Structure and properties of the 5a,6-anhydrotetracycline-platinum(II) dichloride complex: a theoretical ab initio study.

Ab initio structural parameters, relative energies and spectroscopic properties are reported for the 5a,6-anhydrotetracycline-platinum(II) dichloride complex. Distinct coordination modes were analyzed in gas phase and aqueous medium indicating the tricarbonylmethane moiety (O3-Oam-O1) as the most probable Pt(II) complexation sites, supporting experimental proposals. The theoretical (1)H NMR analysis in conjunction with the observed data suggests that the Pt(II) might be bound to the O3-Oam preferentially, even though this site was found slightly unfavorable, based on energetic grounds, relative to O1-Oam. The aquation reaction, which is an important activation step in the action mechanism of cisplatin like molecules, was also investigated showing rate constants (0.5-1 x 10(-5) M(-1) s(-1)) of the same order of magnitude as those for the parent cisplatin compound. This is an important result stressing the importance of tetracyclines-Pt(II) complexes as lead compounds for the development of new platinum based antitumor drugs.