Spectroscopic investigation of complexation between various tetracyclines and Mg2+ or Ca2+

The complexation of various 'normal' tetracyclines and anhydrotetracycline with Mg2+ and Ca2+ was investigated under physiological conditions (aqueous Tris buffer at pH 7.0 and 8.5) by spectrometric titration. The generated sets of UV-Vis absorption and fluorescence spectra were analysed by two techniques, namely (i) classical evaluation which relies on the occurrence of isosbestic points and (ii) multivariate fitting employing the program Specfit. The latter yields not only the conditional binding constants, but also, for each tested kinetic scheme, the absorption or emission spectra of the individual metal?tetracycline complexes. It was found that absolute values of binding constants and deduced spectra are in all cases in favour of a consecutive metal ion complexation yielding 1 : 1 and 2 : 1 metal?tetracycline ion complexes