SUNY at Albany
June 19-23, 2001
DNA Binding to Cr(III) and Cr(III)-tannin Complexes Studied
by FTIR Spectroscopy and Capillary Electrophoresis
Cr(VI) compounds are known to be mutagen and carcinogen and reduced by various cellular reductive agents, glutathion, ascorbic acid, etc., into Cr(V), Cr(IV), and Cr(III) as a final product. During the conversion of the Cr(VI) to Cr(III), DNA is consider to bind chromium to form Cr(III)-DNA adducts inducing mutagenesis. Some of the Cr(V) and Cr(IV) compounds have been reported to cause DNA damages and regarded as mutagenic. However, the effect of Cr(III) compounds on DNA remains unknown. Recent reports suggest that the coordinated ligands play an important role in the toxicity of some of the Cr(III)-organic compounds.
In the present study, we have investigated the interaction of calf-thymus DNA with Cr(III) and Cr(III)-tannin complexes, Cr(III)-gallate and Cr(III)-ethylgallate in aqueous solution at physiological pH with chromium/DNA(P) molar ratios (r) of 1:160, 1:80, 1:40, 1:20, 1:10, 1:4 and 1:2. FTIR spectroscopy and capillary electrophoresis were used to determine the cation binding mode, the binding constant and the structural variations of Cr-DNA complexes in aqueous solution.
Spectroscopic results showed that at low cation concentrations (r=1/80 to 1/20). Cr(III) binds directly to the guanine N-7 and the backbone phosphate group (chelation). At higher cation concentration (r>1/10), DNA condensation was observed. On the other hand, at low metal ion concentration, the Cr(III)-tannin complexes bind indirectly to the guanine of the G-C base pairs with minor perturbations of the backbone phosphate group. At high concentration of the Cr(III)-tannin compounds (r>1/10), DNA aggregation occurred. No major DNA conformational changes occurred upon Cr(III) and Cr(III)-tannin interactions and DNA remains in the B-family structure.
Scatchard analysis of Cr(III)-DNA complexes following capillary electrophoresis showed the presence of two types of bindings, one is of high affinity with K=2.3 x 105 M-1 and the other is of low affinity site with K=3.6 x 104 M-1. In contrast, Cr(III)-tannin-DNA adducts showed one type of binding with K= 4 to 6 x 104 M-1. These results in combination with those from the FTIR suggest that Cr(III) binds directly to the guanine and the nearest phosphate group (chelation), while the Cr(III)-tannin complexes bind to guanine through water or tannin ligands (1,2).
- H. Arakawa, R. Ahmad, M. Naoui and H. A. Tajmir-Riahi, J. Biol. Chem. 275, 1150-1153 (2000).
- H. Arakawa, N. Watanabe and H. A. Tajmir-Riahi, Bull. Chem. Soc. Japan,
in press (2001).
H. Arakawa, 1N. Watanabe and H. A. Tajmir-Riahi*
Department of Chemistry-Biology, University of Québec at Trois-Rivières,
C.P. 500, Trois-Rivières, Québec, Canada G9A 5H7
1Department of Arts & Science, Hokkaido Institute of Technology, Teine-ku, Sapporo 006-8585, Japan
*Phone (819)-376-5052 (ext. 3310)