The anticancer drugs adriamycin and daunomycin have each been crystallized with the DNA sequence d(CGATCG) and the three-dimensional structures of the complexes solved at 1.7- and 1.5-A resolution, respectively. These antitumor drugs have significantly different clinical properties, yet they differ chemically by only the additional hydroxyl at C14 of adriamycin. In these complexes the chromophore is intercalated at the CpG steps at either end of the DNA helix with the amino sugar extended into the minor groove. Solution of the structure of daunomycin bound to d(CGATCG) has made it possible to compare it with the previously reported structure of daunomycin bound to d(CGTACG). Although the two daunomycin complexes are similar, there is an interesting sequence dependence of the binding of the amino sugar to the A-T base pair outside the intercalation site. The complex of daunomycin with d(CGATCG) has tighter binding than the complex with d(CGTACG), leading us to infer a sequence preference in the binding of this anthracycline drug. The structures of daunomycin and adriamycin with d(CGATCG) are very similar. However, there are additional solvent interactions with the adriamycin C14 hydroxyl linking it to the DNA. Surprisingly, under the influence of the altered solvation, there is considerable difference in the conformation of spermine in these two complexes. The observed changes in the overall structures of the ternary complexes amplify the small chemical differences between these two antibiotics and provide a possible explanation for the significantly different clinical activities of these important drugs.

The crystal structure of a daunomycin-d(CGTACG) complex has been solved by X-ray diffraction analysis and refined to a final R factor of 0.175 at 1.2-A resolution. The crystals are in a tetragonal crystal system with space group P4(1)2(1)2 and cell dimensions of a = b = 27.86 A and c = 52.72 A. The self-complementary DNA forms a six base pair right-handed double helix with two daunomycin molecules intercalated in the d(CpG) sequences at either end of the helix. Daunomycin in the complex has a conformation different from that of daunomycin alone. The daunomycin aglycon chromophore is oriented at right angles to the long dimension of the DNA base pairs, and the cyclohexene ring A rests in the minor groove of the double helix. Substituents on this ring have hydrogen-bonding interactions to the base pairs above and below the intercalation site. O9 hydroxyl group of the daunomycin forms two hydrogen bonds with N3 and N2 of an adjacent guanine base. Two bridging water molecules between the drug and DNA stabilize the complex in the minor groove. In the major groove, a hydrated sodium ion is coordinated to N7 of the terminal guanine and the O4 and O5 of daunomycin with a distorted octahedral geometry. The amino sugar lies in the minor groove without bonding to the DNA. The DNA double helix is distorted with an asymmetrical rearrangement of the backbone conformation surrounding the intercalator drug. The sugar puckers are C1,C2'-endo, G2,C1'-endo, C11,C1'-endo, and G12,C3'-exo. Only the C1 residue has a normal anti-glycosyl torsion angle (chi = -154 degrees), while the other three residues are all in the high anti range (average chi = -86 degrees). This structure allows us to identify three principal functional components of anthracycline antibiotics: the intercalator (rings B-D), the anchoring functions associated with ring A, and the amino sugar. The structure-function relationships of daunomycin binding to DNA as well as other related anticancer drugs are discussed.

The molecular structure of triostin A, a cyclic octadepsipeptide antibiotic, has been solved complexed to a DNA double helical fragment with the sequence CGTACG (C, cytosine; G, guanine; T, thymine; A, adenine). The two planar quinoxaline rings of triostin A bis intercalate on the minor groove of the DNA double helix surrounding the CG base pairs at either end. The alanine residues form hydrogen bonds to the guanines. Base stacking in the DNA is perturbed, and the major binding interaction involves a large number of van der Waals contacts between the peptides and the nucleic acid. The adenine residues in the center are in the syn conformation and are paired to thymine through Hoogsteen base pairing.

Two members of the quinoxaline antibiotic family, echinomycin and triostin A, form crystals complexed to a DNA fragment with the sequence d(CpGpTpApCpG). The crystal structure of both complexes was solved by X-ray diffraction to near-atomic resolution. The two structures are similar to each other with differences in some details due to the shorter cross bridge of echinomycin. Both molecules act as bis intercalators surrounding the d(CpG) sequence at either end of the double helix. Alanine forms sequence-specific hydrogen bonds to guanines in the minor groove. The two central AT base pairs are held together by Hoogsteen base pairing with adenine in the syn conformation in both complexes. An octahedrally hydrated magnesium ion is found in the crystal lattice that plays an important role in organizing the lattice as well as stabilizing the complex by hydrogen bonding both to base pairs of DNA and to the quinoxaline ring nitrogen atoms in the major groove side of the DNA double helix. A functional description of the various amino acids in quinoxaline antibiotics is given, together with possible modifications that might affect biological activity.

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