Benzylpenicillin amide

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... 

Penicillin V is a narrow-spectrum penicillin and has similar antibacterial activity to benzylpenicillin. It is active against many streptococcal infections, but it is inactivated by penicillinases. Flucloxacillin is a penicillinase-resistant antibiotic and is effective against infections caused by penicillin-resistant staphylococci. In comparison to penicillin V, attachment of carbocyclic/heterocyclic ring directly to the C6 carbonyl group confers resistance to beta-lactamases due to steric hindrance around the amide group. 

All penicillins (Fignre 74) are composed of a thiazolidine ring attached to a beta-lactam, which in turn carries a free amide gronp (0=CNH) on which a substitution and an attachment (R) are made. In the case of benzylpenicillin, the R is a benzyl gronp. Penicillin may be metabolized by amidase to 6-aminopenicillanic acid, which has antibacterial activity, or by penicillinase (bacterial beta-lactamase), to penicilloic acid, which is devoid of antibacterial activity bnt is antigenic in natnre and acts as a sensitizing structure. The main sonrce of bacterial resistance to penicillin is in fact the prodnction of penicillinase by the microorganisms. 

It is still not understood how penicilloic acids could so rapidly produce in vivo the bi- or plurivalent antigens required for elicitation of anaphylactic reactions, unless the reaction rates of penicilloic acids with proteins in vivo are efficiently catalyzed (Levine and Ovary 1961 Schneider et al. 1973). Penicilloic acid may also act as inhibitor of reactions of BPO specificity. This has also been demonstrated in hemagglutination, precipitation, and guinea pig PCA (Josephson 1960 Torii and Horiuchi 1961 de Weck 1962a,b Levine 1963 Batchelor and Dewdney 1968). In general, penicilloic acid is a worse inhibitor than BPO amides but markedly more efficient than benzylpenicillin. 

The prodrug, acyl methyl phosphate benzylpenicillin (96), can release ben-zylpenicillin by hydrolysis of the acyl phosphate (half-life 90 h at pH 7.5 and 25°C). Moreover, (96) can also interact with yff-lactamase by two mechanisms it can either be a substrate, with the hydroxyl group of serine-70 of )8-lactamase cleaving the lactam ring or can act as an irreversible inhibitor. The amino side-chain of Lys-234, which normally stabilizes the carboxylate group of the antibiotic, can react at the acyl group to form a covalent amide bond to the protein, with methyl phosphate as the by-product [108]. 

A simple reaction to see if either of these effects is apparent is the hydrolysis of the P-lactam antibiotics. The alkaline hydrolysis of benzylpeni-cillin opens the P-lactam ring to give benzylpenicilloate [30]. With respect to hydroxide ion, benzylpenicillin shows a reactivity similar to that of ethyl acetate. The p/L -value of the protonated amine in the thiazolidine derivative [30] is 5.2 and because of this weakly basic nitrogen the leaving group ability of the amine is expected to be improved. Therefore, in order to assess any special reactivity of the P-lactam antibiotics, the dependence of the rate of hydrolysis of simple amides and P-lactams upon substituents must be known. 

The observed pseudo first-order rate coefficient for the hydrolysis of benzylpenicillin is first order in hydroxide ion up to 2 M sodium hydroxide. Above this concentration it begins to level off (Minhas and Page 1982). This is probably attributable to ionisation of the benzylamido side chain. Presumably hydroxide ion attack on the penicillin with a 6-amido anion side chain is retarded. In support of this, the observed rate constant for the hydrolysis of phenoxymethylpenicillin shows a non-linear dependence upon hydroxide ion above 0.1 M sodium hydroxide (Minhas and Page, 1982 Pratt et al., 1983). The more electron-withdrawing phenoxymethyl group decreases the pA j-value of the amide side chain to 13.3. The observed first-order rate eonstants for the hydrolysis of 6-aminopenicillanic acid are, as expected, linear in hydroxide ion concentration. 

If the mechanisms of the reactions of penicillin involves the intermediate formation of a keten [47] or penicillenic acid [38], then the products of the reaction should show deuterium incorporation at C(6) if the reactions are carried out in DjO. The nmr spectra of benzylpenicilloic acid and penicilloyl amides obtained from the hydrolysis and aminolysis of benzylpenicillin in DjO in the presence of copper(II) ion shows no incorporation of deuterium at C(6). This indicates that the elimination-addition mechanism is not a major pathway for either of these reactions (Gensmantel et al., 1978). 

Zinc(II) and tris-buffers are effective catalysts for the aminolysis of benzylpenicillin. It is suggested that this is due to formation of a ternary complex in which the metal ion binds both penicillin and tris. Nucleophilic attack of the ionised hydroxyl on bound tris forms a penicilloyl ester which may then react with tris to form a penicilloyl amide (Schwartz, 1982 Tomida and Schwartz, 1983). A kinetically equivalent mechanism, however, would simply involve nucleophilic attack of tris on the zinc-penicillin complex. 

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