MOSSBAUER-SPECTROSCOPY ON NONEQUILIBRIUM STATES OF MYOGLOBIN - A STUDY OF R-T RELAXATION

A frozen solution of Fe-57-enriched metmyoglobin was irradiated by x r ays at 77 K. Mossbauer spectra showed a reduction of Fe(III) high spin by thermalized electrons and a production of a metastable Fe(II) low spin myoglobin complex with H2O at its sixth coordination site. The re laxation of the intermediate was investigated by Mossbauer spectroscop y as a function of temperature and time. The relaxation process starts above 140 K and is fully completed at similar to 200 K. At temperatur es between 140 and 200 K, the relaxation lasts for hours and is nonexp onential in time. Up to 180 K, the process can be described satisfacto rily by a gamma distribution of activation enthalpies with an Arrheniu s relation for the rate coefficient. The temperature and time dependen ce of the Mossbauer parameters indicates structural changes in the act ive center of the protein as early as 109 K that continue for several hours at higher temperatures. Above 180 K, structural rearrangements i nvolving the whole protein molecule lead to a shift and narrowing of t he barrier height distribution.