We recently have demonstrated that tetraalkylammonium (TAA + ) ions, H(CH 2 ) n 4 N + , strongly stabilize the parallel c-MYC G-quadruplex while not affecting the thermal stabilities of the antiparallel human telomeric G-quadruplex or duplex DNA. Based on these observations, we suggested that TAA + ions, especially the ones with long aliphatic chains, associate and form stable complexes with the c-MYC G-quadruplex. In the present work, we expand on our discovered paradigm of selective G-quadruplex recognition by performing NMR spectroscopic and volumetric characterization of the complex between tetrapentylammonium (TPeA + ) and the c-MYC G-quadruplex. The binding of TAA + to the c-MYC G-quadruplex is optically silent which necessitates the use of non-optical observables, such as volume and compressibility, for its characterization. Our NMR results revealed that TPeA + forms a stable complex with the c-MYC G-quadruplex in which the ligand is positioned above the G9-G13-G18-G22 quartet and surrounded by the 3′-(T23-A24-A25) flanking region. Our densimetric and ultrasonic velocimetric measurements revealed that the binding of TPeA + to the c-MYC G-quadruplex is associated with increases in volume, ΔV, and adiabatic compressibility, ΔK S , of 11±3 cm 3 mol -1 and (60±4)×10 -4 cm 3 mol -1 bar -1 , respectively. We used the value of ΔV in conjunction with changes in solvent-accessible surface area, ΔS A , and molecular volume, ΔV M , computed from structural data to evaluate the number of water molecules released to the bulk from the hydration shells of the interacting species, Δn h ; the binding is accompanied by a release of 55 water molecules. Combining the values of ΔK S and Δn h , we estimated a ∼5% decrease in G-quadruplex dynamics as expressed by its mean-square fluctuations of the intrinsic volume, . Such a moderate change in dynamics is consistent with the external mode of binding of TPeA + to the host G-quadruplex and the absence of structural changes in the latter.
Li et al. (Sun,) studied this question.