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 temperatures in the range 4.2-60 K. The observed kinetics display non-exponential behaviour which was monitored over periods of a few days. It is shown that the time dependence of the kinetics can be reduced to a single universal function of the temperature-dependent variable (t/τ(1/2)(T))(β(T)). The half-decay time τ(1/2)(T) and the scaling parameter β(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 barrier or a fluctuating barrier.