The structure of a DNA octamer d(GCGTACGC) cocrystallized with the bisintercalator antibiotic triostin A has been solved. The DNA forms an unwound right-handed double helix. Four base pairs are of the Watson-Crick type while four are Hoogsteen base pairs, including two A · T and two G · C base pairs. This is the first observation in an oligonucleotide of Hoogsteen G · C base pairs where the cystosine is protonated. It is likely that these also occur in solutions of DNA complexed to this antibiotic.
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