SELECTIVE LOCALIZATION AND ROTATIONAL IMMOBILIZATION OF UNIVALENT CATIONS ON QUADRUPLEX DNA

The quadruplex structure of the oligomer d(T2G4T) is more stable in th e presence of K+ than in the presence of Na+. This enhanced stability correlates with the preferential binding of K+ to a small number of sp ecific sites on the quadruplex. In contrast, Na+ and K+ compete on an equal footing for atmospheric binding. Both K-39+ and Na-23+ are, when specifically bound, significantly inhibited in their rotational mobil ity, so that the quadrupolar relaxation reflects the molecular tumblin g of the oligomer, which occurs on the time scale of nanoseconds. This rotational immobilization is in distinct contrast to the high rotatio nal mobility of atmospherically bound cations. On the other hand, all NMR-visible K-39+ in solution is in rapid exchange among all environme nts (free, specifically bound, and atmospherically bound) implying tha t the lifetime of specifically coordinated K-39+ must be significantly shorter than a millisecond. A similar conclusion holds for Na-23+. Th e oligomer d(T2G4T) forms two distinct Hoogsteen base-paired structure s in NaCl solution, separated by a large kinetic barrier. Neither of t hese structures is as stable with respect to base pair opening as is t he quadruplex structure formed in KCl solution. Only one of these two structures is associated with rotational immobilization of bound Na-23 +.