Rate and proposed mechanism of anhydrotetracycline epimerization in acid solution

The pathway through which the toxic tetracycline degradation product epianhydrotetracycline forms in solution was studied using high-performance liquid chromatography and circular dichroism, taking advantage of the large difference in ellipticity between the reactant and the product at 285 nm. The epimerization of anhydrotetracycline followed a reversible first-order process, and both analytical methods yielded the same rate constants. The rate constants indicate that anhydrotetracycline epimerization is faster than tetracycline epimerization. The equilibrium favored anhydrotetracycline, and the activation energies for the forward and reverse rates were almost the same as those for tetracycline epimerization. The epimerization was catalyzed by phosphate. Activation energies in 0.1 and 1 M phosphate were essentially the same. The equilibrium constants for both anhydrotetracycline and tetracycline favored the natural configuration rather than the epi series. Possible rationalization based on conformational and hydrogen bonding effects is presented.