POLYCHROMATIC KINETICS OF CONFORMATIONAL AND SPIN RELAXATION OF REDUCED INTERMEDIATES OF MYOGLOBIN

Mossbauer spectroscopy was used to study the relaxation of a non-equil ibrium myoglobin state produced at 77 K by reduction of metmyoglobin F e(III) with thermalized electrons. The intermediate is characterized w ith the metMb (r) conformation of the protein globule and a low spin h eme Fe(II) with a water molecule on the sixth coordination site. The i ntermediate is stable at 77 K but relaxes with increasing temperature into equilibrium deoxymyoglobin with dissociation of the bond Fe(II) - H2O and the transition of the iron into the high spin state. In the t emperature range 147 - 195 K the relaxation kinetics is nonexponential in time and can be described in terms of the polychromatic kinetics. A fitting of the kinetics data supposing a gamma-distribution of activ ation enthalpies of the relaxation shows a shift and a narrowing of th e distribution at T > 180 K. The effect of structural rearrangements a nd equilibrium conformational fluctuations in the protein on the shape of the observed barrier distribution is discussed.