Discrimination of Nitroxyl and Nitric Oxide by Water-Soluble Mn(III) Porphyrins

The water-soluble manganese(III) meso-tetrakis (N-ethylpyridinium-2-yl) porphyrin (MnIIITEPyP) and manganese(III) meso-(tetrakis(4-sulfonato-phenyl)) porphyrinate (MnIIITPPS) are able to chemically distinguish between HNO and NO donors, reacting with the former in a fast, efficient, and selective manner with concomitant formation of the {MnNO}7 complex (k on(HNO) ≈ 105 M-1 s-1), while they are inert or react very slowly with NO donors. DFT calculations and kinetic data suggest that HNO trapping is operative at least in the case of MnIIITPPS, while catalytic decomposition of the HNO donors (sodium trioxodinitrate and toluene sulfohydroxamic acid) seems to be the main pathway for MnIIITEPyP. In the presence of oxygen, the product MnIITEPyP(NO) oxidizes back to MnIIITEPyP, making it possible to process large ratios of nitroxyl donor with small amounts of porphyrin.