Adsorption and circular dichroism of tetracycline on sodium and calcium-montmorillonites

The interaction of tetracycline (TC) with a Na+-montmorillonite and a Ca2+-montmorillonite in aqueous media was investigated using a batch technique complemented with X-ray diffraction (XRD) and circular dichroism (CD). The adsorption of TC decreases by increasing the pH in both cases, although Ca2+-montmorillonite is a more effective adsorbent than Na+-montmorillonite in the pH range 6–8.5. In both cases, TC locates in the interlayer spacing, increasing the d0 0 1 basal spacing from nearly 13.7 Å to nearly 22.0 Å. CD data of TC solutions indicates that increasing the pH induces structural changes from the twisted conformation to the extended conformation of the TC molecule. The presence of Ca2+ in the solution enhances this effect in the pH range 6–10.5. Adsorption on Na+-montmorillonite and Ca2+-montmorillonite significantly induces the adoption of the extended conformation of TC. Chelation of TC with Ca2+ ions in the interlayer is also demonstrated. Therefore, Ca2+ ions in the interlayer behave as active sites for TC adsorption on montmorillonite, leading to the formation of montmorillonite-Ca2+-TC complexes. The conformational changes that TC suffers upon adsorption may have important effects on the bioavailability and antimicrobial activity of the antibiotic.

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► Molecular changes of TC adsorbed on Na+ and Ca2+-montmorillonite were studied.
► Cation exchange and Ca-complexes occur when TC are adsorbed on Ca2+-montmorillonite.
► TC adsorption promotes the extended conformation of the molecule.