Acid-base mechanism

Lewis acid-base mechanism, 233, 234 Lewis acid catalysts, 13, 546 Lewis-acid-catalyzed ammonolysis, of nylon, 571 Lewis acids, 224 catalysis by, 68-69 Lewis bases, 338... 

Fig. 15.12 (a) Acid-base mechanism of alcohol dehydrogenation. Reprinted with permission from [70], Copyright (1993) Pergamon (Elsevier), (b) Redox mechanism of cyclohexanol ODH. Reprinted with permission from [74]. Copyright (1998) Elsevier. 

The activated methyl groups of 5,7-dimethyl-l,8-naphthyridin-2-one (106) can be selectively metallated, reaction with two equivalents of butyllithium giving (107), while with two equivalents of sodamide in liquid ammonia (108) is obtained. Both (107) and (108) gave appropriate products on treatment with electrophiles (81JOM(213)405>. A possible explanation of this result involves a coordination mechanism for the formation of (107) and an acid-base mechanism giving (108). 

Although acid-base effects can be important in adhesion our studies indicate that bonding to metals through silane coupling agents is not by an acid-base mechanism, but probably through Si-O-M oxane bonds. As with glass, the hydrolysis and formation of oxane bonds are true equilibria, but the individual equilibrium constants are not known. 

This mechanism is similar to that of the serine protease a-chymotrypsin. In comparison to a-chymotrypsin, however, which features a general acid-base mechanism, the antibody requires a hydroxide ion (OH-) to initiate the deacylation step. The enzyme liberates both products P1 and P2 early in the catalytic cycle and very rapidly whereas the antibody liberates both products at the end of the catalytic cycle only. Even given all the similarities between enzymes and antibodies, one thus cannot expect complete analogy down to mechanistic details. However, the specificity of enzymes and antibodies can be very similar, as Table 18.1 corroborates. 

In the case of beef liver catalase, distal histidine (His74) is believed to serve as a general acid-base catalyst to facilitate the heterolytic 0—0 bond of hydroperoxide bound to the heme (Scheme 2) (21). The Asnl47 residue located near the heme could assist the heterolysis by making the distal site into a polar atmosphere. The same acid-base mechanism has been attributed to peroxidases... 

The results of a computational study revealed that the sedolisins, a family of ser-inecarboxyl peptidases, may evoke different catalytic machineries than do classical serine proteases in achieving transition-state stabilization. The family is characterized by a unique catalytic triad, Ser-Glu-Asp, that operates primarily through a general acid-base mechanism.76... 

A bifunctional acid base mechanism (Figure 14.5) was proposed to explain the high selectivity to O-alkylation of rare-earth phosphates. The absence of C-alkylated products by methylation of ga iacol tends to suggest that the guaicolate surface intermediate has a different behaviour on LaP04 and LaP04 doped with cesium hydrogenophosphate. In this latter case, the excellent selectivity to O-alkylation could be due to the softness of the cesium ion (39). 

Fig. 14.5 Acid base mechanism for the alkylation of phenols with methanol over LaP04 doped with Cs2HP04...

The relative contribution of the acid-base mechanism stated above and this radical mechanism cannot be determined individually. The concentration of the basic sites remaining after milling for 1 and 3 h is lO and 10 times higher, respectively, than radical concentration. This does not straightforwardly mean, however, that the contribution of the radical mechanism is orders of magnitudes smaller, because we can measure only the concentration of remaining radical species on a separately milled sample. The amount of the radical species actually consumed by recombination has not yet been successfully determined. 

Herschlag D. Ribonuclease revisited. Catalysis via the classical general acid-base mechanism or a triester-like mechanism. J. Am. Chem. Soc. 1994 116 11631-11635. 

The prospect of separating lithium isotopes on a large scale by the extraction of lithium from aqueous solutions is not very promising. In the system we have studied, reflux could be accomplished by an acid-base mechanism however, because of the small separation factor, an extremely large reflux ratio would be required. This would necessitate a very large plant using enormous quantities of acid and base, and the cost would be excessive. 

This would seem an ideal reaction to catalyze with a general acid/base mechanism. The enzyme has a bell shaped pH profile with pKa s at 7 9 and has his and lys residues in the active site. 

According to Dadyburjor et al. [16] the acid character of the paraffins and the olefins decrease with their number of carbon atoms, the olefins shown the lower acid character. On the basis of an acid-base mechanism, one will expect that the higher the carbon number of the paraffin the lower the selectivity to olefins on catalysts with acid character but the higher the selectivity to olefins on catalyst with a basic sites. 

Prevention of deamidation If the deamidation occurs by a general acid-base mechanism then the optimum pH for a peptide formulation will usually be about 6, where both rates are at their minimum. If the deamidation occurs through the cyclic imide intermediate it is, in principle, preferable to formulate at a low pH since this type of deamidation is base-catalysed. This may not be feasible in practice, however, since other routes of degradation tend to predominate at lower pH and a compromise must then be sought. It is well established that deamidation rate may be affected markedly by buffer components and hence care must be taken in the choice of buffer used in the control of pH. In general it is found that the phosphate anion is most problematic in its effect on the rates of deamidation. 

R1-NH-CO-R2 + H2O - RI-NH2 + HOOC-R2 where R1 and R2 denote the rest of the protein chain. HIV protease achieves catalysis by a combined acid-base mechanism. There are two aspartic acid residues in the active site, only one of which is protonated. At the time of the reaction, the protein is firmly bound in the active site as is the water molecule which is to be split ("lytic water ). The role of the unprotonated aspartate is... 

Fluoroanisoles 95 and 97 each have two different sites that could be lithiated.19 In each case, the site next to oxygen can be lithiated if a coordination mechanism is operative and the site next to fluorine can be lithiated if an acid-base mechanism is operative. If o/t/zo-fluoroanisolc 97 is... 

Fig. 8. Reaction mechanism of (6-4) photolyase. The enzyme binds to DNA containing a (6-4) photoproduct and flips out the dinucleotide adduct into the active site cavity, where the open form of the photoproduct is converted to the oxetane intermediate by a light-independent general acid-base mechanism. Catalysis is initiated by light MTHF absorbs a photon and transfers energy to FADH , which then transfers an electron to the oxetane intermediate bond rearrangement in the oxetane radical regenerates two canonical pyrimidines, and back-electron transfer restores the flavin radical to catalytically competent FADH form. The repaired dipyrimidine flips back into the DNA duplex, and the enzyme is dissociated from the substrate.

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