title: Structure of poly (I).poly (A).poly (I).
abstract: The molecular structure of poly (I).poly (A).poly (I) has been determined and refined using the continuous intensity data on layer lines in the x-ray diffraction pattern obtained from an oriented fiber of this polymorphic RNA complex. The polymer forms a 12-fold right-handed triple-helix of pitch 39.7A and each base-triplet is stabilized by quasi Crick-Watson-Hoogsteen hydrogen bonds. The ribose rings in all the three strands have C3'-endo conformations. The final R-value for this best structure is 0.24 and the x-ray fit is significantly superior to all the alternative structures where the different chains might have different furanose conformations. This all-purine triple-helix, counter-intuitively, has a diameter roughly 3A shorter than that of DNA and RNA triple-helices containing a homopurine and two complementary homopyrimidine strands. Its compact, grooveless cylindrical shape is consistent with the lack of lateral organization.
authors.affiliation: Whistler Center for Carbohydrate Research, Purdue University, West Lafayette, IN 47907, USA. chandra@purdue.edu
authors.forename: R
authors.initials: R
authors.lastname: Chandrasekaran
authors.forename: A
authors.initials: A
authors.lastname: Giacometti
authors.forename: S
authors.initials: S
authors.lastname: Arnott
mesh_terms: D000225:Adenine / Q000737:chemistry; D004247:DNA / Q000737:chemistry*; D006860:Hydrogen Bonding; D008958:Models, Molecular; D015233:Models, Statistical; D009690:Nucleic Acid Conformation; D011061:Poly A / Q000737:chemistry*; D011069:Poly I / Q000737:chemistry*; D012266:Ribose / Q000737:chemistry; D014961:X-Ray Diffraction
chemical_list: C051802:triplex DNA; D011061:Poly A; D011069:Poly I; D012266:Ribose; D004247:DNA; D000225:Adenine