Acceleration and Inhibition of the Hydrolysis of Penicillin G by Dimerization and Cyclodextrin Inclusion.

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  • Acceleration and Inhibition of the Hydr

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Quantitative treatments for the equilibrium and kinetics of dimerizing, cyclodextrin (CD)-binding, and hydrolyzing systems in unbuffered and buffered solutions have been developed and applied to hydrolysis and pH data on the penicillin G (PCG)-CD system. By dimerization of PCG, the acidic hydrolysis of PCG is accelerated, whereas the basic hydrolysis is inhibited. This result is explicable in terms of electrostatic interactions of the PCG dimer with hydrogen ion and hydroxide ion, similar to the micellar effect on chemical reactions. In buffered solutions, the acidic hydrolysis of PCG is inhibited by all of α-, β, and γ-CDs. This is explained in terms of the reaction mechanism and the enolation of the secondary hydroxyl group of CD. γ-CD inhibits the acidic hydrolysis most effectively of the three CDs, since its binding constant is the greatest among them. The dimer of PCG can be incorporated into γ-CD, but not incorporated into α- and β-CDs. In a 154 mmol dm13 potassium chloride solution, the acidic hydrolysis of 5 mmol dm-3 PCG is enhanced by α- and β-CDs. This striking result can be explained by the catalysis of hydrogen carbonate ion. A commercial sample of α-CD catalyzes the acidic hydrolysis of PCG linearly with the concentration of α-CD, whereas a purified sample catalyzes the same reaction, following Michaelis-Menten-like kinetics. CDs, particularly γ-CD, may be used as an additive for the stabilization of PCG.

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