Low temperature recombination of CO with haemoglobina

In this paper we apply the theory of non-adiabatic multiphonon group transfer processes to describe the low-temperature recombination reaction between CO and haemoglobin (COHb). The COHb recombination is accompanied by an electronic transition from an S = 2 high spin "free" state to an S = 0 low spin "bound" state, with the iron heme moving from a domed-type structure into a planar structure, while the characteristic low frequency of the relevant vibrational mode(s) involving the heme iron coupled to the protein is <ω> ≅ 50 cm^-1. We were able to provide a quantitative account of the temperature dependence of the recombiantion rate over the temperature range 2-40 K, which exhibits a continuous transition from nuclear tunnelling to an activated process. These low temperature kinetic data allow for the exploration of microscopic mechanisms of biological reactions.