Redox Mechanism of NO in Water-Soluble Iron Porphyrin

Abstract

Nitric oxide that plays a role in controlling blood pressure showed an anodic voltammetric wave catalyzed by meso-tetra(N-methyl-4- pyridyl) iron(III) pentachloride ((Fe III (TMPyP)) 5a ) in a phosphate buffer solution (pH 7.4). The current, 10 times larger than the diffusion- controlled current of NO without the iron porphyrin, can be utilized for quantitative determination of NO in aerobic physiological environments. It did not include any oxidation current of nitrite ion due to blocking by the adsorption of the complex. Spectro- electrochemical measurements suggested a formation of iron-nitrosyl complex, which was responsible for the catalytic oxidation of NO. Intermediates of the catalytic oxidation-reduction with (Fe III (TMPyP)) 5a were detected by spectroelectrochemical techniques. They are composed of an oxidation cycle and a reduction cycle, during which NOis generated. Since nitric oxide (NO) was reported to be an endothelium- derived relaxing factor for controlling blood pressure in 1987 (1), the research has been explosively developed. It has been recog- nized that NO generated with an enzyme in living tissues plays important roles in contracting and relaxing blood vessels (2-3), neurotransmission (4) and antiplatelet aggregation. In order to get biological functions of NO in perspective, detection techniques for NO have been developed in biological systems (5-13). They have to solve problems of a short life of NO, low concentrations and an increase in spatial resolution. A possible technique of satisfying these difficulties is an amperometric monitoring (14-19) at a microelectrode. Porphyrin-coated electrodes show a promise of high sensitivity and real-time measurements, when they are covered with Nafion membrane in order to avoid anionic contaminants. Unfortunately, there are such drawbacks in physiological applications as nonlinear relation between the current and concentration of NO, stability of the electrode to prolonged NO exposure, and effects of electrochemical contami- nants (18, 19). In order to solve these problems, it is necessary to know the reaction mechanisms of NO with porphyrins. We reported the electrooxidation of NO catalyzed by (Fe III (TMPyP)) 5+ (20) in which NO was coordinated with (Fe III (TMPyP)) 5+ to form an iron-nitrosyl complex. The previous electrochemical measurement shows that the current at 0.8 V was 10 times larger than the diffusion-controlled current of dissolved NO without the iron porphyrin. The catalytic effect, however, includes complications of slow processes in that it is associated with unexpected irreversible redox peaks. This article is planned to cover the previously reported catalytic mechanism (20) with the oxidation and reduction mechanism including slow processes. Newly found observations here are a block of the oxidation of nitrite in the presence of the complex and the reproduction of NO by catalytic reductions.