title: Kinetics and mechanism of G-quadruplex formation and conformational switch in a G-quadruplex of PS2.M induced by Pb²⁺.
abstract: DNA sequences with guanine repeats can form G-quartets that adopt G-quadruplex structures in the presence of specific metal ions. Using circular dichroism (CD) and ultraviolet-visible (UV-Vis) spectroscopy, we determined the spectral characteristics and the overall conformation of a G-quadruplex of PS2.M with an oligonucleotide sequence, d(GTG(3)TAG(3)CG(3)TTG(2)). UV-melting curves demonstrate that the Pb(2+)-induced G-quadruplex formed unimolecularly and the highest melting temperature (T(m)) is 72°C. The analysis of the UV titration results reveals that the binding stoichiometry of Pb(2+) ions to PS2.M is two, suggesting that the Pb(2+) ions coordinate between adjacent G-quartets. Binding of ions to G-rich DNA is a complex multiple-pathway process, which is strongly affected by the type of the cations. Kinetic studies suggest that the Pb(2+)-induced folding of PS2.M to G-quadruplex probably proceeds through a three-step pathway involving two intermediates. Structural transition occurs after adding Pb(NO(3))(2) to the Na(+)- or K(+)-induced G-quadruplexes, which may be attributed to the replacement of Na(+) or K(+) by Pb(2+) ions and the generation of a more compact Pb(2+)-PS2.M structure. Comparison of the relaxation times shows that the Na(+)→Pb(2+) exchange is more facile than the K(+)→Pb(2+) exchange process, and the mechanisms for these processes are proposed.
authors.affiliation: CAS Key Laboratory of Soft Matter Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, P R China.
authors.forename: Wei
authors.initials: W
authors.lastname: Liu
authors.forename: Hong
authors.initials: H
authors.lastname: Zhu
authors.forename: Bin
authors.initials: B
authors.lastname: Zheng
authors.forename: Sheng
authors.initials: S
authors.lastname: Cheng
authors.forename: Yan
authors.initials: Y
authors.lastname: Fu
authors.forename: Wei
authors.initials: W
authors.lastname: Li
authors.forename: Tai-Chu
authors.initials: TC
authors.lastname: Lau
authors.forename: Haojun
authors.initials: H
authors.lastname: Liang
mesh_terms: D002942:Circular Dichroism; D004247:DNA / Q000737:chemistry*; D054856:G-Quadruplexes; D007700:Kinetics; D007854:Lead / Q000737:chemistry*; D009691:Nucleic Acid Denaturation; D011188:Potassium / Q000737:chemistry; D012964:Sodium / Q000737:chemistry
chemical_list: D007854:Lead; D004247:DNA; D012964:Sodium; D011188:Potassium