Promoter site Proton

FIGURE 6-9 Amino acids in general acid-base catalysis. Many organic reactions are promoted by proton donors (general acids) or proton acceptors (general bases). The active sites of some enzymes contain amino acid functional groups, such as those shown here, that can participate in the catalytic process as proton donors or proton acceptors. 

In Fig. 2 we have represented both the r Acetone values and the total site density (nj) as a function of catalyst composition. Qualitatively, the variation of r Acetone with increasing A1 content is similar to that followed by nj thereby suggesting that acetone conversion depends on both acid and base sites. Pure MgO was the most active catalyst whereas AI2O3 showed the lowest activity. This is because Al-0 pairs are much less active than Mg-0 pairs for promoting the proton abstraction and carbanion stabilization steps involved in aldol condensation reactions. We have showed [1] that the acetone aldolization rate is controlled on basic catalysts by the number of metal cation-oxygen anion surface pairs. Mg-rich Mg AlOx oxides are less active than MgO because they exhibit a lower base site density and also poor acidic properties. In contrast, Al-rich Mg AlOx oxides are more active than AI2O3 due to a proper combination of acid and base sites. 

Lateral interactions between the adsorbed molecules can affect dramatically the strength of surface sites. Coadsorption of weak acids with basic test molecules reveal the effect of induced Bronsted acidity, when in the presence of SO, or NO, protonation of such bases as NH, pyridine or 2,6-dimethylpyridine occurs on silanol groups that never manifest any Bronsted acidity. This suggests explanation of promotive action of gaseous acids in the reactions catalyzed by Bronsted sites. Just the same, presence of adsorbed bases leads to the increase of surface basicity, which can be detected by adsorption of CHF. ... 

The effects of organic molecules and phosphate on the adsorption of acid phosphatase on various minerals, and kaolinite in particular, have been investigated by Huang et al. [97]. The Langmuir affinity constant for AcP adsorption by kaolinite follows the series tartrate (K — 97.8) > phosphate (K= 48.6) > oxalate (K — 35.6) > acetate (K= 13.4). At low concentration, acetate even promoted the adsorption of acid phosphatase. It was considered that competitive interactions between anionic adsorbates can occur directly through competition for surface sites and indirectly through effects of anion adsorption on the surface charge and protonation. 

GSTs contain a site that accommodates GSH ("GSH binding domain") [31], where the proton of the GSH s thiol group is abstracted, promoting the nucleophilic conjugation of the thiolate to electrophilic substrates. The resulting adducts become more water-soluble and are then eliminated by a phase II detoxification mechanism. 

Thus, the role of zinc in the dehydrogenation reaction is to promote deprotonation of the alcohol, thereby enhancing hydride transfer from the zinc alkoxide intermediate. Conversely, in the reverse hydrogenation reaction, its role is to enhance the electrophilicity of the carbonyl carbon atom. Alcohol dehydrogenases are exquisitely stereo specific and by binding their substrate via a three-point attachment site (Figure 12.7), they can distinguish between the two-methylene protons of the prochiral ethanol molecule. 

A weakening of the critical metal-oxygen bonds occurs as a consequence of the protonation of the oxide ions neighboring a surface metal center and imparting charge to the surface of the mineral lattice. The concentration (activity) of D should reflect that three of such oxide or hydroxide ions have to be protonated. If there is a certain numer of surface-adsorbed (bound) protons whose concentration (mol nr2) is much lower than the density of surface sites, S (mol 2), the probability of finding a metal center surrounded with three protonated oxide or hydroxide ions is proportional to (CJ/S)3. Thus, as has been derived from lattice statistics by Wieland et al. (1988), the activity of D is related to (C )3, and the rate of proton-promoted dissolution, Rh (mol nrr2 lr1), is proportional to the third power of the surface protonation ... 

The amide and peptide linkages are much more difficult to hydrolyze than the ester grouping. Both free and metal bound groups hydrolyze with second-order rate constants approximately 10 -10 less than for the corresponding esters. There are two potential sites for coordination in the -CONHR residue, namely at the carbonyl O in 13 and at the amide N in 14 where ionization of the amide proton is induced (Sec. 6.4.3). Cu + promotes hydrolysis of glycinamide at low pH where it is present as 13. However it inhibits hydrolysis at high pH, where it is 14, to such a degree that hydrolysis cannot be observed. ... 

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