Direct measurement of interaction forces between a platinum dichloride complex and DNA molecules

The interaction forces between a platinum dichloride complex and DNA molecules have been studied using atomic force microscopy (AFM). The platinum dichloride complex, di-dimethylsulfoxide-dichloroplatinum (II) (Pt(DMSO)(2)Cl-2), was immobilized on an AFM probe by coordinating the platinum to two amino groups to form a complex similar to Pt(en)Cl-2, which is structurally similar to cisplatin. The retraction forces were measured between the platinum complex and DNA molecules immobilized on mica plates using force curve measurements. The histogram of the retraction force for lambda-DNA showed several peaks; the unit retraction force was estimated to be 130 pN for a pulling rate of 60 nm/s. The retraction forces were also measured separately for four single-base DNA oligomers (adenine, guanine, thymine, and cytosine). Retraction forces were frequently observed in the force curves for the DNA oligomers of guanine and adenine. For the guanine DNA oligomer, the most frequent retraction force was slightly lower than but very similar to the retraction force for lambda-DNA. A higher retraction force was obtained for the adenine DNA oligomer than for the guanine oligomer. This result is consistent with a higher retraction activation energy of adenine with the Pt complex being than that of guanine because the kinetic rate constant for retraction correlates to exp(F Delta x - Delta E) where Delta E is an activation energy, F is an applied force, and Delta x is a displacement of distance.