Dissociation nitric oxide

Back and Mui17 studied the reaction with isotopically labeled nitric oxide to test the proposal that active nitrogen contained vibra-tionally excited molecules capable of dissociating nitric oxide. This reaction was proposed by Winkler and his co-workers133,435... 

The brown nitrogen dioxide gas condenses to a yellow liquid which freezes to colourless crystals of dinifrogen tetroxide. Below 150°C the gas consists of molecules of dinifrogen tetroxide and nitrogen dioxide in equilibrium and the proportion of dinifrogen tetroxide increases as the temperature falls. Above 150°C nitrogen dioxide dissociates into nitric oxide and oxygen. 

Nitric oxide is dissociatively chemisorbed at Ru(0001) at 295 K, with Zambelli et al.n establishing the role of a surface step in the dynamics of the dissociation process. Figure 8.3 shows an STM image taken 30min after exposure of the ruthenium surface to nitric oxide at 315 K. There is clearly a preponderance of dark features concentrated around the atomic step (black strip), which are disordered nitrogen adatoms, while the islands of black dots further away... 

Of crucial significance in deciding between various models have been estimates of the number of copper atoms required to transform the surface into a (2 x 3)N phase. This was the approach adopted by Takehiro et al 2 in their study of NO dissociation at Cu(110). They concluded that by determining the stoichiometry of the (2 x 3)N phase that there is good evidence for a pseudo-(100) model, where a Cu(ll0) row penetrates into the surface layer per three [ll0]Cu surface rows. It is the formation of the five-coordinated N atoms that drives the reconstruction. The authors are of the view that their observations are inconsistent with the added-row model. The structure of the (2 x 3)N phase produced by implantation of nitrogen atoms appears to be identical with that formed by the dissociative chemisorption of nitric oxide. 

The rate of the dissociation has been measured by the uptake of oxygen or nitric oxide.12 13 In the case of triphenylmethyl the dis-... 

The activation parameters AS" and for formation and dissociation of the nitric oxide adduct of cobalamin... 

Ru(edta)(H20)] reacts very rapidly with nitric oxide (171). Reaction is much more rapid at pH 5 than at low and high pHs. The pH/rate profile for this reaction is very similar to those established earlier for reaction of this ruthenium(III) complex with azide and with dimethylthiourea. Such behavior may be interpreted in terms of the protonation equilibria between [Ru(edtaH)(H20)], [Ru(edta)(H20)], and [Ru(edta)(OH)]2- the [Ru(edta)(H20)] species is always the most reactive. The apparent relative slowness of the reaction of [Ru(edta)(H20)] with nitric oxide in acetate buffer is attributable to rapid formation of less reactive [Ru(edta)(OAc)] [Ru(edta)(H20)] also reacts relatively slowly with nitrite. Laser flash photolysis studies of [Ru(edta)(NO)]-show a complicated kinetic pattern, from which it is possible to extract activation parameters both for dissociation of this complex and for its formation from [Ru(edta)(H20)] . Values of AS = —76 J K-1 mol-1 and A V = —12.8 cm3 mol-1 for the latter are compatible with AS values between —76 and —107 J K-1mol-1 and AV values between —7 and —12 cm3 mol-1 for other complex-formation reactions of [Ru(edta) (H20)]- (168) and with an associative mechanism. In contrast, activation parameters for dissociation of [Ru(edta)(NO)] (AS = —4JK-1mol-1 A V = +10 cm3 mol-1) suggest a dissociative interchange mechanism (172). 

Fig. 3. Schematic representation of the solvent reorganization that may occur upon the dissociation of nitric oxide from Fem(Por) in aqueous solution.

N-diazeniumdiolates spontaneously dissociate at physiological pH to release nitric oxide (NO) by stable first order kinetics with half-lives ranging from 2 s to 20 h [209, 210]. They are blessed with many attributes that make them an especially attractive starting point for designing solutions to important clinical problems, namely they are stable as solids, have structural diversity, a controlled rate of release of NO on hydrolysis, and a rich derivatization chemistry that facilitates targeting of NO to specific sites of need, a critical goal for therapeutic uses of a molecule with natural bioeffector roles in virtually every organ [208]. 

This model was fitted to the data of all three temperature levels, 375, 400, and 425°C, simultaneously using nonlinear least squares. The parameters were required to be exponentially dependent upon temperature. Part of the results of this analysis (K6) are reported in Fig. 6. Note the positive temperature coefficient of this nitric oxide adsorption constant, indicating an endothermic adsorption. Such behavior appears physically unrealistic if NO is not dissociated and if the confidence interval on this slope is relatively small. Ayen and Peters rejected this model also. 

D. S. Bohle, C. H. Hung, Ligand-Promoted Rapid Nitric Oxide Dissociation from Ferrous Porphyrin Nitrosyls , J. Am. Chem. Soc. 1995,117, 9584-9585. 

Fabisiak, J.P., Tyuiin, V.A., Tyurina, Y.Y., Sedlov, A., Lazo, J.S., and Kagan, V.E., 2000, Nitric oxide dissociates lipid oxidation from apoptosis and phosphatidylserine externalization during oxidative stress. Biochemistry 39 127-138. 

Clearly the molecular events with iron were complex even at 80 K and low NO pressure, and in order to unravel details we chose to study NO adsorption on copper (42), a metal known to be considerably less reactive in chemisorption than iron. It was anticipated, by analogy with carbon monoxide, that nitric oxide would be molecularly adsorbed on copper at 80 K. This, however, was shown to be incorrect (43), and by contrast it was established that the molecule not only dissociated at 80 K, but NjO was generated catalytically within the adlayer. On warming the adlayer formed at 80 K to 295 K, the surface consisted entirely of chemisorbed oxygen with no evidence for nitrogen adatoms. It was the absence of nitrogen adatoms [with their characteristic N(ls) value] at both 80 and 295 K that misled us (43) initially to suggest that adsorption was entirely molecular at 80 K. 

Irradiation of an aqueous solution at 296 nm and pH values from 8 to 13 yielded different products. Photolysis at a pH nearly equal to the dissociation constant (undissociated form) yielded pyrocatechol. At an elevated pH, 2-chlorophenol is almost completely ionized photolysis yielded cyclopentadienic acid (Boule et al., 1982). Irradiation of an aqueous solution at 296 nm containing hydrogen peroxide converted 2-chlorophenol to catechol and 2-chlorohydroquinone (Moza et al, 1988). In the dark, nitric oxide (10 vol %) reacted with 2-chlorophenol forming 4-nitro-2-chlorophenol and 6-nitro-2-chlorophenol at yields of 36 and 30%, respectively (Kanno and Nojima, 1979). 

However, the latter residue is in no sense equivalent to the Tyr 25 of the P. pantotrophus enzyme. The Tyr 10, which is not an essential residue (19), is provided by the other subunit to that in which it is positioned close to the di heme iron (Fig. 6). In other words, there is a crossing over of the domains. A reduced state structure of the P. aeruginosa enzyme has only been obtained with nitric oxide bound to the d heme iron (20) (Fig. 6). As expected, the heme c domain is unaltered by the reduction, but the Tyr 10 has moved away from the heme d iron, and clearly the hydroxide ligand to the d heme has dissociated so as to allow the binding of the nitric oxide (Fig. 6). This form of the enzyme was prepared by first reducing with ascorbate and then adding nitrite. 

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