A Protective Protein Matrix Improves The Discrimination Of Nitroxyl From Nitric Oxide By Mn-Iii Protoporphyrinate Ix In Aerobic Media

The selectivity of Mn-II/III porphyrinates toward nitroxyl or nitric oxide donors provides a convenient starting point for the development of new materials for the speciation of these nitrogen-containing redox relatives. In the present report, we describe the insertion of Mn-III protoporphyrinate IX in apomyoglobin and its chemical behavior toward HNO or NO donors, either under anaerobic or aerobic conditions. For comparison and discussion, the Mn-III porphyrinate, devoid of the protein matrix, was studied in parallel. The Mn-III reconstituted globin successfully reacted with the nitroxyl donor trioxodinitrate, while it was unreactive toward NO or NO donors, in good agreement with previously reported data on water soluble Mn-III porphyrinates. The estimated association rate constant for the reaction with the nitroxyl donor was of the same order of magnitude for the reconstituted globin and the free porphyrinate, suggesting that the protein environment is not involved in the reaction mechanism. In contrast, the reaction product exhibited enhanced stability in the presence of dioxygen only when the porphyrinate was included in the protein matrix: this feature is ascribed to the role of the distal residues on the metal centered reactivity. This behavior is required for spectroscopic detection under biologically relevant conditions. (C) 2011 Elsevier Inc. All rights reserved.