CATALYSIS BY CU2-OXIDE RELEASE FROM S-NITROSOTHIOLS (RSNO)( OF NITRIC)

The decomposition of a range of S-nitrosothiols (thionitrites) RSNO, b ased on cysteine derivatives, yields in water at pH 7.4 nitrite ion qu antitatively. If oxygen is rigorously excluded then no nitrite ion is formed and nitric oxide can be detected using an NO-probe. The reactio n is catalysed by trace quantities of Cu2+ (there is often enough pres ent in distilled water samples) and also to a lesser extent by Fe2+, b ut not by Zn2+, Cu2+, Mg2+, Ni2+, Co2+, Mn2+, Cr3+ Or Fe3+. The rate e quation (measuring the disappearance of the absorption at ca. 350 nm d ue to RSNO) was established as v = k[RSNO].[Cu2+] + k' over a range of [Cu2+] typically 5-50 mu mol dm(-3). The constant term k' represents the component of the rate due to residual Cu2+ the solvent and buffer components, together with the spontaneous thermal reaction. Decomposit ion can be virtually halted by the addition of EDTA. Reactions carried out in the presence of N-methylaniline gave a quantitative yield of N -methyl-N-nitrosoaniline, but a negligible yield when oxygen was rigor ously excluded. Values of the second-order rate constant k were obtain ed for a range of S-nitrosothiols. Reactivity is highest for the S-nit rosothiols derived from cysteamine and penicillamine, when Cu2+ can be complexed both with the nitrogen atom of the nitroso group and the ni trogen atom of the amino group, via a six-membered ring intermediate. If there is no amino (or other electron donating group) present, react ion is very slow (as for RSNO derived from tert-butyl sulfide). N-Acet ylation of the amino group reduces the reactivity drastically as does the introduction of another CH2 group in the chain. There is evidence of a significant gem-dimethyl effect. Kinetic results using the S-nitr osothiols derived from mercaptoacetic, thiolactic and thiomalic acids suggests that coordination can also occur via one of the oxygen atoms of the carboxylate group. EPR experiments which examined the Cu2+ Sign al showed no spectral change during the reaction suggesting that the m echanism does not involve oxidation and reduction with Cu(2+)reversibl e arrow Cu+ interconversion.