We describe the crystal structure of d(GCGAATTCG) determined by x-ray diffraction at atomic resolution level (0.89 Å). The duplex structure is practically identical to that described at 2.05 Å resolution (Van Meervelt, L., Vlieghe, D., Dautant, A., Gallois, B., Pré cigoux, G., and Kennard, O. (1995) Nature 374, 742-744), however about half of the phosphate groups show multiple conformations. The crystal has three regions with different solvent structure. One of them contains several ordered Mg ؉2 ions and can be considered as an ionic crystal. A second region is formed by a network of ordered water molecules with a polygonal organization that binds three duplexes. The third region is formed by channels of solvent in which very few ordered solvent molecules are visible. The less ordered phosphates are found facing this channel. The latter region provides a view of DNA with highly movable charges, both negative phosphates and counterions, without a precise location.Two periods could be distinguished in the x-ray diffraction studies of DNA: the era of fiber diffraction of polymeric DNA/ polynucleotides and the time of single crystal x-ray structures of short fragments. The data available from fiber diffraction were limited and allowed no detailed analysis. The goal of DNA crystallography is, therefore, the study of detailed DNA structures. However, crystallographic data are also limited in some important aspects of DNA structure. For example, it is well known (1) that the solvent environment plays an important role in the conformational behavior and interactions of DNA. To study this feature in detail, it is necessary to work with x-ray data at atomic resolution level. Until recently, such data were available only for Z-DNA fragments. This situation is now changing. Synchrotron radiation of increasing power, together with better detectors and cryotechniques, make possible a third period of DNA crystallography: the epoch of high resolution DNA structures.We recently presented (2) a preliminary account of the structure of the B-DNA fragment (GCGAATTCG) solved at atomic resolution (0.89 Å). This structure had already been solved at 2.05-Å resolution (3, 4). In our previous analysis we described the water spine in the minor groove and compared it with related structures. Here we present a detailed analysis of the same oligonucleotide, which has multiple conformations in several parts of the phosphodiester backbone. We also describe the organization of the solvent, which in some regions shows well resolved water molecules, whereas in other regions it remains disordered, despite the fact that our crystals have been studied at 120 K. Thus, a frozen sample at atomic resolution, instead of showing a highly ordered duplex, demonstrates multiple conformations and regions of disordered solvent.
MATERIALS AND METHODSCrystallization-The d(GCGAATTCG) nonamer was crystallized, using a batch method, in sitting drops containing 0.5 mM DNA duplex, 1 mM acridine-(Arg 4 ) drug-peptide aduct, 20 mM sodium cacodylate buffer, pH 7, 100 mM...