Local mobility of nucleic acids as determined from crystallographic data. II. Z-form DNA

Directions and magnitudes of the local mobility of the Z-DNA hexamer duplex CpGpCpGpCpG have been determined by crystallographic refinement of anisotropic displacement parameters using the observed X-ray diffraction data. The cytidine and guanosine residues demonstrate different modes of mobility, implying that a dinucleotide is the smallest repeating unit in terms of flexibility as well as structure. Directions of librational and translational mobility of the cytidine and guanosine residues of Z-DNA are similar to those observed for the same nucleotides in B-DNA. This suggests that the local mobility of DNA is primarily determined by the individual nucleotide type and by the constraints of Watson-Crick base-pairing, rather than by helical form. Differences in the magnitudes of mobility may be responsible for some of the different physical properties of B-DNA and Z-DNA. The B to Z transition is discussed in terms of the observed flexibilities of these two helical forms.