Structure of d(CGCGAATTCGCG) in the presence of Ca2+ ions

The dodecamer d(CGCGAATTCGCG) was the first oligonucleotide to be crystalli zed as a B-DNA duplex. Its structure was analyzed in detail in the early 19 80s. Here we show that, in the presence of Ca2+, it crystallizes in a diffe rent way (R3 space group). The dodecamers form parallel columns of straight duplexes with ten base pairs in the B form. The terminal cytosines in each molecule are disordered, whereas the terminal guanines are placed in the m inor groove of neighbor duplexes. The central GAATTC region is practically identical to that found in the classic structure of the same dodecamer crys tallized in the P2(1)2(1)2(1) space group in the presence of Mg2+ and sperm ine. Its structure is thus independent of the crystallization conditions wh ich have been used.