Circular dichroism spectra of tetracycline complexes with Mg+2 and Ca+2

The study of Ca+2 and Mg+2 complexes of tetracycline in buffered solution was undertaken to determine their stoichiometry and the chelation sites. Circular dichroism was used to follow complex formation. Modified tetracyclines, in which potential complexation sites were blocked, were used to determine the participation of particular sites in complexation. Calcium formed a 2:1 metal-ion to ligand complex, while the magnesium complex formed at a 1:1 ratio. Formation of the calcium complex involved addition of one metal ion to the C-10, C-11 site with subsequent addition of a second metal ion at the C-12, C-1 site. The magnesium chelate occurred at the C-11, C-12 beta-diketone site.