Unstable Hoogsteen base pairs adjacent to echinomycin binding sites within a DNA duplex

The bisintercalation complex present between the DNA octamer [d(ACGTACGT)]2 and the cyclic octadepsipeptide antibiotic echinomycin has been studied by one- and two-dimensional proton NMR, and the results obtained have been compared with the crystal structures of related DNA-echinomycin complexes. Two echinomycins are found to bind cooperatively to each DNA duplex at the CpG steps, with the two quinoxaline rings of each echinomycin bisintercalating between the C.G and A.T base pairs. At low temperatures, the A.T base pairs on either side of the intercalation site adopt the Hoogsteen conformation, as observed in the crystal structures. However, as the temperature is raised, the Hoogsteen base pairs in the interior of the duplex are destabilized and are observed to be exchanging between the Hoogsteen base pair and either an open or a Watson-Crick base-paired state. The terminal A.T base pairs, which are not as constrained by the helix as the internal base pairs, remain stably Hoogsteen base-paired up to at least 45 degrees C. The implications of these results for the biological role of Hoogsteen base pairs in echinomycin-DNA complexes in vivo are discussed.