Recent advances in DNA synthesis methods have made it possible to carry out single-crystal x-ray analyses of double-stranded DNA molecules of predetermined sequence, with 4 to 12 base pairs. At least one example has been examined from each of the three known families of DNA helix: A, B, and Z. Each family has its own intrinsic restrictions on chain folding and structure. The observed solvent positions in these crystal structures have confirmed earlier fiber and solution measurements, and have led to proposals explaining the transitions from B to A and from B to Z helices. Prospects are improving for an understanding of the mode of bending of DNA in chromatin, and the way in which specific DNA sequences are recognized by drug molecules and repressor proteins.

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The DNA tetramer d(ICpCpGpG) or ICCGG crystallizes as a double-stranded 4-base pair (bp) segment of an A helix. Two such tetramer helices are packed together in the crystal with local helix axes nearly coincident, simulating an 8-bp helix, and four such octamers make up the tetragonal unit cell. Restrained energy and reciprocal space refinement has led to an R factor of 20.5% at 2.1 A resolution. The ICCGG helix has a twist corresponding to 10.7 bp per turn, a 19 degree base tilt and a 2.3 A rise per base pair along the helix axis. The mean propeller twist of 18 degree is comparable with, and has the same rotational sense as that observed in the B-DNA dodecamer CGCGAATTCGCG at similarly high alcohol concentration. Backbone phosphate groups in A-DNA are extensively hydrated, including a network across the opening of the major groove, whereas base edge N and O groups in major and minor grooves are less hydrated than in B-DNA. The minor groove spine of hydration observed in B-DNA is totally absent. These observations of relative hydration confirm and extend the model for the B- to A- helix transition proposed earlier on the basis of the B helix structure.

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