The DNA fragment d(CpGpCpGpCpG) crystallises as a left-handed double helical molecule with Watson-Crick base pairs and an antiparallel organisation of the sugar phosphate chains. The helix has two nucleotides in the asymmetric unit and contains twelve base pairs per turn. It differs significantly from right-handed B-DNA.

The structure of the crystalline daunomycind(CpGpTpApCpG) complex has been solved by x-ray diffraction analysis. The DNA forms a six-base-pair right-handed double helix with two daunomycin molecules intercalated in the d(CpG) sequences. The daunomycin aglycone chromophore is oriented at right angles to the long dimension of the DNA base pairs and the cyclohexene ring rests in the minor groove. Substituents on this ring have hydrogen bonding interactions to the base pairs above and below the intercalation site. These appear to be specific for anthracycline antibiotics. The amino sugar lies in the minor groove of the double helix without bonding to the DNA. The DNA double helix is distorted in a novel manner in accommodating the drug.An important group of antibiotics are those that interact with DNA and have antitumor activity. Daunomycin, an anthracycline antibiotic, has been found to inhibit virus multiplication and shows considerable activity against tumors; it was the first antibiotic to show activity against acute leukemia in man. It has an aglycone chromophore containing four fused rings and an amino sugar (Fig. 1). The closely related antitumor agent adriamycin (14-hydroxydaunomycin) is widely used in treating various solid tumors. These compounds have been the subject of intensive chemical and biological research since their discovery 17 years ago. More than 500 compounds of this type have been synthesized or isolated from nature and tested for activity (1, 2). These agents are believed to act by binding to DNA and inhibiting both DNA replication and transcription. Several studies suggest that the unsaturated chromophore intercalates between DNA base pairs (1, 2). However, the complexity of the molecule makes its mode of interaction with DNA a matter of considerable interest.Here we report the crystal structure of daunomycin with a self-complementary DNA fragment, d(CpGpTpApCpG). The structure has been solved by x-ray diffraction methods. A sixbase-pair fragment of double-helical DNA was found with two molecules of daunomycin bound to it, plus 80 mM spermine, and 30 mM cacodylate buffer (pH 6.5) was equilibrated with 30% (vol/vol) 2-methyl-2,4-pentanediol. After 1 week, attractive bright red-orange, tetragonal, rod-like crystals appeared. Spectroscopic analysis of the dissolved crystal revealed a 1:1 ratio of hexanucleotide to daunomycin. X-ray diffraction studies indicated a tetragonal crystal system with space group P4 212 or P43212, a = b = 27.92 A, c = 52.89 A.The asymmetric unit contained one hexanucleotide and one daunomycin molecule. Intense meridional reflections at 3.3 A along the c axis strongly suggested that the bases were stacked perpendicular to that direction. Three-dimensional data were collected on a Nicolet x-ray diffractometer to a resolution of

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