Dynamic Features in the Mössbauer Spectra of Heme Proteins

We report Mossbauer studies of the mean square displacements 〈x2(T)〉, the mean square velocities, 〈v 2(T)〉, and the quadruple splitting ∆E Q (T) of heme proteins that probe the motions at the iron active center. Accurate measurements of the recoilless fraction f(T), the isomer shift δ(T) and the quadrupole splitting ∆E Q (T) on 57Fe-enriched oxy- and carbon monoxy-myoglobin (MbO2 and MbCO) in frozen aqueous solution have led to the following conclusions: (i) A harmonic model \(\left\langle {x^2 } \right\rangle = \rlap{--} h/(6m)\sum {(\beta _i^2 /\omega _i } )\) ctanh (ħω i /2k B T) represents 〈x 2〉 quite well up to 180K with a single mode, ω ~25cm -1. The frequency of this vibration is in the range of the delocalized low-frequency modes predicted for globular proteins [1]. The larger contribution of this mode in MbCO than in MbO2, \(\beta _i^2 /3 \simeq 0.29\) vs . ~ 0.13, is consistent with a stronger coupling to the iron in MbCO. (ii) Assuming that the temperature dependence ot δ(T) is entirely due to the second order Doppler shift a harmonic model for 〈v 2(T)〉 fits the data well up to 250K with a single mode, ω ~ 220cm -1 for MbO2 and ω ~ 190cm -1 for MbCO. The frequencies of these vibrations are similar to those reported for the iron-histidine stretching mode in Mb [2].