Origin of observed changes in 14N hyperfine interaction accompanying R → T transition in nitrosylhemoglobin

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Theoretical investigations of electronic distributions in eight different structural forms of nitrosylhemoglobin were carried out to study the changes in 14N hyperfine interaction observed with the transition from R to T structures under the influence of inositol hexaphosphate or changing pH. Four of the eight forms studied consisted of protonated and deprotonated Npros in the proximal imidazole ligand with linear and bent Fe—N—O structures. Two other forms had a straight Fe—N—O structure and Fe—Im bond stretched by 0.5 and 1.0 Å. The other two systems we have studied are five-liganded NO-heme with bent and straight Fe—N—O structures. Our investigations show that arrangements of energy levels did not differ significantly among all the structures, the unpaired electron always occupying an antibonding orbital with dz2 symmetry. The protonated and deprotonated systems with either linear or bent Fe—N—O structure showed substantial hyperfine interaction of the 14N nuclei of the NO group and the Nε atom of the proximal imidazole, indicating that a 9-line electron spin resonance hyperfine pattern (R structure) would be expected in all four cases. On the other hand, the extensions of the Fe—Im bond produce a sizeable decrease in the 14Nε hyperfine interaction, indicating that an extension beyond 1.0 Å would provide a 3-line hyperfine pattern close to that found for the five-liganded NO-heme system. Our results thus provide quantitative support for the model of severe extension or cleavage of the Fe—Nε bond proposed in the literature for explaining the R-to-T transition of the α-chain of nitrosylhemoglobin.

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