- Title
- Nitrosation of aminothiones and decomposition of S-nitroso species
- Creator
- Dorado, Joyeth B.
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2015
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Nitrosation reactions sparked interest in the biochemical field following the discovery of nitric oxide (NO) as a signalling molecule in many biological functions in the 1990s. This prompted a flurry of research in different areas of biochemistry including S-nitrosothiols as NO· donors in vivo as well as in N-nitrosamines due to carcinogenic properties. Diverse industrial functions of nitrosation chemistry are also found in azo-dye production and in chemical gassing of emulsion explosives. In the latter, a substrate and a nitrosating agent react to generate nitrogen (N₂) gas bubbles in situ providing sensitisation of the emulsion prior to detonation. The decomposition of nitrosated compounds to either NO· or N₂often poses challenges due to differing roles of products. Therefore, it is important to determine the kinetics and mechanisms of nitrosation in substrates as well as decomposition fates of nitroso species to N₂ and NOx to optimize processes that are relevant to specific applications. This work studied nitrosation reactions in a class of compounds that contain sulfur and an amine group, using a two pronged approach combining experimental measurements with theoretical calculations. These species are particularly interesting due to the inherent structural flexibility that allows intra- and inter-molecular transnitrosation. In these compounds, initial nitrosation occurs at the sulfur atom due to its greater nucleophilicity and the subsequent ON⁺ transfer to a deprotonated amine group results in rapid release of N₂. Previous studies proposed that nitrous acid initiated reactions with amino-thiones proceed via two parallel pH-dependent pathways. The first pathway leads to the formation of benign N₂consequent of the S- to N-transnitrosation in S-nitroso compounds and was earlier studied within the narrow range of pH 3.8−4.1 for thiourea. The second pathway forms disulfide and toxic NO· and is thought to occur at a lower pH but has not been studied in detail. Despite the long history of kinetic studies for S-nitrosation of aminothiols and aminothiones, there remain significant discrepancies between kinetic rate constants in reported literature. At high acidities where the nitrosating agent is either nitrous acidium and/or nitrosonium ion, rate constants are perceived to approach a diffusion limit of 7 000 M⁻¹∙s⁻¹ for neutral species. Revisiting analysis on the true nature of nitrosating agent reveals that the nitrosonium ion pathway is not adequate to explain the kinetic rate constants observed for neutral species. Moreover, the expected encounter limit for the nitrous acidium pathway is two orders in magnitude higher than the observed kinetic rate constants in literature. The early part of this work delved on kinetics of S-nitrosation of thioacetamide and thiourea. In acidic solutions, reactions proceed via nucleophilic attack on the electrophilic nitrous acidium or nitrous acidium by the electron dense sulfur sites. Using stopped-flow UV-vis spectrophotometry, the equilibrium constants were found to increase with temperature and enthalpies of -27.7 and -29.6 kJ for exothermic S-nitrosation of thioacetamide and thiourea, respectively. The theoretical calculations via a thermochemical cycle are in good agreement with reaction free energies from experiments and errors are as low as -2–4 kJ using the solvation method SMD in conjunction with hybrid meta exchange-correlation functional M05-2X and high-accuracy multi-step method CBS-QB3 for gas-phase calculations. Kinetic rates for both substrates increase with acidity and temperature and activation energies of 54.9 (TA) and 66.1 kJ∙mol⁻¹ (TU) at the same temperature range confirm activation-controlled reactions. Gas formation pathways in the decomposition of RSNO⁺ species are also elucidated. For the first time, quantified evidence for gas phase products obtained through gas chromatography and NOₓ chemiluminescence analyses are presented. Decomposition proceeds exclusively via NO· formation pathways at pH 1 and below. With decreasing acidity, molecular nitrogen formation becomes an equally important product of S-nitrosothiourea while NO· remains the only product for S-nitrosothioacetamide. Theoretical calculations for reaction enthalpies elucidate the gas formation pathways and proposed mechanisms fitted to experimental data afford kinetic rate constants. In both species, the S–N bonds split homolytically with activation energies of 70.9 and 118 kJ·mol⁻¹ for S-nitrosothioacetamide and S-nitrosothiourea, respectively. These activation energies for RSNO⁺ species are found lower than bond dissociation energies for neutral S-nitroso compounds, consistent with shorter S–N bond lengths in the latter. The methyl substitution in S-nitrosothioacetamide enables lower activation energies with the bimolecular reaction of RSNO⁺ and RS occurring within the diffusion controlled regime at an activation energy of 17.6 kJ·mol⁻¹. For S-nitrosothiourea, a further bimolecular reaction of two RSNO⁺ molecules occurs irreversibly with an activation energy of 84.4 kJ·mol⁻¹. This is expectedly higher than the 60.5 kJ·mol⁻¹ activation in the RS and RSNO⁺ reaction as it involves two cationic species and a concerted scission of two S-N bonds resulting in two NO· molecules and a stable CC’-dithiodiformamidinium ion. In the case of oxygen analogues urea and acetamide, gas formation pathways are different and driven by the initial nitrosation reaction despite the structural similarity and close pKₐ values expected at the oxygen and sulfur sites. In sulfur-containing species, nitrosation occurs readily at the sulfur site and subsequent decomposition proceeds exclusively via NO· formation pathways at pH 1 and below. With decreasing acidity, nitrogen formation becomes an equally important product of S-nitrosothiourea while NO· remains the only product for S-nitrosothioacetamide with S–N transnitrosation contributing to at least 50 % of total N₂yields at pH 2.0. In oxygen analogues, different gassing behaviour is observed whereby acetamide remains unreactive while direct N-nitrosation is clearly the preferred pathway in urea — leading to the formation of N₂, CO₂and NH₄₊ ions. By quantification of gas-phase products, definitive evidence for equimolar concentrations of CO₂and N₂were shown. There is a consistent excess of N₂, albeit less than 5 %, attributed to a parallel reaction from decomposition of urea. The presence of NH₃, observed by FTIR spectroscopy, confirms decomposition of urea which potentially exposes two NH₃molecules (from subsequent hydrolysis of cyanic acid) to nitrosation for every CO₂molecule formed. In all reactions, nitrous acid decomposition is seen to occur in parallel, contributing to significant NOₓ yields consisting of NO· and NO₂. Theoretical calculations for reaction enthalpies elucidate the gas formation pathways and support the experimental results favouring S-nitrosation in thioacetamide and thiourea due to d-orbital participation in chemical bonding. Finally, nitrosation reactions of alkyl nitrites are studied, particularly the gas formation pathways in the presence of 2,6-dichlorobenzene-1-carbothioamide (chlorthiamide), ethanedithioamide (dithiooxamide) and 2-propylpyridine-4-carbothioamide (prothionamide). Spectroscopic properties of these compounds and the precipitates formed from the reaction are presented. Combined UV-vis stopped flow spectrophotometry was not successful in identifying the S–N bond for S-nitroso adducts owing to the very high molar absorptivity of the substrates and the fast precipitate formation interfering with analysis. Using the well-known reaction of thiourea and cysteine with nitrous acid in acidic aqueous solutions, in situ Raman scans suggest that the detection limit of S–N bonds is higher than the solubility limit of substrates in 25% methanolic solutions (i.e., higher than 50 mM). Prothionamide was found to decompose to sulfur, a similar behaviour observed in the nitrosation of thioacetamide. Due to poor solubility of these compounds, reactions are studied in methanolic mixtures where the active nitrosating agent is likely methyl nitrite. With added substrates at pH of 1 and below, NO· formation was found as the dominant pathway of decomposition that is unaffected by pH and aminothione structure. The gassing trends resemble the gassing profile of methyl nitrite. Thus, if nitrosation occurs in these substrates, the subsequent decomposition contributes little to the total NO· yield and remains obscured by the fraction generated by homolytic cleavage of O–N bond in the alkyl nitrite.
- Subject
- nitrosation; S-nitrosation; S-nitroso species; NO formation; Alkyl nitrites; NO+ transfer
- Identifier
- http://hdl.handle.net/1959.13/1296469
- Identifier
- uon:19264
- Rights
- Copyright 2015 Joyeth B. Dorado
- Language
- eng
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