Position of bond cleavage

In a reaction where bond formation is to the same element as in bond cleaving simple product analysis does not usually yield the position of fission. The alkaline hydrolysis of 2, 4 -dinitrophenyl 3,5-dimethyl-4-hydroxybenzenesulphonate(Eqn. 56) can involve S-O or Ar-O fission and the problem is solved by labelling the solvent 

Enrichment of 2,4-dinitrophenol from hydrolysis of 2, 4 -dinitrophenyl-3,5-dimelhyl-4-hydroxybenzene-sulphonate [38] 

Water enrichment 0% incorpn. Observed enrichment 100% incorpn.  

Oxygen-18 incorporation may be used to eliminate more sophisticated problems than the bond cleavage one. The hydrolysis of iV-sulphonylanthranilate (Eqn. 57) 

Enrichment of anthranilic acid from the hydrolysis of A -sulphonylanthranilic acid [39] 

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How the position of bond cleavage can be determined is illustrated by consideration of thermally initiated ligand (L) substitution in Re2(CO),(,. This can occur by a dissociative mechanism involving Re — CO bond breakage,... 

The results of that experiment allow one to synthesize a-D-ribose-l-[l80U]-phosphate which can be employed to determine the position of bond cleavage by other enzymes whose role is transfer of phosphate (Pj) to water or to another acceptor. We report results on a. the position of bond cleavage in R-l-P by PNP from human erythrocytes and E. coli as well as by alkaline phosphatase, acid phosphatase, formic acid andb. the position of bond making in ribose-5-phosphate by phosphoglucomutase. The earlier experiment from this laboratory employed the equilibration ... 

Position of Bond Cleavage PNP (EC 2.U.2.1) from human erythrocytes (homogeneous, purified by formycin B affinity chromatography) as well as from E, coli were allowed to equilibrate a mixture of R-l-[1 0lj]-P, pl Oij, hypoxanthine and inosine at pH 7-00 in 10 mm NMR tubes. The chemical shift differences of the 31p nuclei of the two R-l-P s (13 9 Hz for the human erythrocytic and 13.1 Hz for the E. coli enzyme) as well as of the two P3 resonances (13 9 Hz for erythrocytic and 13-7 Hz for E. coli source) clearly indicated C-0 bond cleavage by these enzymes as well. In addition, no evidence was found over the time course of the NMR measurements (l hr) for purine nucleoside phosphorylase catalyzed exchange of pl o + H2O (solvent) J randomized P. Therefore,... 

Position of Bond Cleavage or Formation in Some Other Phosphate Transfers... 

For a discussion of the mechanism of acetal hydrolysis, experimental information about the position of bond cleavage is needed. According to the results of oxygen-18 labeling experiments, bond cleavage in the formation and hydrolysis of n-butyl benzal takes place between oxygen and aldehyde-carbon [170, 171], viz. 

For example, a number of studies have been made on the metal ion-catalyzed hydrolyses of acetyl phosphate and acetyl phenyl phosphate, " but the role of the metal ion in these processes remains uncertain. Investigations of catalysis by exchange labile metal ions e.g. Ca" and Mg") have yielded conflicting results and both the nature and distribution of the kinetically significant species, as well as the positions of bond cleavage, have yet to be determined unequivocally. Chelation, charge neutralization and attack by metal-bound hydroxide have variously been proposed as important factors in acyl phosphate hydrolysis. 

Mega, T.L. Van Etten, R.L. The 0-18 isotope shift in C-13 nuclear magnetic-resonance spectroscopy. 12. Position of bond cleavage in the acid-catalyzed hydrolysis of sucrose. J. Amer. Chem. Soc. 1988, 110, 6372-6376. 

There has also been a recent report of radical displacement of an ordinary carboxylate ester ( ). Numerous examples of such reactions involving percarbo-xylate functions have already been mentioned (cf. Fig. 1, eqs. 1, 9-12) but in the cases where the position of bond cleavage has been determined all have involved attack on peroxidic oxygen. The example reported for an ester would have to involve cleavage of the 0-C bond (49). 

It has been suggested317 that the position of ring cleavage in such cyclic, (tetraisopropyldisiloxane-l,3-diyl) derivatives on partial hydrolysis could possibly be rationalized in terms of the acidity of the carbon-bound hydroxyl group that is formed on hydrolysis of a silicon-oxygen bond. 

A large body of scientific evidence suggests that the loss of food palatability as a result of lipid oxidation is due to the production of short chain compounds from the decomposition of the hydroperoxides. The volatile compounds produced from the oxidation of edible oils are influenced by the composition of the hydroperoxides and the positions of oxidative cleavage of double bonds in the fatty acids." ... 

Because the dissociation will have a positive entropy of activation, the value of the enthalpy of bond cleavage can be higher than the free energy of activation. 

In all cases discussed above the charged fragments of bond cleavage - electrons and positively or negatively charged ions - remain in the material. In order to be emitted the electrons or ions must get an additional energy, that enables them... 

Several models have been proposed to account for the non-random distribution of oligosaccharides formed when polysaccharides are hydrolysed by a-amylase. The preferred-attack model assumes that the probability of bond-cleavage depends on the position of the bond in the chain the repetitive (or multiple-attack) model assumes that a-amylase can form a cage-like complex with the substrate and attack it several times during a single encounter the multiple-enzyme (or dual-site) model assumes that the substrate is hydrolysed by the combined actions of exo- and en /o-enzymes. The effects of pH, inhibitors, and the chain length of the substrates have been studied in an attempt to decide which of the three models best fits the action of a-amylase. The effects of these variables on either the distribution of products or the action pattern of the enzyme were incorporated into the models, which were then used to interpret experimental data obtained with porcine pancreatic a-amylase. 

The activation volume of degradation reactions which can occur at high temperatures should be positive because bond cleavage increases the volume of the activated complex more than the formation of double bonds from single bonds decreases it. 

With electrons flowing from ethylene to zirconium the Zr—CH3 bond weakens the carbons of ethylene become positively polarized and the methyl group migrates from zirconium to one of the carbons of ethylene Cleavage of the Zr—CH3 bond is accom panied by formation of a ct bond between zirconium and one of the carbons of ethylene m Step 3 The product of this step is a chain extended form of the active catalyst ready to accept another ethylene ligand and repeat the chain extending steps... 

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