Research - Research Design, One Key Element Crystal structure of d(CCAGTACTGG)2 viewed parallel to the helix axis (left) and perpendicular to the helix axis (right). {Structure solved and published by C. L. Kielkopf, S. Ding, P. Kuhn, and D. C. Rees, Conformational Flexibility of B-DNA at 0.74 Å Resolution: d(CCAGTACTGG)2. J. Mol. Biol. 296, 787-801 (2000)} Central to our design is the use of DNA samples (primarily oligodeoxynucleotides) that are structurally well defined. The key samples are crystalline. By employing crystals of DNA, for which the structure has been solved by x-ray diffraction, we maximize our knowledge regarding base sequence, DNA conformation, hydration state, counter ions, packing, and purity. These qualities are illustrated by the example shown above. The oligomer sequence is d(CpCpApGpTpApCpTpGpG); the conformation is B-DNA with a rise of 3.3 Å per residue; there are 13.1 H2O per residue (50.1 % of the crystal is solvent); there are 8 Ca++ per duplex (given that the duplex has no terminal phosphates, there are 8 PO4/duplex.); and the duplexes are packed end-to-end forming continuous cylinders that run parallel to one another. Three features are particularly noteworthy. First, base stacking is seamless at the point of abutment between two decamers; with regard to base stacking, therefore, this DNA has the properties of high molecular weight DNA (104-106 bp). The base-stacking length depends on crystal dimensions, typically 0.01-1.0 mm. By choosing crystals of other oligomers, the length of base stacking is readily limited to the length of one oligomer. Second, in this example all the bases lie in the same plane, optimizing the information that can be obtained by EPR. And third, solvent channels run continuously through the crystals, making it possible to diffuse small molecules such as O2 through the crystal.