Stabilization and detection of nitroxyl by iron and cobalt porphyrins in solution and on surfaces

Abstract

Nitroxyl (HNO/NO-) is a small short-lived molecule that has been suggested to be produced by nitric oxide (NO) synthases under certain conditions. As for NO , biologically relevant targets of HNO are mainly heme-proteins and therefore, it has been difficult to discriminate the physio-pathological role of each molecule conclusively. Therefore, accurate discrimination between them is still an unresolved matter. On the other hand, there is only scarce information about nitroxyl-metalloporphyrin complexes. Hence, there is growing interest in obtaining and characterizing stable heme model nitroxyl complexes. In this review we show how HNO and NO can be discriminated electrochemically by a Co porphyrin attached to a gold surface, and how nitroxyl can be stabilized by coordination to an electron-poor Fe porphyrin. The Co porphyrin with four anchors, cobalt(II)-5,10,15,20-tetrakis[3-(p-acetylthio-propoxy)phenyl]porphyrin [Co(P)] was covalently attached to gold electrodes, and its reactions with NO and HNO donors were studied electrochemically. By fixing the potential to values that oxidize CoIII(P)NO-, HNO can be selectively detected by amperometric techniques. On the other hand, the one-electron chemical reduction of FeII(TFPPBr8)NO (TFPPBr8= 2 ,3,7,8,12,13,17,18-octa-β-bromo-5,10,15,20-[tetrakis-(pentafluorophenyl)]porphyrin) with cobaltocene yields the significantly stable {FeNO}8nitroxyl anion complex, [Co(C5H5)2]+[Fe(TFPPBr8)NO]-, which was isolated and characterized by several spectroscopies and DFT calculations. This species is intermediate between FeIINO-and FeINO , which is contrasted with the predominant FeIINO-character of known non-heme {FeNO}8complexes.