STRUCTURE OF THE B-DNA OLIGOMERS D(CGCTAGCG) AND D(CGCTCTAGAGCG) IN NEW CRYSTAL FORMS

We present the structure of the dodecamer CGCTCTAGAGCG and the related octamer CGCTAGCG, both in the B form, determined by single crystal X- ray diffraction. Two different crystal forms of the octamer have been obtained, with either three or four duplexes in the asymmetric unit. T he dodecamer crystallizes in the P2(1) space group with two duplexes i n the asymmetric unit. Very few such structures have been previously r eported, while the octamer structure is the first one determined with three duplexes in the asymmetric unit. It is also the first octamer wi th standard Watson-Crick base pairs to be crystallized in the B form. The crystal structure is stabilized in both cases by interactions betw een the guanines in the two terminal base pairs of each duplex. This i nteraction is similar to that found in most dodecamers which have been previously studied, but here it is found in a new unit cell (for the dodecamer) and in one octamer. In the dodecamer cytosine-stacking inte ractions between neighbor duplexes are also present. The two dodecamer duplexes in the asymmetric unit show different patterns of bending, w hile the octamer molecule has a rather straight helical axis. The resu lts presented confirm the strong conformational variability of the TA pyrimidine-purine step and demonstrate a clear alternating structure f or the (CT/GA)(n) sequence.