Smith, James N. ; Dasgupta, Tara P
Author Affiliation, Ana.
Department of Chemistry
Kinetics and mechanism of the decomposition of S-nitrosoglutathione by l-ascorbic acid and copper ions in aqueous solution to produce nitric oxide
Date of Publication
S-Nitrosothiols serve as a good source of nitric oxide (·NO) mainly due to the ease of cleavage of the S-N bond which consequently produces ·NO. The reductive decomposition of S-nitrosoglutathione (GSNO) by L-ascorbic acid (vitamin C) yields ·NO which was monitored both electrochemically (using NO-probe) and spectrophotometrically. The rate of reaction and ·NO release was found to be pH dependent in a manner which drastically increases with pH demonstrating that the L-ascorbic acid dianion (A2-) is by far the most reactive species of L-ascorbic acid (H2A). The derived rate expression (measuring the disappearance of the absorption at ca. 336 nm due to GSNO) was established as rate = –d[GSNO]t/dt = ((ka[H+]2 + kb[H+]K1 + kcK1K2)/([H+]2 + K1[H+] + K1K2))[GSNO]t[H2A]t. ka, kb, and kc are second-order rate constants via the H2A, HA-, and A2- pathways, respectively, while K1 and K2 represent the first and second equilibrium dissociation constants of L-ascorbic acid. There is little or no reaction at low pH (below 5.5), where H2A is a predominant species, and as a result the rate constant (ka) via this route was found to be negligible. At 25 ºC, kb = 5.23 ± 1.47 x 10-3 dm3 mol-1 s-1 and kc = 1.22 ± 0.04 x 103 dm3 mol-1 s-1, activation parameters DH‡b = 54.4 ± 4.3 kJ mol-1, DS‡b = -106 ± 16 J K-1 mol-1, DH‡c = 80.5 ± 7.5 kJ mol-1, DS‡c = 84 ± 7 kJ mol-1. The experimental rate and activation parameters suggest that this redox process follows an outer-sphere electron transfer mechanism. GSNO is relatively stable in the dark, aqueous medium and even in the presence of trace quantities of Cu2+. Induced catalytic decomposition of GSNO only becomes significant above ca. 10 µM Cu2+, but after this it shows linear dependency. To nullify any catalysis by Cu2+ or any other transition metal ions, EDTA was added to all experimental reactions except those where catalysis by Cu2+ was studied.....