Observation of a Dynamic G-Tetrad Flip in Intramolecular G-Quadruplexes

Abstract

A MYC sequence forming an intramolecular G-quadruplex with a parallel topology was modified by the incorporation of 8-bromoguanosine ( Br G) analogues in one of its outer G-tetrads. The propensity of the Br G analogues 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 nuclear magnetic resonance spectroscopic studies. When substituting three of the four G-tetrad residues with Br G analogues, two coexisting quadruplex conformational isomers with an all- syn and all- anti outer G-quartet are approximately equally populated in solution. A dynamic interconversion of the two quadruplexes with an exchange rate ( k ex ) of 0.2 s –1 is demonstrated through the observation of exchange crosspeaks in rotating frame Overhauser effect spectroscopy and nuclear Overhauser effect spectroscopy experiments at 50 °C. The kinetic properties suggest disruption of the corresponding outer G-tetrad but not of the whole quadruplex core during the tetrad flip. 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.