The structural features of the drug-DNA adducts resulted from treatment of DNA with the platinum based mononuclear drug cisplatin and the binuclear drug [{trans-PtCl(NH3)(2)}(2)H2N(CH2)(4)NH2]Cl-2 or bis(platin) have been investigated by atomic force microscopy (AFM). Reduction in the contour length of the DNA fragments has been observed after cisplatin treatment while, compaction and aggregation are found to be the primary structural modifications following treatment with the binuclear drug. The intermolecular interaction upon bis(platin) treatment leads to observation of highly condense aggregates without a distinct sight of single isolated DNA molecule. These differences in drug binding indicate that unlike the mononuclear drug cisplatin, bis(platin) causes extensive interhelical/intermolecular cross-linking through its multiple linking sites. To our knowledge, this is the first report of a comparative AFM study to monitor the effects of a mono- and a binuclear platinum anti-cancer drug on DNA structure. These observations should provide clues towards explaining the distinct biological activities of the two drugs. (c) 2005 Elsevier Inc. All rights reserved.