Mössbauer Spectroscopic Investigations of Photodissociated Myoglobin‐CO at Low Temperatures

Myoglobin‐CO (MbCO) has been photodissociated at low temperatures. The photoproduct Mb * has been identified by Mössbauer spectroscopy as a ferrous high‐spin complex with Mössbauer parameters which are different from those of deoxy‐myoglobin. From the time‐dependent change of the linewidth of the Mb * Mössbauer spectrum at 5 K over a time interval of 8 h, we conclude that there exist several slightly different Mb * conformations with different recombination characteristics. In order to obtain a convenient time resolution of the recombination behavior, we have investigated the time‐dependence of the Mb * absorption within a time interval of 10 2 –5 · 10 4 s after the photo‐dissociation, with a Mössbauer drive of constant velocity. The resulting recombination data have then been analyzed with various approximation steps. It is shown that the interpretation of experimental data on the basis of two independent exponentials leads to pre‐exponential frequency factors which are of the order of A 1 = 10s −1 and A 2 = 10 5 ± 5 · 10 2 s −1 . The attempt to interpret the experimental data on the basis of a distribution of energies seems more plausible and is in agreement with the existence of several slightly different Mb * conformations. The corresponding frequency factor A is about 4 s −1 and is therefore in disagreement with the value of A (of about 10 7 s −1 ) which was derived from optical recombination data by Austin et al. within a time interval of 10 −6 –10 3 s after the photodissociation. Typical activation energies for the recombination process Mb * + CO → MbCO are 8 kJ/mol. At low temperatures ( T < 46 K) the recombination behavior is explained by quantum mechanical tunnelling.