To determine what topological changes antiparasitic heterocyclic dications can have on kinetoplast DNA, we have constructed ligation ladders, with phased A5 and ATATA sequences in the same flanking sequence context, as models. Bending by the A5 tract is observed, as expected, while the ATATA sequence bends DNA very little. Complexes of these DNAs with three diamidines containing either furan, thiophene or selenophene groups flanked by phenylamidines were investigated along with netropsin. With the bent A5 ladder the compounds caused either a slight increase or decrease in the bending angle. Surprisingly, however, with ATATA all of the compounds caused significant bending, to values close to or even greater than the A5 bend angle. Results with a mixed cis sequence, which has one A5 and one ATATA, show that the compounds bend ATATA in the same direction as a reference A5 tract, that is, into the minor groove. These results are interpreted in terms of a groove structure for A5 which is largely pre-organized for a fit to the heterocyclic amidines. With ATATA the groove is intrinsically wider and must close to bind the compounds tightly. The conformational change at the binding site then leads to significant bending of the alternating DNA sequence.
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