CO binding to hemoglobin: Spin-tunneling temperature dependence, Fe electronic states, and electronic effects on nonadiabatic dynamics

Abstract

The electronic spin change of the iron in heme proteins that occurs upon the binding of carbon monoxide is investigated theoretically. A group theoretic approach is taken to determine an Fe electronic level scheme that is consistent with the experimentally observed temperature dependence of the rate of binding of CO. We show that a nearly degenerate ground state orbital doublet used with the standard theory for the spin transition involving second-order spin–orbit coupling gives a temperature dependence consistent with the experimental data. Measurements, using techniques independent of the recombination experiments, have produced evidence for this ground state orbital doublet, and we describe additional experimental methods for testing this model. The effect of this electronic transition on the nonadiabaticity of the reaction dynamics as a function of temperature is also calculated.