Chemical rearrangement

Several processes are unique to ions. A common reaction type in which no chemical rearrangement occurs but rather an electron is transferred to a positive ion or from a negative ion is tenued charge transfer or electron transfer. Proton transfer is also conunon in both positive and negative ion reactions. Many proton- and electron-transfer reactions occur at or near the collision rate [72]. A reaction pertaining only to negative ions is associative detaclunent [73, 74],... 

Hulbert, P. B. Grover, P. L. Chemical rearrangement of phenol-epoxide metabolites of polycyclic aromatic hydrocarbons to quinone-methides. Biochem. Biophys. Res. Commun. 1983, 117, 129-134. 

In contrast, the pentavalent Sb-119 ions at the interfaces are weakly bonded to the oxide ion layer of the hematite surfaces in neutral and slightly acidic region, while in the acidic region most of the adsorbed Sb-119 ions are in the zeroth or first metal ion layers of the substrate forming Sb-O-Fe bonds. The pentavalent Sb-119 ions having once been incorporated into the surface metal ion sites retain their chemical form, even when the pH of the aqueous phase is raised above 7. Heating of suspensions at 98°C results in chemical rearrangement of the hematite surfaces to yield pentavalent Sb-119 ions in the second or deeper metal ion layers. 

Phosphorus nuclei have been used for many years in in vivo NMR, especially for intracellular pH measurements. However, because most organic phosphates have similar chemical shifts, compound identification can be difficult without special attention being paid to culture conditions in the NMR tube.15 Carbon NMR also yields significant results because of the large chemical shift dispersion and narrow lines of this nucleus. 13C spectra reflect most of the chemical rearrangements that may take place between substrate and final product. 

Hofmann rearrangement org chem A chemical rearrangement of the hydrohalides of N-alkylanilines upon heating to give aminoalkyl benzenes. haf man. res ran)-mont ... 

Figure 7.22 Metabolism of bromobenzene. The bromobenzene 2,3-oxide and 3,4-oxide may undergo chemical rearrangement to the 2- and 4-bromophenol, respectively. Bromobenzene 3,4-oxide may also be conjugated with glutathione, and in its absence react with tissue proteins. An alternative detoxication pathway is hydration to the 3,4-dihydrodiol via epoxide hydrolase.

Metabolic activation via cytochromes P-450 to reactive metabolites. A reactive intermediate has been suggested as some inhibitors of cytochromes P-450 decrease the toxicity, and some inducers of the monooxygenases increase toxicity. However, other inducers and inhibitors do not change toxicity, and these treatments also affect the renal concentration of cephaloridine in a manner consistent with the effect on toxicity. As the p-lactam ring is unstable, a chemical rearrangement to produce a reactive intermediate is also possible. 

Thus, the susceptibility is the result of accumulation of the drug in the target organ to reach concentrations not achieved in other tissues. This is then followed by what is probably a combination of events such as formation of a reactive intermediate, possibly a free radical, stimulation of lipid peroxidation and depletion of GSH, and then peroxidative damage to cell membranes and mitochondria. Whether metabolic activation by cytochromes P-450, or chemical rearrangement, or reductive activation, or all the three are involved is not currently clear. 

Thiamine pyrophosphate plays an important role in the cleavage of bonds adjacent to a carbonyl group, such as the decarboxylation of a-lceto acids, and in chemical rearrangements in which an activated acetaldehyde group is transferred from one carbon atom to another (Table 14-1). The functional part of TPP, the thiazolium ring, has a relatively acidic proton at C-2. Loss of this... 

A variety of interesting chemical rearrangements occur in the catabolic pathways of amino acids. It is useful to begin our study of these pathways by noting the classes... 

MECHANISM FIGURE 22-18 Tryptophan synthase reaction. This enzyme catalyzes a multistep reaction with several types of chemical rearrangements. An aldol cleavage produces indole and glyceraldehyde 3-phosphate this reaction does not require PLP. Dehydration of serine forms a PLP-aminoacrylate intermediate. In steps and this condenses with indole, and the product is hydrolyzed to release tryptophan. These PLP-facilitated transformations occur at the /3 carbon (C-3) of the amino acid, as opposed to the a-carbon reactions described in Figure 18-6. The /3 carbon of serine is attached to the indole ring system. Tryptophan Synthase Mechanism... 

Molecular systems exist in discrete quantum states, the study of which lies in the realm of molecular structure and wave mechanics. Transitions between quantum states occur either by absorption or emission of radiation (spectroscopy) or by collisional processes. There are two main types of collisional transitions which are important in chemical physics these are first, reactive processes in which chemical rearrangement takes place (reaction kinetics), and secondly collisions in which the energy distribution is changed without overall chemical reaction. It may therefore be concluded that the energy transfer processes discussed here are of fundamental importance in all molecular systems, and that the subject, like molecular structure, is enormously varied and complex. 

Gas chromatography—analysis of compounds that can be volatilized without being decomposed or chemically rearranged ... 

An example for location 2 would be sulfuric acid, which, without being metabolized, would indiscriminately destroy nucleic acid and protein, regardless of genetic differences. Reactive metabolites, as potent electrophiles, presumably seek out macromolecular nucleophilic sites in a random fashion covalent interaction of small reactive metabolities with DNA is therefore an example of location 6. The chemical rearrangement of a reactive epoxide to a less reactive phenol would also occur nonenzymatically this is an example of pathway 7. 

The ionization potential of nitric oxide is nine volts, and the ratio of molecules reacting to ion-pairs produced by an energy input of 8.6 calories is 1.6. In Table I it is seen that for a given input of cathode rays the reaction is 2.3 times as great for nitric oxide as for oxygen. The very low yield with carbon dioxide is in perfect agreement with Lind s results with alpha particles.13 Apparently the C02 ions recombine without chemical rearrangement. 

The many-electron wave function of a molecular system is taken as the antisymmetrized product of (pt, and for closed-shell systems it is convenient to represent it by a Slater determinant. Such an approach is known as the restricted Hartree-Fock (RHF) method and is the most widely used method in chemisorption calculations. Its principal drawback is the neglect of Coulomb electron correlation, which is of crucial importance for adequate treatment of chemical rearrangements with varying numbers of electron pairs. 

---