of 5 kcal/mol) and estimates for some parameters are obtained.