A new ZnL2 complex containing two molecules of a tridentate Schiff base derived from 5-methyl-1H-pyrazole (HL) is synthesized and characterized. The photophysical properties of HL and ZnL2 are disclosed and supported by CAMB3LYP DFT/TDDFT calculations. It is shown that there is keto-tautomer stabilization upon excitation with an energetically accessible triplet state in HL, not present in ZnL2, this explaining the differences found in the emissions of the compounds. The intrinsic fluorescence of ZnL2 is used as probe for a detailed study of its binding to human serum albumin. The protein-complex association is thermodynamically favourable and it is shown by fluorescence quenching and time-resolved analysis that the fluorescence quenching involves a mixed mechanism with prevalence of static quenching, which corroborates adduct formation at site I, close to the Trp214 residue. The ability of ZnL2 to bind DNA was also evaluated, as well as its cytotoxic activity against MCF7 (breast), PC3 (prostate) cancer cells and hamster V79 fibroblasts. ZnL2 is a moderate DNA intercalator (Kapp = 3.9 × 104 M-1) and depicts a quite low IC50 value at 48 h against MCF7 cells (IC50 = 530 nM), but much higher for PC3 and V79 cells. The relevance of a more careful speciation evaluation of ZnL2 and other potential metal-based drugs in incubation media used in in vitro tests is highlighted.
Keywords: Antitumor agents; DFT/TDDFT calculations; Fluorescence spectroscopy; Host-guest systems; Tautomerism.
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