Minocycline (MIN) and tigecycline (TIG) complexed with Ca²⁺ substantially more strongly than tetracycline (TC) due to the dimethylamino substituent at C7 that increased electron density at the Ca²⁺ coordination site.

MIN and TIG preferred higher-order (1Ca²⁺–3MIN and 2Ca²⁺–3TIG) complexes at pH 6.8 and 7.5 while TC preferred a 1Ca²⁺–3TC complex at pH 6.8 but a 1Ca²⁺–1TC complex at pH 7.5.

TIG formed a different higher order complex than MIN, suggesting the involvement of the N-t-butylglycylamido substituent at C9 in Ca²⁺ coordination.