Base-mediated hydrolysis, rate

Variations in pH induce acid-base mediated hydrolysis. For base-catalyzed hydrolysis, for example, the rate equation for an organic contaminant is... 

Increases in pH as a direct proportional augmentation of the hydroxyl ion activity leads to a base-mediated hydrolysis process. In this case, the hydroxyl behaves as a nucleophile and is consumed in the reaction. Neutral and alkaline hydrolysis are the most frequent reactions over the common environmental pH ranges. The relation between first-order hydrolysis rate constants and the pH often is presented as a pH rate profile (Wolfe et al. 1990). 

Equation (30) represents a QSAR for the base-mediated hydrolysis of formates and acetates. The correlation is between the second-order alkaline hydrolysis rate constants and the linear combination of the shifts of the vC=0 and vC-O stretching peaks for 12 of the 41 compounds in Table 13.3. 

Chemical/Physical. Anticipated products from the reaction of 1,3-dichlorobenzene with atmospheric ozone or OH radicals are chlorinated phenols, ring cleavage products, and nitro compounds (Cupitt, 1980). Based on an assumed base-mediated 1% disappearance after 16 d at 85 C and pH 9.70 (pH 11.26 at 25 C), the hydrolysis half-life was estimated to be >900 yr (Ellington et al., 1988). 1,3-Dichlorobenzene (0.17-0.23 mM) reacted with OH radicals in water (pH 8.7) at a rate of 5.0 x 10 /M-sec (Haag and Yao, 1992). 

Abiotic hydrolysis of pollutants in subsurface waters is pH dependent. The predominant pathways are acid-catalyzed, base-mediated, and neutral (pH-independent) hydrolysis. The acid-catalyzed hydrolysis reaction rate is dependent on proton concentration increases with a decrease in pH. This behavior occurs because the proton is not consumed in the reaction. 

The above mentioned investigations revealed that the lipase-mediated hydrolysis proceeds at higher reaction rate and, in many cases with better selectivity, if butanoates or pentanoates are employed as substrates instead of acetates. However, the use of enzymatic deacylations is by no means restricted to simple alkanoates. An illustrative and impressive example is found in the hydrolysis of generally base-stable carbohydrate pivaloylates using an esterase from rabbit serum (ERS)[214—217]. For instance, the biocatalyst selectively splits off the 6-pivaloyl group from a-methyl... 

The theoretical basis for such a rationale has been laid in the recent work of Pack et al [161,162]. Using the Poisson-Boltzmann approximation the pH-contour maps on and near the surface of B-DNA ( poly(dG).poly(dC)) have been constructed under simulated conditions of 45 mM tris buffer with 3mM Mg at pH 7.5. Three domains of high ET concentration (>10p.M) are predicted one is spread over the minor groove and two are localised in the major groove near N7(G) and C5(C) for a G.C base pair [114,163]. The reduction in pH by two units would translate into one hundred fold increase in TC production compared to the bulk rate. This is manifested in the accelerated rate of DNA-mediated hydrolysis. Elaborating on the two state model of Islam et al [149] in which the DE is either free or statically bound. Pack and Wong [163(a)] concluded that the catalysis by DNA is primarily an electrostatic effect of acidic domains in the surface grooves of the nucleic acid. While such computations were found satisfactory for a //-BaPDE hydrolysis, they could not adequately reproduce... 

The results (Leete and Chedekel, 1974) indicate that 48% of [2- H](-)-nomicotine and 52% [2- C](+)-nomicotine is incorporated from the labeled nicotine. Thus if (+)-nornicotine is formed from (-)-nicotine, the transformation must involve the loss of the hydrogen from C-2 however, almost the same [ HA C]ratio occurs as in the administered mixture of [2 - H](-)-nicotine and [2 2- C](+)-nicotine, which indicates that in N. glauca (+)- and (-)-nicotine are demethylated at similar rates. If the demethylation had been stereospeciflc for (-)-nicotine, the resultant nomi-cotine would have its [ HA C]ratio doubled. This led the authors to propose a scheme for the formation of (+)-nicotine and (-)-nicotine (Figure 6.11). This mechanism accounts for the partial racemization of the nornicotine derived from (-)-nicotine a Cope elimination of nicotine N -oxide (a) would involve one of the hydrogens at C-3, which would provide the unsaturated compound (b). Elimination of water from this hydroxy amine yields the Schiff base (c), which upon hydrolysis yields formaldehyde or other Cl metabolite and the primary amine (d). Cyclization of this intermediate yields (+)- and ( )-nomicotine. Leete and Chedekel presume that these steps are enzyme mediated, and it is to be expected that the final cyclization would yield a preferential amount of (+)- or (-)-nornicotine however, the mechanism which yields (+)-nornicotine from (-)-nicotine remains unknown. 

The term nucleotide is used generically for both RNA and DNA units. The absence of a 2 -OH group in DNA prevents alkali-mediated cleavage of the 3 -5 phosphodiester cleavage observed in RNA and thus makes DNA more resistant to hydrolysis. Both RNA and DNA contain two types of purines, adenine (A) and guanine (G), and two types of pyrimidine bases (Fig. 1C). The second key difference between RNA and DNA is that while cytosine (C) is present in both RNA and DNA, RNA normally contains uracil (U), while DNA contains 5-methyluracil, called thymine (T), as the other pyrimidine base. The difference in chemical structure is reflected in the intrinsic chemical stability of these nucleic acids. The purine N-glycosyl bond in DNA is more unstable than in RNA, and as a result, purines are released much more easily from DNA by acid catalysis. Furthermore, cytosine deamination to produce U also occurs at a finite rate in DNA. 

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