Rate constants reaction efficiency, enhanced ratios

A different approach was used by Emgenbroich and Wulff [42] to develop imprinted polymers with enantioselective esterase activity. The system was based on the use of an amidinium functional monomer (33), already developed earlier by the same group, but in this case a chiral phosphonate (62) was used as imprinting TSA in order to catalyse the hydrolysis of the corresponding chiral ester (63). The polymer imprinted with the L-enantiomer was able to enhance the esterolytic activity 325-fold when compared with the background and 80 times compared to the non-imprinted polymer. The ratio between the two rates constant, Lai-i/L ii-i) = 1 -4, can be taken as a measure of the enantioselective efficiency of the reaction. 

The magnitude of the NEMCA effect for a given catalytic system is commonly described by two parameters, the rate enhancement ratio, p, (= r/r0, where r and rQ are the electropromoted and unpromoted reaction rate values) and the faradaic efficiency, A, (= (r - r0)/(I/nF)), where I is the current, F is the Faraday constant, and n is the charge of the promoting ion. The magnitude of A can be predicted from the parameter 2Fro/I0, where I0 is the exchange current of the catalyst-support interface [v]. 

Representative for systems exhibiting sigmoidal conversion curves Fig. 1 shows experimental results for the rate constant of the reaction of TS, evaluated from thermal and y-polymerization data according to K = (1 — X) dX(t)/dt, and normalized to the rate constant in the low conversion limit. It is obvious, that at low conversion K depends on X, contrary to what is to be expected for a simple first order reaction. The functional form of KPC) is different for the two modes of polymerization. The overall increase of K with increasing X reveals an autocatalytic reaction enhancement. A measure for its efficiency is the ratio K(X = 0.5)/K(X = 0) which tirnis out to be about 200 for TS under thermal polymerization conditions. This effect is often observed with disubstituted diacetylenes albeit with different kinetic... 

The kinetics of the hydrolysis reactions (43) and (44) was studied in detail by means of polarography [158], and the respective pseudo-first-order rate constants and k2 were determined. In 0.2M KOH at 75°C, the rate constants were k = 3.2 x 10 s and At2 = 1.43 X 10 s , indicating that the second step of hydrolysis (44) is nearly 500 times faster than the first step (43). For this reason, the efficiency of borohydride in enhancing the plating reaction is very low, typically on the order of a few %, and most borohydride is lost by hydrolysis. (The actual efficiency depends on the ratio of substrate area to bath volume, i.e., the loading factor.)... 

To compare the catalytic efficiency of catalysts, it is helpful to compare the enhancement ratios (E.R.). E.R. is calculated by dividing the kcat by the kuncat (the rate constant for the uncatalyzed reaction). Enormous rate enhancements are achieved by enzymes. For example, hydrolytic enzymes often exhibit rate enhancements of 10 -10 2 compared with the spontaneous water-catalyzed or the acid/base-catalyzed reaction at about neutral pH. e For purposes of comparison, the kcat, kuncat, and E.R. values for two hydrolytic abzymes, as well as CCMP fluorohydrolase chromatographed on G-15 Sephadex gel is presented in Table 3. The fluorohydrolase, chromatographed on G-15 Sephadex, has an E.R. four times that of the two abzymes. In addition, the enhancement ratios of natural DFPases from various sources as well as CCMP fluorohydrolase are presented in Table 4. In all cases, the initial E.R. (before any purification) is higher for semisynthetic fluorohydrolases than for any of the natural unpurified DFPases. 

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