X-ray analysis of the complex of netropsin with the B-DNA dodecamer of sequence C-G-C-G-A-A-T-TBrC-G-C-G reveals that the antitumor antibiotic binds within the minor groove by displacing the water molecules of the spine of hydration. Netropsin amide NH furnish hydrogen bonds to bridge DNA adenine N-3 and thymine 0-2 atoms occurring on adjacent base pairs and opposite helix strands, exactly as with the spine of hydration. The narrowness of the groove forces the netropsin molecule to sit symmetrically in the center, with its two pyrrole rings slightly non-coplanar so that each ring is parallel to the walls of its respective region of the groove. Drug binding neither unwinds nor elongates the double helix, but it does force open the minor groove by 0.5-2.0 A, and it bends back the helix axis by 8°across the region of attachment. The netropsin molecule has an intrinsic twist that favors insertion into the minor groove of B-DNA, and it is given a small additional twist upon binding. The base specificity that makes netropsin bind preferentially to runs of four or more ACT base pairs is provided not by hydrogen bonding but by close van der Waals contacts between adenine C-2 hydrogens and CH groups on the pyrrole rings of the drug molecule. Substitution of one or more pyrroles by imidazole could permit recognition of G'C base pairs as well, and it could lead to a class of synthetic "lexitropsins," capable of reading any desired short sequence of DNA base pairs.Netropsin and its close relative distamycin ( Fig. 1) are antiviral antitumor antibiotics that, although too toxic for clinical use, have received extensive study as the paradigms of base-specific yet non-intercalative DNA-binding drug molecules. First isolated from Streptomyces netropsis in 1951 (1, 2), netropsin exerts its biological activity by binding tightly to double-helical B-DNA, interfering with both replication and transcription (3, 4). It shows little or no affinity for single-stranded DNA or RNA or for double-stranded RNA or DNA-RNA hybrids (3-5), suggesting that it does not bind to the A helix. It also fails to bind to left-handed Z-DNA; in fact, binding of netropsin to DNA favors A-to-B and Z-to-B helix transitions (6, 7).Chemical protection studies (3,4,8) and Overhauser NMR experiments (9) indicate that netropsin does not intercalate between base pairs, but it binds within the minor groove of the intact double helix, using hydrogen bonds between netropsin amide NH and exposed adenine N-3 and thymine 0-2 on the floor of the minor groove. The drug molecule attaches to clusters of four or more A'T or ITC, but not to G-C, base pairs (3, 10, 11). Alternating A-T-A-T regions bind netropsin less well than continuous runs of A or T (12, 13). Binding involves both an electrostatic component from the two cationic ends and hydrogen bonds from the central three amide NH groups, although neither aspect is absolute- FIG. 1. The netropsin molecule can be regarded as being assembled from (left to right): guanidinium, amide, methylpyrrole, amide, methylpyrr...

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One of the questions that constantly is asked regarding x-ray crystal structure analyses of macromolecules is: To what extent is the observed crystal structure representative of the molecular conformation when free in solution, and to what degree is the structure perturbed by intermolecular crystal forces? This can be assessed with DNA oligomers because of an unusual aspect of crystallization self-complementary oligomers should possess a twofold symmetry axis normal to their helix axis, yet more often than not crystal of such oligomers do not use this internal symmetry. The two ends of the helix are crystallographically distinct though chemically identical. Complexes of DNA oligomers with intercalating drugs such as triostin A tend to use their twofold symmetry when they crystallize, whereas complexes with non-intercalating, groove-binding drugs ignore this symmetry unless the drug molecule is very small. A detailed examination of crystal packing in the dodecamer C-G-C-G-A-A-T-T-C-G-C-G provides an explanation of all of the foregoing behavior in terms of the mechanism of nucleation of DNA or DNA-drug complexes on the surface of a growing crystal. Asymmetry of the ends of the DNA helix is the price that is paid for efficient lateral packing of helices within the crystal. The actual end-for-end variation in standard helix parameters is compared with the experimental noise level as gauged by independent re-refinement of the same oligonucleotide structure where available, and with the observed extent of variation of these same parameters along the helix. Oligomers analyzed are the B-DNA dodecamer C-G-C-G-A-A-T-T-C-G-C-G, the A-DNA octamer G-G-T-A-T-A-C-C, and the phosphorothioate analogue of the B-DNA hexamer G-C-G-C-G-C. End-for-end variation, presumably the result of crystal packing is typically double the experimental noise level, and half the variation in the same parameter along the helix. Analysis of crystal packing in the phosphorothioate hexamer, which uses the same P212121 space group as the dodecamer, shows that the highly unsymmetrical B1 vs. BII backbone conformation probably is to be ascribed to crystal packing forces, and not to the sequence of the hexamer.

When cisplatin [cis‐ diamminodichloroplatinum (II)] is diffused into pre‐grown crystals of the B‐DNA double‐helical dodecamer C‐G‐C‐G‐A‐A‐T‐T‐C‐G‐C‐G, it binds preferentially to the N7 positions of guanines, with what probably is an aquo bridge between Pt and the adjacent O6 atom of the same guanine. The entire guanine ring moves slightly toward the platinum site, into the major groove. Only three of the eight potential cisplatin binding sites on guanines actually are occupied, and this differential reactivity can be explained in terms of the relative freedom of motion of guanines toward the major groove. This shift of guanines upon ligation may weaken the glycosyl bond and assist in the depurination that leads to mismatch SOS repair and G.C. to T.A. transversion.