title: Cobalt hexammine induced tautomeric shift in Z-DNA: the structure of d(CGCGCA)*d(TGCGCG) in two crystal forms.
abstract: We report here the crystal structure of the DNA hexamer duplex d(CGCGCA)*d(TGCGCG) at 1.71 A resolution. The crystals, in orthorhombic space group, were grown in the presence of cobalt hexammine, a known inducer of the left-handed Z form of DNA. The interaction of this ion with the DNA helix results in a change of the adenine base from the common amino tautomeric form to the imino tautomer. Consequently the A:T base pair is disrupted from the normal Watson-Crick base pairing to a 'wobble' like base pairing. This change is accommodated easily within the helix, and the helical parameters are those expected for Z-DNA. When the cobalt hexammine concentration is decreased slightly in the crystallization conditions, the duplex crystallizes in a different, hexagonal space group, with two hexamer duplexes in the asymmetric unit. One of these is situated on a crystallographic 6-fold screw axis, leading to disorder. The tautomeric shift is not observed in this space group. We show that the change in inter-helix interactions that lead to the two different space groups probably arise from the small decrease in ion concentration, and consequently disordered positions for the ion.
authors.affiliation: Department of Crystallography and Biophysics, University of Madras, Guindy Campus, Chennai 600 025, India.
authors.forename: S
authors.initials: S
authors.lastname: Thiyagarajan
authors.forename: S S
authors.initials: SS
authors.lastname: Rajan
authors.forename: N
authors.initials: N
authors.lastname: Gautham
mesh_terms: D020029:Base Pairing; D003035:Cobalt / Q000737:chemistry*; D018360:Crystallography, X-Ray; D043542:DNA, Z-Form / Q000737:chemistry*; D006860:Hydrogen Bonding; D008958:Models, Molecular; D009838:Oligodeoxyribonucleotides / Q000737:chemistry*
chemical_list: D043542:DNA, Z-Form; D009838:Oligodeoxyribonucleotides; C493798:d(CGCGCA)-d(TGCGCG) DNA hexamer duplex; C032300:hexaamminecobalt(II); D003035:Cobalt