Title
Observation of a Dynamic G-Tetrad Flip in Intramolecular G-Quadruplexes
Author
Beatrice Karg, Linn Haase, Andrea Funke, Jonathan Dickerhoff, Klaus Weisz
Year
2016
Journal
Biochemistry
Abstract
A MYC sequence forming an intramolecular G-quadruplex with a parallel topology was modified by the incorporation of 8-bromoguanosine (BrG) analogs in one of its outer tetrads. The propensity of the BrG analogs to adopt a syn glycosidic torsion angle results in an exceptional monomolecular quadruplex conformation featuring a complete flip of one tetrad while keeping a parallel orientation of all G-tracts as shown by circular dichroism and NMR spectroscopic studies. When substituting three out of four G-tetrad residues for BrG analogs, two coexisting quadruplex conformational isomers with an all-syn and all-anti outer G-quartet are about equally populated in solution. A dynamic interconversion of the two quadruplexes with an exchange rate of kex = 0.2 s-1 is demonstrated through the observation of exchange crosspeaks in ROESY and NOESY experiments at 50 °C. The kinetic properties suggest disruption of the corresponding outer G-tetrad but not of the whole quadruplex core. Conformational syn-anti isomers with homopolar and heteropolar stacking interactions are nearly isoenergetic with a transition enthalpy of -18.2 kJ/mol in favor of the all-syn isomer.
Instrument
J-810
Keywords
Circular dichroism, DNA structure, Stereochemistry, Biochemistry