Reaction rate equimolar substrate

The four constants in Equation 17.14 may be estimated by fitting the equation to the measured initial reaction rate data presented in Figure 17.3. Because equimolar concentrations of the two substrates, PCP and H202, were used in the experiments, Equation 17.14 may be simplified as follows ... 

FIGURE 17.3 Experimental data (symbols) showing the variation of reaction rate V with equimolar substrate concentration ([H202] = [PCP]) for different initial enzyme concentrations [ ),]. Also shown are the theoretical curves (lines) calculated according to Equation 17.15 with the constants of set A as given in Table 17.1. 

There was found to be a correlation between changes in the initial substrate ratio and the pH of the liquid phase [10], and it was demonstrated that even a slightly unequal substrate ratio can have a large effect on rate. For example, a two-fold higher initial rate could be achieved in a reaction mixture with 60% L-Leu-NH2 (p K, around 7.8) and 40% Z-L-Gln (p K, around 3.6) as compared to a reaction with equimolar substrates [10]. However, an unequal substrates ratio means that at the equilibrium an excess of one unreacted substrate will be present, so other methods are generally preferred. 

Because all of the HOOH is consumed in the experiments of Table 4-2, the reaction efficiency for the Fe(n)-HOOH system represents a crude measure of the relative rate of reaction for the adduct with substrate and HOOH. The relative order of the reaction rates for the substrates of Table 4-2 has been determined via a series of competition experiments with equimolar amounts of RH and Ph2SO. Hence, the extent of oxidative conversion of RH relative to Ph2SO from the slow addition of dilute HOOH (0.5 mmol) to an MeCN solution that contains 0.5 mmol of RH, 0.5 mmol of Ph2SO, and 0.5 mmol of Fe(II) has been measured the results are summarized in Table 4-3 as relative amounts reacted. The bond energies for the weakest R-H bond of the substrate also are tabulated. ... 

In this case study, an enzymatic hydrolysis reaction, the racemic ibuprofen ester, i.e. (R)-and (S)-ibuprofen esters in equimolar mixture, undergoes a kinetic resolution in a biphasic enzymatic membrane reactor (EMR). In kinetic resolution, the two enantiomers react at different rates lipase originated from Candida rugosa shows a greater stereopreference towards the (S)-enantiomer. The membrane module consisted of multiple bundles of polymeric hydrophilic hollow fibre. The membrane separated the two immiscible phases, i.e. organic in the shell side and aqueous in the lumen. Racemic substrate in the organic phase reacted with immobilised enzyme on the membrane where the hydrolysis reaction took place, and the product (S)-ibuprofen acid was extracted into the aqueous phase. 

In none of the cases mentioned were substrates that were polymerized to different degrees used at equimolar concentrations, so that, in some cases, it is not clear to what extent the differences in the rate of cleavage reflect the effective concentration of the terminal bonds (as assumed by Mill154) and to what extent they reflect the differences in the enzyme reaction-mechanism. More satisfactory information would be obtainable by comparing the values of the maximum velocities. 

In the presence of an equimolar amount of Cu(II) salt, the rate profile shows a steadily increasing reaction velocity as the pH increases, until a practical limit is attained, as a result of the precipitation of copper hydroxide. There is no indication that the rate might level off or decrease at higher pH, as is true for the metal-free ligand. On this basis it seems that the metal ion combines with the substrate in such a way as to increase its reactivity toward the adjacent carboxylate group, as indicated in Figure 3 (formulas XXI and XXIII). Since increasing pH... 

Although the reaction system contains several Ti-tartrate complexes, the species containing equimolar amounts of Ti and tartrate is the most active catalyst. The reaction is much faster than Ti(IV) tetra-alkoxide alone or Ti-tartrates of other stoichiometry and exhibits selective ligand-accelerated catalysis (64). The rate is first order in substrate and oxidant and inverse second order in inhibitor alcohol, under pseudo-first-order conditions in catalyst. The crystal and molecular structures... 

Several kinds of evidence indicate that the reactions are catalytic rather than stoichiometric. When the reaction is followed to completion, linear first order plots are obtained for at least 90% of the reaction 7>. At the ratio of substrate to polymer employed, about 1 1 by weight, nonlinear first order plots would be predicted for a stoichiometric reaction. When a second aliquot of substrate is added after completion of the reaction, the first order rate constant noted with the second aliquot is essentially identical to that of the original7). The liberation of acetate and p-nitrophenol in equimolar proportions is also consistent with an inference of catalysis 7>. 

High pressure continuously operated reactor. The design of the continuously operated apparatus is shown in Figure 2. An air operated high pressure pump delivered CO2 in the system. The gaseous fluid was dried when passing through columns packed with molecular sieves. The flow rate of C02 was 1.0 L per min. Equimolar solution of substrates (oleic acid and oleyl alcohol) was pumped into the system with an HPLC pump. Carbon dioxide and substrates were equilibrated in the saturation column. The reaction was performed in a... 

The present contribution reports the first results of a systematic approach to a study of the reaction and the products obtained in the liquid-phase ozonolysis of unsaturated substrates containing halogenated double bonds. As a model case the ozonolysis of trans-2,3-dibromo-2-butene was studied. Ozonolysis occurs at a slower rate than that of pure hydrocarbon olefins and less than equimolar amounts of ozone are sufficient for the quantitative conversion of the substrate. Double bond cleavage is not the overriding reaction, but the original double bond is to a large extent converted into C—C single bonds which are fairly stable towards further ozone attack. 

The effect of a substituent on the reactivity of a particular centre may be quantified in terms of the partial rate factor, The partial rate factor is defined as the rate of substitution at a given position relative to that in any one position in benzene itself. Partial rate factors may be calculated by treating an equimolar mixture of benzene and the substituted benzene with insufficient reagent to complete the reaction. Analysis of the products will then show which substrate has reacted with more of the reagent and at which centre. 

True catalysis was proven by three criteria (Table XI see p. 381). When three successive equimolar aliquots of OAA were reacted to completion with an equivalent amount of polymer (on a lysine residue basis), identical rates ( 7 relative %) were observed with each aliquot (Table XI). A progressive drop in rate would be expected from a stoichiometric reaction (33). Second, linear first-order plots were obtained from the interaction of equimolar amounts of substrate and polymer nonlinear plots would be predicted for a stoichiometric reaction (34-36). Third, in the experiments of Table XI at least three equivalents of COg were liberated for each equivalent of lysine residue present. The importance of avoiding an excess of polymer, in order to employ the first two criteria, was pointed out. 

In the authors judgment, catalytic action in the strict sense of the term, implying regeneration of the catalyst, has been rigorously proven in two cases. With other reactions, however, true catalysis has not been unequivocally established. Michaelis-Menten kinetics, indicative of the formation of a substrate-polymer complex, has been shown with each type of reaction. Some of the activities have been very weak, with reaction times for incomplete conversion of substrate measured in days. Others, although not as rapid as enzyme-catalyzed rates, are relatively fast, e.g., the catalytic decarboxylation of OAA by equimolar amounts of lysine residue in polymeric form was over 90% complete in less than 1 hour. 

Equimolar mixtures of toluene and benzene were passed over beds of DHY at low temperatures (25-60°) in experiments where the two aromatics of different reactivity competed for the electrophilic deuterium (75). The distribution of deuterium between toluene and benzene (apparent ACeHsCHs/fcCeHe) and among the ring positions of the toluene-di samples was determined. A plot of log pt vs selectivity factor (St) for these data from the competitive experiments at 25-60° (Fig. 20, black circles) falls on the line obtained from a study of 47 electrophilic substitution reactions by H. C. Brown and associates (83). The partial rate factors pt and mt give the rate of substitution of the para position and one of the meta positions in toluene, relative to the rate of substitution of one of the six equivalent ring positions of benzene. Points a, b, c, d, and e fall quite close to the line, which represents a linear free energy relationship in both positional and substrate selectivity. 

Initiation/Termination Control of Enzyme Reaction. The enzymatic hydrolysis of sucrose was studied at 25 C through a monitoring of the formation rate of reducing sugar products (an equimolar mixture of glucose and fructose) in an aqueous suspension of the enzyme-loaded capsules. The suspension (50 mL) initially contained 100 mM of the substrate and 13.5% (v/v) of the capsules with a total of 200 mg of the encapsulated enzyme. On/off control was tested by investigating the batch reaction kinetics at pH 5.5 (at which the reaction occurs) and at pH 4.5 (at which the reaction stops). The adjustment of pH was made by quick additions of a small amount of 2M HCl or NaOH. 

Deuterium exchange with the solvent does occur during the reaction, but at a rather slow rate. A symmetrical reaction intermediate must exist, since the rate of incorporation of tritium from the solvent into D-madelate as the substrate yields equimolar amounts of D-and L-product [50]. Thus the data are consistent with the formation of an a-carbanion intermediate with an enzymatic base group acting as the proton acceptor [50]. The proton transfer has to be rate-limiting, as indicated by the approximately 5-fold primary isotope effect for deuterium. In the enzyme-substrate complex, the epimerization occurs with a rate constant of the order of 10 s ... 

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