The crystal structure of a DNA octamer d(GCGTACGC) complexed to an antitumor antibiotic, triostin A, has been solved and refined to 2.2 A resolution by x-ray diffraction analysis. The antibiotic molecule acts as a true bis intercalator surrouding the d(CpG) sequence at either end of the unwound right-handed DNA double helix. As previously observed in the structure of triostin A - d(CGTACG) complex (AH.-J. Wang, et. al.Science, 225, 1115-1121 (1984)), the alanine amino acid residues of the drug molecule form sequence-specific hydrogen bonds to guanines in the minor groove. The two central A o T btfse pairs are in Hoogsteen configuration with adenine in the syn conformation. In addition, the two terminal G o C base pairs flanking the quinoxaline rings are also held together by Hoogsteen base pairing. This is the first observation in an oligonucleotide of. Hoogsteen G o C base pairs where the cytosine is protonated. The principal functional components of a bis-intercalative compound are discussed.
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