The three-dimensional structure of d[CGTA'CG], where A' = [2-NH2-A], was determined to atomic (1.35 A) resolution by single isomorphous replacement. The d[CGTA'CG] hexamer crystallizes in space group P3221, and is not isomorphous with other DNA hexanucleotides. Despite completely different crystal packing, the essential characteristics of the Z-DNA conformation are maintained. The structure was determined by single isomorphous replacement using a triammine platinum fragment. Thus, this study also demonstrates, for the first time, the feasibility of the use of this reagent for the direct phasing of DNA crystal structures.

The stoichiometry of the interaction of reporter molecules, 1, 2,4-(NOa)aC6HsNH(CHz)aN+H-(CH,), Br-, and 2 , ~,~-(N~z)~C~H~NH(CH~)~N+(CH~)~(CHZ)~N+(CH~)~. 2Br-, with salmon sperm DNA is reported. A spectrophotometric technique is used to determine the apparent binding constant, k ' R . Moreover, proton magnetic resonance studies on the free and DNA-bound reporter molecules, together with viscosity measurements of DNA solution in the presence and absence of reporter molecules, permit the following conclusions: (1) H-bonding, electrostatic, and "hydrophobic type" forces are involved in the binding process; (2) a site-binding model adequately explains the nucleic acid reporter complex; (3) pmr studies show that the rate of tumbling of the reporter molecule in the DNA complex is considerably diminished with a concomitant upfield chemical shift which is probably indicative of shielding due to an aromatic ring current effect; the results are consistent with an intercalation model; (4) viscosity measurements support the pmr studies and indicate that the reporter molecules 1 and 2 are intercalated.(1) (a) Henry Rutgers Scholar (1968)(1969); (b) to whom correspondence should be addressed at the

The structure of the synthetic dodecamer (d[CGTGAATTC(O6Me)GCG])2 has been determined to a resolution of 2.25 A and refined to a final R factor of 16.7%. The volume of the unit cell is significantly smaller by 16% than the original Drew and Dickerson parent dodecamer [Drew, H. R., Wing, R. M., Takano, T., Broka, C., Tanaka, S., Itakura, K., & Dickerson, R. E. (1981) Proc. Natl. Acad. Sci. U.S.A. 78, 7318-7322]. The double helix is in a different position in the unit cell, rotated by -85.9 degrees, and translated by 9.9 A around the helical axis with respect to the parent structure. The intermolecular arrangement of helices, characterized by double hydrogen bonded guanine-guanine minor groove interactions, remains conserved. The molecular geometry exhibits several significant changes that are related to the changed position of the helix and the presence of two mismatched base pairs with O6-methylguanine. Both mispairs are found in a symmetrical T(anti).(O6Me)G(anti) conformation, and the methyl groups are in proximal orientation. The hydration pattern of the structure is different and can be related to changes in the minor groove geometry. An incorrect model that was isomorphous to the parent dodecamer could be refined to a low R factor. Characteristics of the refinement and of the geometry that are indicative of incorrect structures have been analyzed.

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