Solution NMR characterization of the electronic structure and magnetic properties of high-spin ferrous heme in deoxy myoglobin from Aplysia limacina

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Solution 1H NMR has been used to elucidate the magnetic properties and electronic structure of the prosthetic group in high-spin, ferrous deoxy myoglobin from the sea hare Aplysia limacina. A sufficient number of dipolar shifted residue signals were assigned to allow the robust determination of the orientation and anisotropy of the paramagnetic susceptibility tensor, χ. The resulting quantitative description of dipolar shifts allows a determination of the contact shifts for the heme. χ was found to be axial, with Δχax = -2.07 × 10-8 m3/mol, with the major axis tilted (∼76°) almost into the heme plane and in the general direction of the orientation of the axial HisF8 imidazole plane which coincides approximately with the β-,δ-meso axis. The factored contact shifts for the heme are shown to be consistent with the transfer of positive π spin density into one of the two components of the highest filled π molecular orbital, 3eπ, and the transfer of negative π-spin density, via spin-spin correlation, into the orthogonal excited-state component of the 3eπ molecular orbital. The thermal population of the excited state leads to strong deviation from the Curie law for the heme substituents experiencing primarily the negative π-spin density. The much larger transfer of negative spin density via the spin-paired dπ orbital into the excited state 3eπ in high-spin iron(II) than in low-spin iron(III) hemoproteins is attributed to the much stronger correlation exerted by the four unpaired spin on the iron in the former, as compared to the single unpaired spins on iron in the latter.

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Journal of the American Chemical Society

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