Circular Dichroism Measurements of the Tetracyclines IV. 5-Hydroxylated Derivatives

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Abstract
From extensive circular dichroism measurements of a variety of tetracycline (TC) derivatives in the presence and absence of a variety of chelating ions, it is concluded that 5-hydroxytetracycline possesses an identical solution conformation to that possessed by all fermentation-derived, bioactive TC species in dilute aqueous solutions at p H 7.5 or below. These conditions resemble those encountered under normal physiological circumstances. In alkaline solutions, the conformation of of 5-hydroxytetracycline diverges from the non-5-hydroxylated subclass of TC species. This divergence is rationalized as due to a buttressing effect of colinear peri -substituents at positions 4, 5, and 6 and is assisted by the possibility of hydrogen bonding between the 5 and 12a hydroxyl groups. Chelation, in alkaline solutions, with Mg ions “locks” the molecule into a conformation which has been detected by others in nuclear magnetic resonance studies in concentrated non-aqueous solvents and in X-ray studies with the 5, 12a-diacetoxy analogue. Parallel studies with a variety of model TC species and Ca ions provide strong support for the hypothesis that both Ca and Mg ions bind at the BCD juncture and that, above p H 7.5, the Ca ion binds to the A ring between 4-NMe 2 and 12a-OH, whereas Mg ion does not do this. If a 5-OH group is present, the Ca ion still binds as before, whereas the Mg ion will bind to the AB rings between 5-OH and 12a-OH. These factors are discussed in conjunction with the various conformations potentially available to the molecule.