Hemoglobin recombination spin selectivity

Abstract

Group theory and the spin Hamiltonian are employed in a theoretical treatment of the spin dependence of the tunneling recombination of CO with hemoglobin following photolysis. Second order spin-orbit coupling between the 1A1 and 5T2 states of the octahedral five-coordinate Hb heme iron is considered in the presence of lower D2 symmetry fields. It is assumed that the totally symmetric 1A1 character that is mixed into the lowest spin multiplet of 5T2 is responsible for recombination to the 1A1 state of the recombined hemoglobin. The problem of identifying which spin multiplet, 5B1, 5B2, 5B3, of 5T2 in lower D2 symmetry lies lowest has been solved. When the field and temperature dependence of the HbCO tunneling recombination rate is calculated, 5B2 is in best agreement with experiments on HbCO. With the formulation developed for diamagnetic HbCO, it is relatively simple to include a small exchange interaction between the NO and Hb iron heme while assuming that the recombination of HbNO and HbCO is otherwise similar. The effect of an exchange interaction as small as a few cm−1 is formally similar to the application of a very large magnetic field and predicts an increase of the recombination rate in agreement with experiment.