| Abstract
| - The synthetic heme−thiolate complex (SR) in methanol binds nitric oxide (kon = (2.7 ± 0.2) ×106 M-1 s-1 at 25 °C) to form SR(NO). The binding of NO to the SR complex in a noncoordinating solvent,such as toluene, was found to be almost 3 orders of magnitude faster than that in methanol. The activationparameters ΔH⧧, ΔS⧧, and ΔV⧧ for the formation of SR(NO) in methanol are consistent with the operationof a limiting dissociative mechanism, dominated by dissociation of methanol in SR(MeOH). In the presenceof an excess of NO, the formation of SR(NO) is followed by subsequent slower reactions. The substantiallynegative activation entropy and activation volume values found for the second observed reaction step supportan associative mechanism which involves attack of a second NO molecule on the thiolate ligand in theinitially formed SR(NO) complex. The following slower reactions are strongly accelerated by a large excessof NO or by the presence of NO2- in the SR/NO reaction mixture. They can be accounted for in terms ofdynamic equilibria between higher nitrogen oxides (NOx) and reactive SR species, which lead to the formationof a nitrosyl−nitrite complex of SR(FeII) as the final product. This finding is clearly supported by laser flashphotolysis studies on the SR/NO reaction mixture, which do not reveal simple NO photolabilization fromSR(FeIII)(NO), but rather involve the generation of at least three photoinduced intermediates decaying withdifferent rate constants to the starting material. The species formed along the proposed reaction pathwayswere characterized by FTIR and EPR spectroscopy. The results are discussed in terms of their relevancefor the biological function of cytochrome P450 enzymes and in context of results for the reaction of NOwith imidazole- and thiolate-ligated iron(III) hemoproteins.
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