Benzylpenicillins, hydrolysis

TABLE 8.4 Rate Constants of Benzylpenicillin Hydrolysis Catalyzed by TEM /3-Lactamase ... 

Step 1 Benzylpenicillin is first converted to the corresponding penicilloic acid (a dicarboxylic acid) by carrying out the hydrolysis with sodium hydroxide solution, as follows ... 

Fig. 5.20. Modes of coordination of transition metal ions with /3-lactam antibiotics. Complex A In penicillins, the metal ion coordinates with the carboxylate group and the /3-lactam N-atom. This complex stabilizes the tetrahedral intermediate and facilitates the attack of HO-ions from the bulk solution. Complex B In benzylpenicillin Cu11 binds to the deprotonated N-atom of the amide side chain. The hydrolysis involves an intramolecular attack by a Cu-coordinated HO- species on the carbonyl group. Complex C In cephalosporins, coordination of the metal ion is by the carbonyl O-atom and the carboxylate group. Because the transition state is less stabilized than in A, the acceleration factor of metal ions for the hydrolysis of cephalosporins is lower than for penicillins. Complex D /3-Lactams with a basic side chain bind the metal ion to the carbonyl and the amino group in their side chain. This binding mode does not stabilize the tetrahedral transition complex and, therefore, does not affect the rate of... 

The metabolic fate of the benzylpenicillin-human serum albumin conjugate was studied in rats [149]. The conjugate was taken up by the liver, where it underwent enzymatic cleavage to form benzylpenicilloic acid. Thus, the benzylpenicilloic acid excreted in urine may be formed either by direct hydrolysis of the /3-lactam ring, or by catabolism of protein conjugates formed in vivo. 

H. Chaimovich, V. R. Correia, P. S. Araujo, M. V. Aleixo, I. M. Cuccovia, A Quantitative Analysis of the Effect of Hexadecyl-Trimethylammonium Bromide Micelles on the Rate of Alkaline Hydrolysis of Benzylpenicillin , J. Chem. Soc., Perkin Trans. 2 1985, 925-928. 

Following the realization that the presence of phenylacetic acid in the fermentation led to a simplification of the mixture of penicillins produced by the fungus due to preferential uptake of this acid and its incorporation into benzylpenicillin (4), a wide variety of other acids were added to the growing culture. Inclusion of the appropriate acids in the culture medium thus afforded, respectively, phenoxymethylpenicillin (5, penicillin V), phenethicillin (6), > propicillin (7), and phenbencillin (8). These modifications served to increase stability of the lactam bond towards hydrolysis and thus conferred some degree of oral activity. 

Extracts containing benzylpenicillin were prepared for analysis in buffer at pH 6.5 at 25°C, the rate constant for the hydrolysis of benzylpenicillin under these conditions is 1.7 x 10" s. What is the maximum length of time the solutions can be stored before analysis so that no more than 1 % decomposition occurs. 

Since cephalosporin C is only one-thousandth as active as benzylpenicillin, its use is very limited. However, it is remarkably resistant to enzymatic hydrolysis and becomes highly concentrated in the urine, which makes it useful in urinary tract infections caused by Gram-negative organisms. 

The zinc complex of (HOCH2)3CNH2(tris) is a most effective catalyst for the hydrolysis and aminolysis of benzylpenicillin, 1 and it is suggested that this observation may be of some relevance to the mode of action of zinc-dependent /3-lactamases. 

Relative Rates of Hydrolysis of Various Substrates (Rate of Hydrolysis of Benzylpenicillin = 100)... 

A further example is seen in the rapid hydrolysis of benzylpenicillin (3.6) by cop-per(n) which is thought to proceed through an intermediate of type 3.7. Notice that in this case the activation of the carbonyl group to nucleophilic attack seems to be through... 

Figure 3-14. The copper(u)-directed hydrolysis of benzylpenicillin (3.6) showing the postulated intermediate, 3.7.

Theoretical studies were reported of the thiolysis by 2-mercaptoethanol and 2-mer-captoethylamine of simple models of the bicyclic ring systems of penicillin and cephalosporin88 and of the alkaline hydrolysis of sanfetrinem (93)89 The major determinant in allergies induced by penicillins is the penicilloyl group bound to the amino group of the Lys residues present in the carrier protein. Now a study is reported of the polyelectrolyte polyethylenimine (94) as a model of the carrier protein and its catalysis of the aminolysis of benzylpenicillin.90... 

Before interest developed in the structure of penicillin, the imidazo[5,1-b ]thiazole ring system was unknown, and even today, almost all of the examples which have been described are those prepared in connection with penicillin studies. Very mild acidic hydrolysis converts benzylpenicillin methyl ester (293) into benzylpenicillic acid (294) by a mechanism suggested... 

The chemoselective hydrolysis of an acetoxy group in the presence of a 7-lactone ring has been reported in the presence of PLE (eq 9). In a benzylpenicillin, PLE catalyzes the chemoselective hydrolytic opening of the P-lactam ring, the methoxycarbonyl moiety remaining uneffected (eq 10). ... 

Many dmgs associate to form micelles in aqueous solution (see section 6.3) and several smdies have been reported of the effect of this self-association on stability. In micellar solutions of benzylpenicillin (500 000 units cm ) the apparent rate of the hydrogen-ion-catal-ysed degradation was increased twofold, but that of water- and hydroxide-ion-catalysed hydrolysis was decreased twofold to threefold. Consequently, the pH profile was shifted to higher pH values and the pH of minimum degradation was found to be 7.0 compared to 6.5 for monomeric solution (8000 units cm ). When compared at the respective pH-rate profile minima, micellar benzylpenicillin was reported to be 2.5 times as stable as the monomeric solutions under conditions of constant pH and ionic strength. 

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