Electronic structure of azidomyoglobin and associated magnetic and hyperfine properties

The electronic structure of azidomyoglobin has been investigated for understanding the observed magnetic and hyperfine properties of this system. The results of our investigation show that a configuration with five electrons in d-like molecular orbital states, as in the case of ferricytochrome c but unlike nitrosylhemoglobin, provides a satisfactory explanation of the observed strongly rhombic g-tensor, the ^57mFe quadrupole splitting from Mössbauer measurements and the porphyrin ¹⁴N quadrupole interactions. For the magnetic hyperfine interactions of the ^57mFe and porphyrin ¹⁴N nuclei, there are significant differences between theory and experiment. For the ^57mFe nucleus, after incorporating the influence of spin-orbit effects, which leads to unquenching of the orbital angular momentum through admixture of excited state configurations to the ground state one, very good agreement is found with single crystal Mössbauer data. For ¹⁴N hyperfine interactions associated with the pyrrole group however, where spin-orbit effects are expected to be much less pronounced, the theoretical values of the hyperfine constants are found to be less than a fifth of those derived from ENDOR measurements. It is suggested that the difference between theory and experiment could be bridged through incorporation of exchange polarization contribution to the ¹⁴N hyperfine interaction from the sizeable valence electron spin density (about 65 per cent of the total) on the iron atom. The need for additional experimental measurements is pointed out, among them ENDOR measurements to determine the hyperfine properties of the azide nitrogens for which the end nitrogens are predicted from the present work to have sizeable magnetic hyperfine constants (about –10 MHz).This work was supported by National Institute of Health Grant HL15196