Nitrosyl complexes of the iron(III) forms of myoglobin, human hemoglobin, Glycera dibranchiata hemoglobins (Hbm and Hbh), and model iron(II) and iron(III) synthetic porphyrins including octaethylporphyrin (OEP) have been prepared. The iron(III) heme proteins are electron spin (paramagnetic) resonance (ESR) silent, while hexacoordinate solution structures are indicated for [Fe(OEP)(NO)2]ClO4 and for Hbm(II)NO, which has an ESR spectrum similar to that of Mb(II)NO and the hexacoordinate iron(II) model complex Fe(OEP)NO(BzIm). The splitting of the alpha- and beta-bands in the optical spectrum of Mb(III)NO and Hbh(III)NO contrasts markedly with the sharp, single bands observed in that of Hbm-(III)NO. The nondegeneracy of the dxz and dyz orbitals in Mb(III)NO and Hbh(III)NO is attributed to the influence of the distal histidine. Circular dichroism spectra were obtained for Hbm(III)NO, Hbm(II)NO, Hbh(III)NO, Hbh(II)NO, Mb(II)NO, and Mb(III)NO. The vicinal chiral center contribution that governs the heme protein CD leads to low Kuhn anisotropies, which have been used to assign certain electronic transitions. The Hb(III)NO spectrum is not stable but transforms into that of Hb(II)NO. This autoredox process follows kinetics that are first order in FeIIINO. The relative rates of autoreduction (25 degrees C, 1 atm NO) are Mb(III)NO less than Hbm(III)NO less than Hb alpha(III)NO less than HbA(III)NO. At high NO partial pressure or after "recycling" of HbA, the rates of reduction decrease. The first step in the reaction of NO with the ferric heme is the reversible formation of the formally iron(III) adduct. This reacts with another molecule of NO, generating the final heme(II)-NO via nitrosylation of NO itself or of an endogenous nucleophile. Kinetic and spectroscopic evidence shows involvement of trans-heme-(NO)2 in the reaction. The activation parameters delta H and delta S were determined. The overall reaction is photoenhanced.

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In comparison with myoglobin and human and Glycera dibranchiata hemoglobins, the heme distal side amino acid exchanges within the heme environment of elephant tetrameric hemoglobin (Hbe) only slightly affect the electronic and ESR spectra of Hbe(III) and Hbe(II) derivatives, several of which were prepared and characterized by optical and ESR spectroscopy. Addition of 2,3‐bisphosphoglycerate [Gri(2,3)P2] or inositol hexakisphosphate to Hbe(II)NO causes tension in the Fe‐N(proximal His) bond, although the behaviour differs in detail from that of HbA(II)NO. There are two equilibrium states of Hbe having significantly different kinetics for the Hbe(III) Hbe(II) reaction of Hbe(III)NO. This autoreduction occurs in the form of two parallel processes, which collapse into one intermediate rate in the presence of Gri(2,3)P2. The temperature dependences of the rates enable deduction of H0 and S0 for the linked equilibrium, and yield liner Eyring plots for Hbe(III)NO, from which activation parameters were estimated on the basis of a previously described mechanism.

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