Bright acid copper plating process: EPI, Electrochemical Products Inc. New Berlin, Wis.

Abstract

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°C, kb = 5.23 ± 1.47 × 10−3 dm3 mol−1 s−1 and kc = 1.22 ± 0.04 × 103 dm3 mol−1 s−1, activation parameters ΔH‡b = 54.4 ± 4.3 kJ mol−1, ΔS‡b = −106 ± 16 J K−1 mol−1, ΔH‡c = 80.5 ± 7.5 kJ mol−1, ΔS‡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.