O exchange

In the process of O-exchange the nitronium ion mechanism requires that the rate of nitronium ion formation be the rate at which the label... 

TABLE 2.2 Zeroth-order rates of nitration and of O-exchange in aqueous nitric acid at o°C... 

Aromatic thioamides can be prepared as described in the literature by different ways, either by S -> O exchange between the corresponding benzamides and phosphorus pentasulfide in pyridine solution in the presence of triethylamine (65, 646) as strong base, or by action of H2S on the appropriate nitrile with pyridine and triethylamine solvents using the method of Fairfull et al. (34, 374, 503). In this reaction, thioacetamide in acidic medium can also be used as a H2S generator with dimethylform-amide as the solvent (485). 

However, an elegant O nmr study using specifically labelled [CoINHslsl ONO)] " and (Co(NH3)5(ON 0)] established that spontaneous intramolecular O-to-O exchange in the nitrite ligand occurs at a rate comparable to that of the spontaneous O-to-N isomeri /ation/ ° ... 

CO oxidation involving 5) O exchange suface oxygen PdO -CVPd ... 

Fig. 8-16. Electron state density in a semiconductor electrode and in hjrdrated redox partides, rate constant of electron tunneling, and exchange redox current in equilibrium with a redox electron transfer reaction for which the Fermi level is close to the conduction band edge eF(sc) = Fermi level of intrinsic semiconductor at the flat band potential 1. 0 (tp.o) = exchange reaction current of electrons (holes) (hvp)) - tunneling rate constant of electrons (holes).

Armed with the background study of Santer (1959) and the enzymatic evidence for the APS pathway. Peck and Stulberg (1962) undertook to demonstrate the O exchange reactions in cell-free extracts of T. thioparus... 

The tight 7t-bonded intermediate is consistent with a negative volume of activation. It also explains the (unanticipated) O-to-O exchange in the nitrito ligand at a rate comparable to the O-to-N isomerization (Sec. 2.2.2). ATj/Atj = 1.2 ... 

All of this evidence supports the existence of tetrahedral intermediates in a-chymotrypsin-catalysed reactions, but it should be noted that O-exchange with water is not observed in deacylation of cinnamoyl- 0-chymotrypsin, in contrast with the hydrolysis of O-cinnamoyl-N-acetylserinamide where such exchange is detected (Bender and Heck, 1967). Lack of exchange in the enzyme reaction could reflect interactions of the tetrahedral intermediate with the protein. 

O-exchange studies of Ye et al. (1991) support, we believe, the catalysis by nitrite reductase of redox reversibility between nitrite and NO as depicted in the first line of Eq. (3). They observed by analyzing the 0 content of product N2O that all eight strains of denitrifying bacteria studied could catalyze the exchange of 0 between water and nitrite or NO by way of an electrophilic (nitrosyl donor) species of NO. The rates and extent of these exchange reactions depended on whether the bacterium made use of a heme- or Cu-type nitrite reductase. Contrary to the conclusions of Ye et al. (1991), we do not believe that this study otherwise informs about the pathway of denitrification or whether NO is an intermediate. 

Oxygen atom transfer fnechanism of nitrite dehydrogenase from Nitrobacter agilis. The filled-in oxygen atom tepresents O. The lower pathway provides for O exchange with water, a process that competes with O atom transfer from nitrate to nitrite. M represents a molybdenum atom. From Friedman et... 

Shearer, G., and Kohl, D. H. (1988). Nitrogen isotope fractionation and "O exchange in relation to the mechanism of denitrification of nitrite by Pseudomonas stutzeri. J. Biol. Chem. 263, 13231-13245. 

If the rate-determining step of the reaction involves only the protonated ester and a water molecule or molecules, the dependence of the rate of hydrolysis of the protonated ester on the activity of water may be obtained by plotting log 0hs/[BH+] versus log aH2o- Lane s plot is reproduced as Fig. 9. The slope should give the order in water, and is close to 2 for hydrolysis and for O-exchange also. 

THERMODYNAMIC PARAMETERS FOR THE ACID-CATALYZED "O-EXCHANGE OF CARBOXYLIC ACIDS IN WATER... 

KINETIC DATA FOR THE "O EXCHANGE REACTION OF CARBOXYLIC ACIDS IN ENRICHED WATER AT 25°C. IONIC STRENGTH 4.0... 

KINETIC DATA FOR THE HYDROXIDE-CATALYZED "O EXCHANGE REACTION OF ESTERS WITH ENRICHED WATER... 

The S/O exchange represented by the direct transformation of a thiocarbonyl group into a carbonyl group has received scarce attention in the scientific literature, although some hydrolytic methods have been reported432, such as the use of PT catalysts (equation 150) introduced by Alper and coworkers496. This method has broad scope and yields are usually high. 

It is interesting to point out that N-H lactams which are secondary amides react differently (no O-exchange) from acyclic secondary amides. This difference in behavior can however be readily explained because acyclic secondary amides exist in a different conformation 1) from N-H lactams which are locked in the conformation. Indeed, oxygen exchange can occur without conformational change, i, e. via appropriate oxygen and nitrogen... 

H O exchanging with Na. ESCA has also been shown to be an excellent means for investigating coatings on glasses (21). 

Figure 3 2D O-D- O deuteron exchange NMR spectra of DRADA-32 at T= 40 K for fmix = 10 and 30 s. In addition to the diagonal peaks, the O-D- 0<->0- D-O exchange cross-peaks are clearly visible.

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