THEORETICAL NEAR UV AND VIS ELECTRONIC-SPECTRA FOR THE ZN-II-ANHYDROTETRACYCLINE COMPLEX

The UV-VIS electronic spectra for the free ligand and complexed forms of the Zn-II-AHTC system have been analyzed using the configuration in teraction (CI) procedure as implemented in the ZINDO program. The main transitions in the UV and VIS region are assigned and compared with t he experimental data available. The electronic spectrum for the free l igand, the tautomeric form ionized at O-11 [LH dagger(O-11)], is very close to that observed experimentally, with the calculated absorption band centered at 429 nm and the experimental one at 428 nm. This absor ption is attributed to a pi-->pi transition of the BCD ring system. C omparing the visible spectra for the complexed forms with that obtaine d for the free ligand, a blue shift is observed for all structures ana lyzed, except for the complex II-B, where a red shift from 429 nm (fre e ligand) to 436 nm (complexed form) was calculated. Experimentally, a bathochromic shift from 428 nm to 440 nm was observed with the [Zn-II ]/[AHTC] ratio ranging from 0 to 3. These values are very close to tho se calculated for the complex II-EI, in which the AHTC acts asa triden tate ligand. The possibility of the formation of the binuclear complex (M2L) was discarded, once it was observed that the second metal compl exation shifts the VIS band to lower wavelength values. This effect is not observed in the experimental spectrum at high Zn-II concentration .