The structure of 5a,6-anhydrotetracycline and its Mg2+ complexes in aqueous solution.

Semiempirical molecular orbital theory has been used for a systematic scan of the binding positions for a Mg2+ ion with 5a,6-anhydrotetracycline taking both conformational flexibility and possible different tautomeric forms into account. The magnesium ion has been calculated alone and with four or five complexed water molecules in order to simulate the experimental situation more closely. The results are analyzed by comparing the behavior of the title compound with that of tetracycline itself and possible causes for the stronger induction of the Tetracycline Receptor (TetR) by 5a,6-anhydrotetracycline than by tetracycline are considered. Energetically favored 3D-structure of the zwitteranionic 5a,6-anhydrotetracycline magnesium complex in solution.