Evidence for Conformational and Diffusional Mean Square Displacements in Frozen Aqueous Solution of Oxymyoglobin

The temperature dependence of the Mössbauer spectra of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ in the active center of oxymyoglobin is analyzed. The effective mean square displacement $〈x^2〉$ of the iron is a sum of three parts: a vibrational term ${〈x^2〉}_v$, a conformational term ${〈x^2〉}_c$, and a diffusional term ${〈x^2〉}_d$. For $T>\mathrm{}240\mathrm{}$ K diffusional line broadening $\Delta \Gamma \propto {〈x^2〉}_d$ is observed, along with a decrease in quadrupole splitting due to random reorientation of the electric field gradient.