Kinetics and mechanism of the degradation and epimerization of sodium cefsulodin in aqueous solution.

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The kinetics of the degradation and epimerization of cefsulodin in various buffer solutions were studied at 25°C and 0.6 ionic strength. The overall degradation was a pseudo-first-order reaction in the pH region studied. The rate law of the degradation could be approximated in terms of specific acid-base and water catalyzed hydrolysis, that is k1 (total degradation rate constant)=kH+×αH++k0+kOH-×αOH-. The apparent activation energies of the degradation reaction were 20.7, 22.3, 23.0 and 27.7 kcal mol-1 at pH values of 2, 4, 6, and 9, respectively. The epimerization of cefsulodin was proved to be catalyzed by hydroxide ion from the epimerization rate constant-pH profile, solvent effects using ethanol, and the apparent activation energies (which were 27.0 and 26.1 kcal mol-1 for the apparent forward and reverse epimerization reactions at pH 9.0, respectively). The mechanism of epimerization of cefsulodin is proposed to involve removal of the α-proton of the benzyl side chain by hydroxide ion to form an anioic intermediate. Interactions of cefsulodin with amines and aminoglycosides were also examined. The reaction was pseudo-second-order and the second-order rate constants for various amines and aminoglycosides were compared. It was found that intramolecular catalysis is the predominant factor for amines. An equation is proposed for the second-order rate constants of aminoglycosides. Peaks of unknown products in the alkaline reaction solutions could be separated under the high performance liquid chromatography conditions of the current study.

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