Ring crystals of oligonucleotides: Growth stages and X-ray diffraction studies

The hexamer duplex d(CACGCG).d(CGCGTG) produces unusual hexagonal ring shaped crystals when crystallized in the presence of cobalt hexammine chloride. Observation of the growth of these crystals over a period of about 6 days indicated that this morphology resulted from radial dissolution of the crystal from the center of the well-formed hexagonal plates. X-ray diffraction studies were carried out for crystals of four stages in the ring crystal morphology. The structure of the DNA in all four crystals was the Z type DNA double helix. The packing mode of the Z DNA duplex in all four crystal structures was that of the ‘spermine’ form. The crystal morphology changes from hexagonal plates to hexagonal rings without any apparent change in the molecular structure of the oligonucleotide or helical packing of the duplexes.