ELECTRONIC SPIN TUNNELING IN THE BINDING OF CARBON-MONOXIDE TO HEMOGLOBIN

A non-adiabatic quantum tunneling process is investigated as the mechanism for effecting the electronic spin change of the hemoglobin's iron upon the binding of carbon monoxide.; As the carbon monoxide approaches there is a spin state change in the Fe('2+) from S = 2 to S = 0. The Born-Oppenheimer approximation can be used to separate the recombination of the CO to the iron in the heme at low temperatures into a nuclear tunneling and an electronic tunneling. Based upon the spin change of the Fe as well as the size of the tunneling matrix element and the energy splitting of the two states in the transition region, we assume the reaction to be a non-adiabatic electronic Landau-Zener state to state tunneling. The tunneling involves a spin change of the Fe and thus a spin-orbit interaction is used as the perturbation that couples the S = 2 and S = 0 manifolds.; Since the matrix element for the transition is due to spin-orbit coupling the size of the matrix element can be changed, and hence the tunneling rate, by changing the spin magnetic sublevel of the initially CO unbound Fe. This is accomplished by applying a strong magnetic field of approximately 100 000 gauss which will tend to align the Fe spin at low enough temperature. The L vector will be affected only slightly by the external magnetic field since the Zeeman effect on the orbital levels is much smaller (10('-2)) than that of the internal crystal field of the molecule. Hence the crystal field of the heme determines the L quantization axis in each local heme coordinate system. Thus in a random oriented distribution of hemes frozen in place we expect faster CO recombination for those hemes who have their L vector aligned in the direction of the magnetic field than for those hemes whose L vector is perpendicular to the magnetic field.; Hemoglobin has a strong absorption band at 436 nm when CO is bound. This absorption is also orientation dependent for the absorption is predominantly for light polarized in the plane of the dish like heme inside the hemoglobin molecule. We look for an orientation dependence to the recombination rate by looking for a magnetically induced optical dichroism in the sample. The CO molecules that are bound to hemoglobin molecules in a sample are photo-dissociated by an intense laser pulse in the presence of a magnetic field. As the CO molecules recombine with the hemoglobin we monitor the optical dichroism of the sample.; The theory behind the magnetic field dependence is investigated in this work. I also describe the experiment carried out to test this theory and present the results which showed a magnetic field dependence to the recombination rate.