Kinetic and thermodynamic aspects of in vitro interphase transfer of tetracyclines II: influence of divalent metal salts.
With a two-phase in vitro model composed of an aqueous pH 5.5 buffer and 1-octanol, the kinetics of the interphase transfer of tetracycline derivatives were examined in the presence and absence of calcium and magnesium salts, and the contribution of some functional group substituents to the "apparent" free energy changes for partitioning of tetracyclines was evaluated. Only small changes were observed in kf, kb, K0w, and apparent functional group free energy changes, deltaFG deltaF, in the presence of divalent metals as compared to values observed in the absence of these metals. Introduction of the C-6beta hydroxyl group on the tetracycline nucleus decreased the apparent K0w because of a positive apparent deltaFG deltaF contribution, whereas introduction of C-5 hydroxyl, C-6alpha methyl, or C-7 chloro groups increased the apparent K0w through negative apparent deltaFG deltaF contributions.