Hydrolysis electrostatic effects

One of the very few exceptions to the rule that the acidity of the complexed ligand exceeds that of the free ligands involves the Ru(II) complexes shown in Table 6.5. It is believed that back bonding from the filled iig orbitals of Ru(II) to unoccupied tt-antibonding orbitals of the ligands more than compensates for the usual electrostatic effects of the metal that makes the nitrogen less basic. This tt-bonding is less likely with the Ru(III) complex and its is lower than that for the protonated pyrazine (see also Sec. 6.3.3. for the effects of Ru(II) and Ru(III) on hydrolysis of nitriles). ... 

The following relative second-order rate constants have been obtained for hydroxide ion-catalysed hydrolysis glycine ethyl ester, 1 protonated glycine ethyl ester, 41 and the cupric ion complex of glycine ethyl ester, F3 x 10 (Conley and Martin, 1965). The large effect of the cupric ion cannot be due entirely to electrostatic effects, but rather to catalysis by direct co-ordination with the ester function. 

The enhanced reactivity in the cupric ion-catalyzed hydrolysis cannot be due solely to the electrostatic effect of an attack of hydroxyl ion on a positively charged a -amino ester, since the introduction of a positive charge, two atoms from the carbonyl group of an ester, increases the rate constant of alkaline hydrolysis by a factor of 103 (10), whereas there is a difference of approximately 106 between the cupric ion-catalyzed and the alkaline hydrolyses of DL-phenylalanine ethyl ester. The effective charge on the cupric ion-glycine (buffer)-ester complex is +1, so that the factor of 106 cannot be explained by an increase in charge over that present in the case of betaine. Furthermore, the reaction cannot be due to attack by a water molecule on a positively charged a-amino acid ester, since the rate constant of the acidic hydrolysis of phenylalanine ethyl ester is very small. It thus seems... 

Effective concentration 65-72 entropy and 68-72 in general-acid-base catalysis 66 in nucleophilic catalysis 66 Elastase 26-30, 40 acylenzyme 27, 40 binding energies of subsites 356, 357 binding site 26-30 kinetic constants for peptide hydrolysis 357 specificity 27 Electrophiles 276 Electrophilic catalysis 61 metal ions 74-77 pyridoxal phosphate 79-82 Schiff bases 77-82 thiamine pyrophosphate 82-84 Electrostatic catalysis 61, 73, 74,498 Electrostatic effects on enzyme-substrate association rates 159-161... 

Although the contribution of the purine and pyrimidine bases is considered to be relatively minor compared to the electrostatic effects of the phosphates, the enzymic subsites must clearly be able to preferentially distinguish and discriminate the various bases. For example, Mikulski et al. have shown that the nature of the base in the / position exerts a dominant role in the susceptibility of hydrolysis of dinucleotides a clear preference is demonstrated for A and T (22). Also consistent with this are the observations that 5 -mononucleotide binding shows a clear preference for A and T (3, 63), that poly A is more rapidly hydrolyzed than poly C or poly U (3), and that Tp are preferentially released during the early phases of RNA and DNA hydrolysis. 

Complex formation takes place at the amino group of the substrate. In some examples, the increase of the hydrolysis rate of the ester group in the complexed substrate is of the same order of magnitude as in the N protonated substrate. It can be qualitatively understood on the basis of an electrostatic effect which facilitates attack of OH at the carbonyl carbon [272],... 

The influence of electrostatic effects on the rate of hydrolysis of peptides is demonstrated vividly by the studies of Long and co-workers (1963) who made quantitative kinetic studies of the parallel and consecutive reactions which occur on hydrolysis of tripeptides. These workers employed an... 

This method of finding the concentration of ions near the surface was applied by Davies (49,21) to the hydrolysis of ionized films of the ester monocetyl succinate. Table IX shows that the rate constant for this hydrolysis, which increased 300% if calculated using bulk concentrations of the catalytic hydroxyl ion, varied by not more than 36% when evaluated using the surface concentrations deduced from (xxv) and (xxvi). Figure 19 shows a similar effect for the addition of neutral salt, the marked catalysis by which is thus demonstrated to be due entirely to electrostatic effects. The acceleration in the rate of hydrolysis of a film cholesterol formate if the surface bears a negative charge can be predicted on the basis of the Donnan equations (xxv) and (xxvi). Values of 5 of 6 A. and 8 A. have been used, the results being compared with experiment in Fig. 21a. The calculated retardation is shown in Fig. 21b. 

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 relative rate of hydrolysis in acid of any peptide bond and hence the yield of a given peptide is determined mainly by the number of hydrogen ions that can approach the bond. While the rate probably depends on a number of different factors, we may consider two which probably play a major role, namely, electrostatic effects and steric effects. 

The hydrolysis of Th(IV) at total concentrations larger than 0.1-1 mM is dominated by the formation of polynuclear complexes that have high positive charges, e.g. Thj (OH)f and Th (OH) 5. Hence electrostatic effects are expected to play an important role for the stability of these species as shown by the ionic strength dependence of the reaction ... 

The cyclopropane ring in both epimers occupies the same position with respect to the departing group both epimers form a stable bisected cyclopropylcarbinyl cation. The exo endo rate ratio can reflect the contribution ratio of steric factors on the formation of the unsubstituted 2-benzonorbomenyl cation out of the two epimers. The hydrolysis of compounds 269 and 270 has resulted in an exo endo rate ratio of 12 and agrees fairly well with the previous value. A comparison of these data with the value of 15000 for the unsubstituted compounds clearly shows the steric and electrostatic effects to have a low value for secondary systems and in the absence of the ring 7i-participation the exo-isomer solvolyzes faster than its endo epimer only 2- to 10-fold. 

The structure-reactivity relationship for polyamine derivatives in activated ester hydrolysis was previously established [46]. Polyvinylamine (PVA), linear (LPEI) and branched (41% branching) polyethylene imine (BPEI) as well as their dodecyl- and imidazole-substituted derivatives with an approximate and equal degree of substitution (16-20%) were applied as catalysts. The compoundsp-NPA and 4-acetoxy-3-ni-trobenzoic acid (ANBA) as well as some of their homologues were used as substrates. At an excessive catalyst concentration relative to the substrate concentration, reactions proceeded at pseudo first order. In each series of polymers, the reaction rate constant was increased considerably by substitution of dodecyl (hydrophobic site) by imidazolyl (catalytic center) and when a charged substrate (electrostatic effect) was employed. At an equal degree of substitution, the catalytic activity increased in the following order LPEK PVA < BPEI. 

To test the dominance of electrostatic effects in the mineralization model, a mutant of CCMV was constructed (subE) in which all the basic residues on the N-terminus of the coat protein were substituted for glutamic acid (E), thus dramatically altering the electrostatic character of the interior of the assembled protein cage." This mutant was able to catalyze the oxidative hydrolysis of Fe(II) to form an iron oxide nanoparticle encapsulated within the protein cage of the modified virus. High-rcsolution spectral imaging allowed the elemental composition of a protein-mineral composite material to be resolved (1 nm spatial resolution, Fig. 3). This clearly showed that the mineral nanoparticle was completely encapsulated within the protein cage structure. This mutant is able to bind Fe(lT), facilitate its autoxidation... 

The acid or base catalyzed hydrolysis of polyacrylamide (PAM) or Its partially hydrolyzed counterpart (HPAM) In aqueous solutions has been the subject of numerous studies. In part because this system provides an opportunity for evaluating the influence of polymer composition, and steric and electrostatic effects on the course of a simple organic reaction In a convenient solvent medium. 

Most transition states involve charged intermediates, which are stabilized within the active site of an enzyme via ionic bonds in pockets or holes bearing a matching opposite charge. Such charges are derived from acidic or basic amino acid side chains (such as Lys, Arg, Asp, or Glu) ° or are provided by (Lewis acid-type) metal ions, typically Zn +. Computer simulations studies suggested that in enzymes electrostatic effects provide the largest contribution to catalysis [107]. As a prominent example, the tetrahedral intermediate of carboxyl ester hydrolysis is stabilized in serine hydrolases by the so-called oxyanion hole (Scheme 2.1). 

Syntheses of the bicyclo[2,2,l]heptyl carboxylic acids (9) and (10) have been described, and it was argued that the electrostatic effect of the ionized endo-carboxylic acid group on the hydrolysis of (9) should be a reasonable model for... 

The hydrolysis of TiCU, whose ultimate products are TiOa and hydrochloric acid, is at least initially a first-order process. Whereas exchange of fluoride, and reaction of chloride, with [CrOgF]" are both slow processes, at least on the n.m.r. time-scale, there is rapid halide exchange between CrOaFa and CrOaQa. presumably via the known compound CrOaFa. The difference in reactivity between these systems may be due at least in part to electrostatic effects. ... 

It should be noted that the proposed attack of acylate ion at the sahcoyl carbon, unlike attack of water, would not lead to hydrolysis and that attack at the salicyl carbonyl group would be more adversely affected by an electrostatic effect than attack at the acetyl group. 

Nearly 19- and 26-fold lower values of k than k for pH-independent hydrolysis of 2 in CTABr and SDS micelles, respectively, are explained in terms of high concentration of ionic head groups in Stem layer and electrostatic effect on partially anionic transition state. However, such an electrostatic effect cannot explain nearly 190- and 65-fold lower values of k, compared to k for pH-independent hydrolysis of 3. It has been suggested that the influence of hydro-phobic chains is more pronounced for 3 than for 2. But the nearly 3-fold larger value of k i for 3 in SDS micelles than in CTABr micelles remained unexplained. The deaease in kw for 2 from 4.8 x lO- to 2.4 x lO- seer with the increase in [NaCl] from 0.0 to 0.5 M in SDS micelles has been attributed to increased counterion binding (i.e., P value in pseudophase ion-exchange [PIE] model for-... 

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