A theoretical investigation of the near UV and VIS electronic spectra for the fully deprotonated forms of anhydrotetracycline

Abstract

Anhydrotetracycline (AHTC) is the major toxic decomposition product of the antibiotic tetracycline with a characteristic electronic spectrum used for identification purposes. In the present work, the electronic spectra of the fully deprotonated forms (L^2-) of the free AHTC molecule, the main species involved in the metal complexation reaction, have been analysed using the spectroscopic version of the INDO/S method. The polarization and specific solute–solvent interaction were analysed using the standard SCRF continuum model and the supermolecule approach. The theoretical band spectra were simulated by fitting the calculated frequency and oscillator strength values to a Lorentzian type function and directly compared with the experimental aqueous solution spectrum. For the L^2-(10-O) deprotonated molecule, the theoretical spectrum in the near UV region showed three absorption bands located at 215, 261 and 336 nm, which are in very good agreement with the observed values: 220, 268 and 335 nm. All transitions were assigned as π→π* of the BCD ring chromophore with contributions of the A ring for the absorption at 261 nm. The same transitions were located at 222 and 261 nm for the L^2-(11-O) and the shoulder at 236 nm is not observed.