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The crystal structure of the 1:1 complex of netropsin and the B-DNA decamer d(CCIICICCII)(2) has been elucidated and refined to an R factor of 19.6% and an R(free) of 24.7% using 1790 reflections in the resolution range 8-2.4 A. The complex crystallizes in space group C2, with unit-cell parameters a = 62.40, b = 24.47, c = 36.31 A, beta = 110.09 degrees and one molecule of netropsin in the asymmetric unit; the rest of the minor groove is filled with six water molecules. The structure was solved by the molecular-replacement method using the DNA model d(CCCCCIIIII)(2) from the 2:1 netropsin complex by removing both bound netropsins (Chen et al., 1998). Surprisingly, only one netropsin molecule is found to bind to the present decamer, covering residues 2-6 at the upper stream of the duplex. The positively charged guanidinium head is hydrogen bonded through N1H(2) to the O(2) of cytosine 2 and through N10H(2) to N(3) of inosine 6. The three amide N-H groups of the peptides face the minor groove and form three sets of bifurcated hydrogen bonds with the base atoms. The central part of the drug (C3-N8) is nearly conjugated. The preference of the cytosine carbonyl O2 atoms over the inosine N3 atoms in hydrogen bonding is seen. The drug-bound region has more uniform twists, roll angles, propeller twists and minor-groove widths compared with the water-bound region.

The 2.40 A resolution crystal structure of a side-by-side binding of distamycin A molecules to a DNA octamer d(GTATATAC)2 with an extended alternating TA sequence has been determined. The unit-cell parameters are a = 29.55, b = 42.18, c = 43.38 A, beta = 96.56 degrees, space group P21, with two molecules in the asymmetric unit, in contrast to all previous side-by-side distamycin-DNA complexes which have only a single DNA strand and one drug molecule in the asymmetric unit. The structure was solved by the molecular-replacement method and refined to an R index of 21.0% using 3467 reflections [>/= 2sigma(F)]. The minor grooves of the DNA molecules bind two side-by-side antiparallel staggered distamycins spanning about five base pairs and virtually covering the entire length of the DNA. The octamer duplexes exhibit low-high alternations in the helical twist, sugar puckering and the C-O3' and O3'-P torsion angles, similar to the earlier side-by-side complexes containing inosine bases. The molecules are stacked one over the other along the ac diagonal in an infinite pseudo-continuous helical column with no lateral interactions.

The crystal structure of the alternating RNA r(CGUAC)dG with a 3'-terminal deoxy G residue has been determined at 1.76 A resolution. The crystal belongs to the orthorhombic space group C2221, unit-cell dimensions a = 29.53, b = 44.61 and c = 94.18 A, with two independent duplexes (I and II) per asymmetric unit. The structure was solved by the molecular-replacement method. The final R factor was 18.8% using 4757 reflections in the resolution range 8.0-1.76 A. The model contains a total of 496 atoms and 85 solvent molecules. The two duplexes form the repeating unit and stack in the usual head-to-tail (5',3'/5',3') fashion into a pseudocontinuous helical column. Almost all of the 2'-hydroxyl groups are engaged in the three modes of water-mediated interactions to the base N3/O2 atoms, the sugar O4' atoms and the backbone phosphates. Thus, the 2'-hydroxyl group of RNA is probably contributing to the stability of the duplexes.