Hydrolysis rates electrostatic effects

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 effect of the local non-neutral environment (4) should be considered together with the detailed reaction mechanism of the hydrolysis reaction and together with the charge development in the activation process in particular. The electrostatically non-neutral environment offered by ionic micelles is generally thought to be the reason for the observation that rate-retarding effects exerted by anionic surfactants on this type of hydrolysis reaction are typically stronger than those by other surfactants. 

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... 

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 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. 

As mentioned above, various proteins are able to form electrostatic complexes with HA. We thus decided to investi te the effect of the presence of a protein with no catalytic activity towards HA on the kinetics of the HA hydrolysis catalyzed by BT-HAase. For that purpose, we chose to first use BSA. Indeed, BSA is known as to be able to form electrostatic corrrplexes with HA. In addition, BSA is a rrtajor protein of synovial fluid (Scott et al., 2000), a flrrid which is also rich in HA (see Irrtroduction section). Figure 6 shows BSA-deperrdence crrrves (that is to say, irritial hydrolysis rate plotted as a function of the BSA concentration) obtained for variorrs HA concentrations, at pH 4 and at low ionic strength (5 mmol 1 ). We can observe (Figure 6) that (i) the initial rate of HA hydrolysis catalyzed by BT-HAase strongly depertded on the BSA concentration and (ii) the BSA-deperrdence ciuves had all the same shape whatever the HA concerrtration (Lenormand et al., 2009). 

The first domain corresponds to BSA concentrafiorrs ranging from zero to A. When the BSA concentration is nil, nearly all the BT-HAase molecules form electrostatic complexes with HA. The concentration of catalytically active BT-HAase is thus close to zero, which makes the initial hydrolysis rate extremely low. For increasing BSA concerrtratiorrs up to A, the added BSA molecules use the space remairting free on HA molecules to form electrostatic complexes. This has no effect on the initial hydrolysis rate which remains close to zero. It should be noted however that domain 1 exists only for low values of the ratio of the BT-HAase concentration over the HA concentration. 

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