title: Symmetry and molecular structure of a DNA triple helix: d(T)n.d(A)n.d(T)n.
abstract: A structure for the triple helix d(T)n.d(A)n.d(T)n consistent with recent infrared spectral data is proposed, and its salient features are discussed. The present structure preserves the pseudodyad between the Watson-Crick base-paired adenine and thymine strands and in addition has a pseudorotational symmetry relating the Hoogsteen-paired adenine and thymine strands. The simultaneous presence of these two symmetries gives rise to a dyad between the two thymine polynucleotides. These symmetries result in identical backbone conformations for all three strands, unlike any previously proposed model for a triple helix. The proposed structure has an axial rise per residue of 3.26 A and 12 residues per turn obtained from X-ray fiber diffraction [Arnott S., & Selsing, E. (1974) J. Mol. Biol. 88, 509-521]. The present structure is structurally and conformationally similar to double helical B-form DNA and has sugar pucker in the C2'-endo region. This structure is fundamentally different from the one proposed by Arnott and co-workers, which was based on structural and conformational features similar to double helical A-form DNA with C3'-endo sugar pucker. It is stereochemically satisfactory, and it does not have the disallowed nonbonded distances present in the earlier model of Arnott and co-workers. It is energetically much more favorable than their structure. Coordinates of the present structure are given.
authors.affiliation: Laboratory of Mathematical Biology, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892.
authors.forename: G
authors.initials: G
authors.lastname: Raghunathan
authors.forename: H T
authors.initials: HT
authors.lastname: Miles
authors.forename: V
authors.initials: V
authors.lastname: Sasisekharan
mesh_terms: D004247:DNA / Q000737:chemistry*; D004563:Electrochemistry; D008958:Models, Molecular; D009690:Nucleic Acid Conformation; D013816:Thermodynamics; D014961:X-Ray Diffraction
chemical_list: D004247:DNA