Metal complexes of anhydrotetracycline. 2. Absorption and circular dichroism study of Mg(II), AMID, and Fe(III) complexes. Possible influence of the Mg(II) complex on the toxic side effects of tetracycline

Abstract

Anhydrotetracycline (AHTC) is the major toxic decomposition product of the antibiotic tetracycline. The complexation of AHTC to Mg(II), AI(III), and Fe(III) was studied in aqueous medium using absorption and circular dichroism measurements. The study of the Mg(II)-AHTC interactions at pH 7 indicated the formation of the MHL and M 2L species in which an Mg(II) ion is coordinated to the C11 and C12 oxygens of the BCD ring system. In the M 2L species, a second metal ion coordinates to the N4 and 03 positions on ring A, inducing the ligand to adopt the “twisted” conformation. At pH 4, an MHL species is formed with AXIII) by complexation of the metal ion to 011 and 012. At pH 1, Fe(III) forms an MH2L species, probably by coordination of the metal to 012 and 01. The stability constants of all species were calculated. The possible participation of Mg(II) in the mechanism of toxicity of tetracycline is suggested.