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Publication Type
Journal Article
UWI Author(s)
Author, Analytic
Smith, James N. ; Dasgupta, Tara P
Author Affiliation, Ana.
Department of Chemistry
Article Title
Kinetics and mechanism of the decomposition of S-nitrosoglutathione by l-ascorbic acid and copper ions in aqueous solution to produce nitric oxide
Medium Designator
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Connective Phrase
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Journal Title
Nitric oxide
Translated Title
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Reprint Status
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Date of Publication
2000
Volume ID
4
Issue ID
1
Page(s)
57-66
Language
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Connective Phrase
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Location/URL
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ISSN
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Notes
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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 x 10-3 dm3 mol-1 s-1 and kc = 1.22 0.04 x 103 dm3 mol-1 s-1, activation parameters DHb = 54.4 4.3 kJ mol-1, DSb = -106 16 J K-1 mol-1, DHc = 80.5 7.5 kJ mol-1, DSc = 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 M 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.....
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