Interaction of metal ions with nucleic acids and related compounds. II. Studies on Au(III)‐nucleic acid system

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Abstract
The nature of interaction of Au(III) with nucleic acids was studied by using methods such as uv and ir spectrophotometry, viscometry, pH titrations, and melting‐temperature measurements. Au(III) is found to interact slowly with nucleic acids over a period of several hours. The uv spectra of native calf‐thymus DNA 9pH 5.6 acetate buffer containing (0.01MNaCIO4) showed a shift in λ max to high wavelengths and an increase in optical density at 260 nm. There was a fourfold decrease in viscosity (expressed as ηsp/c). The reaction was faster at pH 4.0 and also with denatured DNA (pH 5.6) and whole yeast RNA (pH 5.6). The order of preference of Au(III) (as deduced from the time of completion of reaction) for the nucleic acids in RNA > denatured DNA > DNA. The reaction was found to be completely reversible with respect KCN. Infrared spectra of DNA‐Au(III) complexes showed binding to both the phosphate and bases of DNA. The same conclusions were also arrived at by melting‐temperature studies of Au(III)‐DNA system. pH titrations showed liberation of two hydroxylions atr= 0.12 [r= moles of HAuCl4added per mole of DNA‐(P)] and one hydrogen ion atr= 0.5. The probable binding sites could be N(1)/N(7) of adenine, N(7) and/or C(6)O of guanine, N(3) of cytosine and N(3) of thymine.DNAs differing in their (G = C)‐contents [Clostridium perfingensDNA(G = C, 29%), salmon sperm DNA (G + C, 42%) andMicrococcus lysodeikticusDNA(G + C, 29%), salmon sperm DNA (G = C, 72%)] behaved differently toward Au(III). The hyperchromicity observed for DNAs differing in (G + C)‐content and cyanide reversal titrations indicate selectivity toward ( A + T)‐rich DNA at lw values ofr. Chemical analysis and job's continuous variation studies indicated the existence of possible complexes above and belowr= 1. The results indicate that Au(III) ions probably bind to hte phosphate group in the initial stages of the reaction, particularly at low values ofr, and participation of the base interaction also increases. Cross‐linking of the two strands by Au(III) may take place, but a complete collapse of the doulbe helix is not envisaged. It is probable that tilting of the bases or rotaiton of the bases around the glucosidic bond, resulting in a significant distrotion of the double helix, might take place due to binding of Au(III) to DNA.