| Abstract
| - A solution-state NMR study on 15NH4+ ion movement within d(G3T4G4)2, a dimeric G-quadruplexconsisting of three G-quartets and two T4 loops, rather unexpectedly demonstrated the absence of 15NH4+ion movement between the binding sites U and L along the central axis of the G-quadruplex. Distincttemperature dependences of autocorrelation signals for U and L binding sites have been observed in 15N−1H NzExHSQC spectra which correlate with the local stiffness of the G-quadruplex. The volumes of thecross-peaks, which are the result of 15NH4+ ion movement, have been interpreted in terms of rate constants,T1 relaxation, and proton exchange. 15NH4+ ion movements from the binding sites U and L into the bulksolution are characterized by lifetimes of 139 ms and 1.7 s at 298 K, respectively. The 12 times fastermovement from the binding site U demonstrates that 15NH4+ ion movement is controlled by the structureof T4 loop residues, which through diagonal- vs edge-type orientations impose distinct steric restraints forcations to leave or enter the G-quadruplex. Arrhenius-type analysis has afforded an activation energy of66 kJ mol-1 for the UB process, while it could not be determined for the LB process due to slow rates attemperatures below 298 K. We further the use of the 15NH4+ ion as an NMR probe to gain insight into theoccupancy of binding sites by cations and kinetics of ion movement which are intrinsically correlated withthe structural details, dynamic fluctuations, and local flexibility of the DNA structure.
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