LOW-TEMPERATURE STUDY OF MYOGLOBIN LIGAND REBINDING KINETICS WITH MOSSBAUER-SPECTROSCOPY

We have studied the recombination kinetics of carboxymyoglobin (after photodissociation of the CO ligand) by Mossbauer spectroscopy for temp eratures in the range 4.2-60 K. The observed kinetics display non-expo nential behaviour which was monitored over periods of a few days. It i s shown that the time dependence of the kinetics can be reduced to a s ingle universal function of the temperature-dependent variable (t/tau( 1/2)(T))(beta(T)). The half-decay time tau(1/2)(T) and the scaling par ameter beta(T) are analysed for the presence of tunneling effects. The non-Arrhenius temperature dependence of the half-decay time below 60 K is interpreted as activated tunneling in models with an Eckart barri er or a fluctuating barrier.