Equimolar substrate

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. 

Irradiation of a 1 1 complex crystal (mp 95-97 °C, plates) between 1 and 2, which was prepared by recrystallization of die equimolar substrates from acetonitrile, with a 400-W high-pressure mercury lamp through a Pyrex fdter under nitrogen at room temperature gave [2+2]cycloadduct 3 in 54% yield as a sole product. On the other hand, direct photoirradiation to an acetonitrile solution of 1 and 2 gave another type of [2+2]cycloadduct 5 in 25% yield together with a small amount of 3. 

Figure 6.4 Variation in the RH in systems equimolar substrate concentrations of 25 mM constituted of toluene (2 ml), mycelia (lOg/l), and 50mM, respectively. Empty squares...

Figure 6.5 Variation in the RH measured during the esterification reaction of butyric acid with different alcohols cinnamyl alcohol ( ), 1-phenyl-1-butanol ( ), and Z-L-Ser-OBzl (A). Experimental conditions 2 ml of toluene, 20 mg of lyophilized mycelia, and 50mM equimolar substrates in a 6ml sealed vial.

Shape selective catalysts, such as ZeoHtes of the H-ZSM-5 type, are capable of directing alkyl groups preferentially to the para position (18). The ratio of the catalyst to the substrate also plays a role ia controlling the regiochemistry of the alkylations. For example, selective alkylation of anilines at the para position is achieved usiag alkylatiag ageats and AlCl ia equimolar ratio (19). 

Alternating equimolar copolymers of vinyl acetate and ethylene and alternating copolymers of vinyl acetate and acrylonitrile have been reported (127,128). Vinyl acetate and certain copolymers can be produced directly as films on certain metallic substrates by electroinitiation processes in which the substrate functions as one electrode (129). 

Receptor—substrate-binding constants are typically between 10 and 10, at equimolar ratios, implying that when a receptor—substrate... 

Many of the reactions of BF3 are of the Friedel-Crafts type though they are perhaps not strictly catalytic since BF3 is required in essentially equimolar quantities with the reactant. The mechanism is not always fully understood but it is generally agreed that in most cases ionic intermediates are produced by or promoted by the formation of a BX3 complex electrophilic attack of the substrate by the cation so produced completes the process. For example, in the Friedel-Crafts-type alkylation of aromatic hydrocarbons ... 

Depending on the specific reaction conditions, complex 4 as well as acylium ion 5 have been identified as intermediates with a sterically demanding substituent R, and in polar solvents the acylium ion species 5 is formed preferentially. The electrophilic agent 5 reacts with the aromatic substrate, e.g. benzene 1, to give an intermediate cr-complex—the cyclohexadienyl cation 6. By loss of a proton from intermediate 6 the aromatic system is restored, and an arylketone is formed that is coordinated with the carbonyl oxygen to the Lewis acid. Since a Lewis-acid molecule that is coordinated to a product molecule is no longer available to catalyze the acylation reaction, the catalyst has to be employed in equimolar quantity. The product-Lewis acid complex 7 has to be cleaved by a hydrolytic workup in order to isolate the pure aryl ketone 3. 

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. 

All mammalian cells are virtually capable of producing CO with heme as the main substrate (Fig. 1) [5]. Enzymatic heme metabolism in vivo is mainly catalyzed by heme oxygenase (HO). In the presence of HO, the porphyrin ring of heme is broken and oxidized at the a-methene bridge, producing equimolar amounts of CO, ferrous iron, and biliverdin. Three isoforms of HO have been identified. Inducible HO-1 (32 kDa) is mostly recognized for its upregulation in response... 

One of the most efficient plasmin inhibitor is a2-PI (70 kDa), which is synthesized by the liver, secreted into the blood circulation, where its concentration is 1 pM. It rapidly forms equimolar complex with plasmin, and in this complex, the active site of the enzyme is irreversibly blocked. The complex, thereafter, is removed by the liver. It is remarkable that when plasmin is bound to its substrate (fibrin), it is protected against its primarily inhibitor, a2-PI the rate of inactivation decreases by 400-fold (Fig. 4) [3]. 

An example of the efficient formation of an electron-deficient double bond by RCM was disclosed by a Japanese group in a novel total synthesis of the macrosphelides A (209) and B (208) (Scheme 41) [100]. When the PMB-pro-tected compound 204 was examined as a metathesis substrate, the ring closure did not proceed at all in dichloromethane using catalysts A or C. When the reaction was carried out using equimolar amounts of catalyst C in refluxing 1,2-dichloroethane, the cyclized product 205 was obtained in 65% yield after 5 days. On the other hand, the free allylic alcohol 206 reacted smoothly at room temperature leading to the desired macrocycle 207 in improved yield. 

Upon hydrolysis of G6 to G8 the formation of equimolar product pairs is not observed anymore. With G6 as substrate Gl is formed at least twice as fast as G5 while G4 is formed... 

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 ... 

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