Spectroscopic investigation of the Al(III)-anhydrotetracycline complexation process

The Al3+-anhydrotetracycline (AHTC) complexation process is analyzed using the calculated UV/VIS electronic spectra for the free ligand and several di stinct complexed forms. The results obtained for the free ligand suggest tw o possible chemical processes involving ionization and polarization of the zwitterionic structure (LH2+/-) of the AHTC molecule in aqueous solution. T he presence of the ionized structure (LH-(O-11)) in the medium at a ratio c lose to 50% shifts the VIS transition from 356 nm (pure LH2+/-) to 426 nm i n agreement with the experimental data (428nm). The polarized structure was located analyzing the intramolecular proton transference from O-11 to O-12 A red shift of the lowest energy transition was observed as the proton mov es towards the O-12 site; the best result obtained are for an H-11... O-12 distance equal to 1.2 Angstrom (lambda = 405 nm). The analysis of the coord inated structures showed that the complexation site O-11-O-12 Should be fav orable for the Al3+ ion, with a calculated absorption band centered at 461 nm, a result which is in good agreement with the experimental value of 470 nm. (C) 2000 Elsevier Science B.V. All rights reserved.