title: The influence of Mg++ and Zn++ on polypurine-polypyrimidine multistranded helices.
abstract: We have studied by X-ray diffraction fibres of complexes of polypurine-polypyrimidine with divalent cations. In the presence of Mg++, poly(dC) and poly(dG) form a very stable triple helix at neutral pH, based on G-G-C triplexes, whereas Zn++ prevents its formation, both at neutral and acidic pH. The poly(dC) . poly(dG) complex with Zn++ is of the B form, but its X-ray diffraction pattern shows an unusual intensity distribution. This is probably due to the fact that counterions occupy defined positions on the helix. The A form has not been observed. With poly[d(A-G)].poly [d(C-T)] a different triple helical structure is formed, both with Zn++ and Mg++. Direct, X-ray diffraction evidence for these triple helices is provided here for the first time.
authors.affiliation: Unitat de Quimica Macromolecular ETSEIB, Barcelona, Spain.
authors.forename: J L
authors.initials: JL
authors.lastname: Campos
authors.forename: J A
authors.initials: JA
authors.lastname: Subirana
mesh_terms: D001482:Base Composition; D004247:DNA / Q000737:chemistry; D008274:Magnesium / Q000737:chemistry*; D009690:Nucleic Acid Conformation; D011066:Poly C / Q000737:chemistry*; D011068:Poly G / Q000737:chemistry*; D011089:Polydeoxyribonucleotides / Q000737:chemistry*; D014961:X-Ray Diffraction; D015032:Zinc / Q000737:chemistry*
chemical_list: D011089:Polydeoxyribonucleotides; D011068:Poly G; C023826:poly(dC); C026628:poly(dG); C050366:poly(dG-dA)n.poly(dC-dT)n; D011066:Poly C; D004247:DNA; D008274:Magnesium; D015032:Zinc