The crystal structure of the tetradecanucleotide sequence d(CCCCGGTACC-GGGG) 2 has been determined at 2.5 Å resolution in the tetragonal space group P4 1 . This sequence was designed with the expectation of a four-way junction. However, the sequence crystallized as an A-DNA duplex and represents more than one full turn of the A-helix. The crystallographic asymmetric unit consists of one tetradecanucleotide duplex. The structural parameters of the A-type DNA duplex structure and the crystal-packing arrangement are described. One Mn 2+ ion was identified with direct coordination to the N7 position of G 13 and a water molecule at the major-groove side of the C 2 ÁG 13 base pair.

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Crystal structures of the hexanucleotide d(CACGCG)·d(CGCGTG) were determined in two crystal lattices when different concentrations of the counterion Mn2+ were used in crystallization. The availability of Mn2+ during the crystallization process appears to play an important role in inducing different crystal packings that lead to crystals belonging to the two space groups P2(1) and P6(5). Analysis of the molecular interactions of Mn2+ with the Z-form duplexes shows direct coordination to the purine residues G and A.

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