Nitric oxide reactions, types

Photochemical elimination reactions include all those photoinduced reactions resulting in the loss of one or more fragments from the excited molecule. Loss of carbon monoxide from type I or a-cleavage of carbonyl compounds has been previously considered in Chapter 3. Other types of photoeliminations, to be discussed here, include loss of molecular nitrogen from azo, diazo, and azido compounds, loss of nitric oxide from organic nitrites, and loss of sulfur dioxide and other miscellaneous species. 

Diazotization of 3-(4-aminophenyl)sydnones followed by reaction with 1- or 2-hydroxynaphthalene provides azo dyestuff materials <1998MI209>. A new type of reaction between 4-acetyl-3-arylsydnones and hydrazine yields substituted pyrrolidinones by a cycloaddition process involving loss of nitric oxide (Equation 20) <1999H(51)95, 2001AHC73>. 

The NO/NO+ and NO/NO- self-exchange rates are quite slow (42). Therefore, the kinetics of nitric oxide electron transfer reactions are strongly affected by transition metal complexes, particularly by those that are labile and redox active which can serve to promote these reactions. Although iron is the most important metal target for nitric oxide in mammalian biology, other metal centers might also react with NO. For example, both cobalt (in the form of cobalamin) (43,44) and copper (in the form of different types of copper proteins) (45) have been identified as potential NO targets. In addition, a substantial fraction of the bacterial nitrite reductases (which catalyze reduction of NO2 to NO) are copper enzymes (46). The interactions of NO with such metal centers continue to be rich for further exploration. 

Nitric oxide formation from hydroxyurea requires a three-electron oxidation (Scheme 7.15) [114]. Treatment of hydroxyurea with a variety of chemical oxidants produces NO or NO-related species , including nitroxyl (HNO), and these reactions have recently been extensively reviewed [114]. Many of these reactions proceed either through the nitroxide radical (25) or a C-nitroso intermediate (26, Scheme 7.15) [114]. The remainder of the hydroxyurea molecule may decompose into formamide or carbon dioxide and ammonia, depending on the conditions and type of oxidant (one-electron vs. two electron) employed. 

The homogeneous reaction was found to be catalysed by small percentages of nitric oxide and acetylene dichloride without any detectable change in the overall stoichiometry. This observation suggests the occurrence of additional initiation processes of the type... 

Most free radicals contain odd numbers of electrons and most stable molecules contain even numbers of electrons (nitric oxide and nitrogen dioxide are two important exceptions being stable molecules with odd numbers of electrons). Therefore in the reaction, free radical + stable molecule % another free radical is usually generated. This free-radical diain process is stopped only when one of the following types of processes occurs ... 

The above examples point out at the direct stimulation of apoptosis by nitric oxide. At the same time, the exclusively rapid reaction of NO with superoxide always suggests the possibility of peroxynitrite participation in this process [141] correspondingly, the role peroxynitrite in the stimulation of apoptosis has been considered. Bonfoco et al. [144] has found that the producers of low peroxynitrite concentrations during the exposure of cortical neurons to the low level of NMDA or the use of peroxynitrite donors resulted in an apoptosis in neurons, while the high concentrations of peroxynitrite induced necrotic cell damage. The formation of peroxynitrite is apparently responsible for NO-stimulated apoptosis in superoxide-generating transformed fibroblasts because nontransformed cells, which do not produce superoxide, were not affected by nitric oxide [145]. It is of interest that proapoptotic effect of peroxynitrite may depend on the cell type. Thus, the formation of peroxynitrite enhanced the NO-induced apoptosis in glomerular endothelial cells, while superoxide inhibited the formation of ceramide and apoptosis in these cells exposed to nitric oxide probably due to peroxynitrite formation... 

In addition to the three main types of reaction involved in nitrocellulose decomposition, the author assumes that secondary reactions also occur originating in the chemical combination between oxides of nitrogen and water vapour to produce nitric and nitrous acids, which in turn react with the nitrocellulose. By heating nitrocotton with dilute nitric acid at 40°C, Desmaroux found in this instance that hydrolytic and oxidation reactions predominate, causing a weight loss of +-J of the total loss (see Table 67). 

Because of its fundamental characteristics, nitric oxide is an important species in three types of atmospheric concerns ballistic-missile reentry, polluted air, and the upper atmosphere. Understanding its reactions, therefore, is of consequence in the development of an effective military defense, the solution of urban air-pollution problems, and the exploration of space. The concern of Government, as well as private industry, in these areas is obvious and has led to the appropriation of considerable funds for studies of nitric oxide and its related species. Consequently, a large number of publications has appeared in recent years (more than half our references are 1960 or later). 

Iron-centered paramagnetic complexes formed by reactions between iron salts and nitric oxide in the presence of anionic ligands, and characterized by g = 2.03, were first reported over 20 years ago (22) similar complexes, of the general type [Fe(NO)2X2] +, have subsequently been produced by reactions of iron salts and nitric oxide in the presence of halides and pseudohalides (118), alcohols and alkoxides (119), mercaptides (120, 121), and mercaptopurines and mercaptopy-rimidines (122). 

The reactions of nitric acid are of three types (a) acid-base reactions which are typical of a strong acid (b) oxidation reactions, such as those with metals and organic materials, the latter often involving carbonization (c) substitution reactions such as the replacement of —H by —NO2 in aromatic hydrocarbons, to form nitro compounds, or of hydroxyl hydrogen by —N02 to produce esters of HNO3. 

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