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Cadmium (II) ion-catalyzed degradation of ampicillin in methanol at 20°C has been studied. It has been observed that the rate values tend to saturate when the concentration of ampicillin or the metal ion is increased. The results obtained in the present study suggest that ampicillin degradation occurs through the formation of a 1:1 (SM) and 2:1 (S 2 M) ampicillin-metal complexes. These complexes decompose giving a single product (absorption maximum at 285 nm; (p =1.82×10 4 l mol − 1 cm − 1) that has been isolated and identified (Cd II (L 2 −) 2 (H 2 O) 4 Na 2). The appearance of this product reflects a first order reaction with respect to the 1:1 complex, with a rate constant of 3.87 ×10 − 2 min − 1 and the existence of an equilibrium between the 1:1 and 2:1 initial complexes. The equilibrium constant value, calculated from kinetic data, is 1.7 × 10 3 l mol − 1 .

The penamaldic derivatives of amoxicillin, ampicillin, and penicillins G and V, stabilized with Zn 2؉ , were obtained from a methanolic medium. The enzymatic kinetic results show that the these derivatives elicit reversible inhibition of the enzyme metallo-␤-lactamase from Bacillus cereus, with inhibition constant values determined at pH 7.0 and 25°C.Bacteria have developed various strategies to deactivate ␤-lactam antibiotics, including the production of ␤-lactamase enzymes. These have been grouped in four classes (6, 10). Classes A, C, and D contain a catalytic serine residue in their active sites, and several have inhibitors utilized in therapeutics (4,15,16). Class B metalloenzymes require one or two zinc ions to carry out the hydrolysis of the ␤-lactams (3) and act on a wide range of ␤-lactam antibiotics, including carbapenems and inhibitors of the serine-dependent enzymes (12). Over the past 10 years, several inhibitors of these enzymes have been discovered, for instance, two esters of benzyloxycarbonylmethyl-6-aminopenicillanic acid (17), a group of ␣-amido-trifluoromethyl alcohols and ketones (18,19), a series of thiol ester derivatives of mercaptoacetic and mercaptophenylacetic acids (7,8,13,14), and derivatives of ␤-methylcarbapenem (11). More recently, derivatives of cysteinyl peptides have also been tested (1).Because thiol derivatives inhibit the metallo-␤-lactamases (1, 2), the present study tests the enzymatic inhibition of the metallo-␤-lactamase of Bacillus cereus by the Zn 2ϩ -stabilized penamaldic derivatives obtained from ampicillin, amoxicillin, and penicillins G and V. The type of inhibition is determined, and its parameters are calculated.The penamaldic derivatives ( Fig. 1) stabilized with Zn 2ϩ as 2:1 (ligand-metal ion) complexes were obtained by the procedure described in a previous paper (9). The results of the chemical analysis of the polycrystalline solids obtained and the infrared and 1 H-nuclear magnetic resonance data were compatible with the formation of such complexes. According to the thermogravimetric analysis data, the structures of all solids feature two water molecules of crystallization, and the derivatives of ampicillin and amoxicillin have a molecule of NaCl.The spectra of the penamaldic derivatives in the 50 mM MOPS (morpholinepropanesulfonic acid) medium, at pH 7 and 25°C, scarcely varied over time. Generally, the degradation of the four penamaldic derivatives produced an insignificant change in the rate of absorbance over time compared with that for the degradation of substrate in the enzymatic reaction. This may be attributed to the fact that Zn 2ϩ -stabilized penamaldic derivative compounds have a high constant of formation in an aqueous medium. Moreover, the slight instability of the Zn 2ϩ -stabilized compounds in aqueous solution, presumably because of the enamine moiety, can tautomerize to imines, which undergo ester hydrolysis (5). In the presence of enzymes the spectra of the penamaldic derivatives showed the same changes over time as those observed for the der...

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