A Surface Effect Allows HNO/NO Discrimination by a Cobalt Porphyrin Bound to Gold
journal contributionposted on 02.08.2010, 00:00 by Sebastián A. Suárez, Mariano H. Fonticelli, Aldo A. Rubert, Ezequiel de la Llave, Damián Scherlis, Roberto C. Salvarezza, Marcelo A. Martí, Fabio Doctorovich
Nitroxyl (HNO) is a small short-lived molecule for which it has been suggested that it could be produced, under certain cofactors conditions, by nitric oxide (NO) synthases. Biologically relevant targets of HNO are heme proteins, thiols, molecular oxygen, NO, and HNO itself. Given the overlap of the targets and reactivity between NO and HNO, it is very difficult to discriminate their physiopathological role conclusively, and accurate discrimination between them still remains critical for interpretation of the ongoing research in this field. The high reactivity and stability of cobalt(II) porphyrins toward NO and the easy and efficient way of covalently joining porphyrins to electrodes through S−Au bonds prompted us to test cobalt(II) 5,10,15,20-tetrakis[3-(p-acetylthiopropoxy)phenyl]porphyrin [Co(P)], as a possible candidate for the electrochemical discrimination of both species. For this purpose, first, we studied the reaction between NO, NO donors, and commonly used HNO donors, with CoII(P) and CoIII(P). Second, we covalently attached CoII(P) to gold electrodes and characterized its redox and structural properties by electrochemical techniques as well as scanning tunneling microscopy, X-ray photoelectron spectroscopy, and solid-state density functional theory calculations. Finally, we studied electrochemically the NO and HNO donor reactions with the electrode-bound Co(P). Our results show that Co(P) is positioned over the gold surface in a lying-down configuration, and a surface effect is observed that decreases the CoIII(P) (but not CoIII(P)NO−) redox potential by 0.4 V. Using this information and when the potential is fixed to values that oxidize CoIII(P)NO− (0.8 V vs SCE), HNO can be detected by amperometric techniques. Under these conditions, Co(P) is able to discriminate between HNO and NO donors, reacting with the former in a fast, efficient, and selective manner with concomitant formation of the CoIII(P)NO− complex, while it is inert or reacts very slowly with NO donors.
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gold surfaceamperometric techniquesHNO donorsheme proteinstheory calculationsresults showCoIIIelectrochemical discriminationSCECobalt Porphyrin BoundHNO donor reactions0.4 Vcofactors conditionssurface effectscanning tunneling microscopyphysiopathological rolegold electrodeselectrochemical techniquesnitric oxide