Calf-thymus DNA interaction with Cr(III)-gallate and Cr(III)-ethyl gallate studied by FTIR spectroscopy and capillary electrophoresis

Cr(VI) salts are well known to be carcinogen, and are reduced by various cellular components to form cross-linked Cr(III) products. Organic compounds, such as gallic acid (GA) and ethyl gallate (EGA), reduce Cr(VI) to Cr(III) to form Cr(III)-gallate [Cr(III)-GA] and Cr(III)-ethyl gallate [Cr(III)-EGA] as final products. These Cr(III)-tannin complexes are DNA binders. The interaction of calf-thymus DNA with Cr(III)-GA and Cr(III)-EGA in aqueous solutions at physiological pH were studied at Cr(III)/DNA (phosphate) molar ratios (r) of 1: 160, 1:80, 1:40, 1:20, 1: 10, 1:4, and 1:2 using FTIR spectroscopy and capillary electrophoresis. An analysis by FTIR showed that at low concentrations (r = 1/80 and 1/40), Cr(III)-GA and Cr(III)-EGA mainly bind to the guanine N-7 atom of the G-C base pairs with minor perturbations of the A-T base pairs. At r > 1/20, a partial helix opening occurred with major perturbations of the G-C and A-T base pairs. At r >1/10, aggregation of the Cr(III)-tannin-DNA complexes occurred. However, no DNA conformational changes were observed, and the DNA maintained the B-family structure. The binding constants of the Cr(III)-GA-DNA and Cr(III)-EGA-DNA were estimated to be 3.8 × 104 M-1 and 6.2 × 104 M-1, respectively, by Scatchard plots following capillary electrophoresis. These results suggest that the Cr(III)-tannin complexes are external DNA binders, and do not form a chelate with the DNA.