1982 Volume 30 Issue 6 Pages 2181-2188
Ampicillin, buffered to pH 8.00, in the presence of benzaldehyde was found to degrade according to pseudo-first-order kinetics at 35°C, and its rate constant was smaller than that of ampicillin alone. This stabilization was assumed to be attributable to the formation of ampicillin-benzaldehyde complex. Therefore, pseudo-first-order rate constants of ampicillin in the presence of various amounts of benzaldehyde were determined. It was concluded that a 1 : 1 molar complex was formed between ampicillin and benzaldehyde, and that the reversible complexation reaction was very fast compared to the degradation reaction of ampicillin or the complex. From the degradation behavior in various concentrations of buffer at the same pH, it was clear that the stability constant of the complex was independent of buffer concentration, and that the degradation of complexed ampicillin was affected by general acidbase catalysis in the same way as that of uncomplexed ampicillin. The complexation was not observed at pH<6.00, while the degradation of ampicillin was inhibited by the addition of benzaldehyde at pH>7.00. The suppresion of the degradation of complexed ampicillin seems to be due to the inhibition of hydroxy ion attack on the β-lactam carbonyl group of the complex. Furthermore, it was shown that only the ampicillin anion (uncharged α-amino group) participates in the complexation, because the stability constant of the complex increased with increase of pH.
