Sequence and Solvent Effects on Telomeric DNA Bimolecular G-Quadruplex Folding Kinetics

International audienceTelomeric DNA sequences are particularly polymorphic: the adopted structure is exquisitely sensitive to the sequence and to the chemical environment, for example solvation. Dehydrating conditions are known to stabilize G-quadruplex structures, but information on how solvation influences the individual rates of folding and unfolding of G-quadruplexes remains scarce. Here, we used electrospray mass spectrometry for the first time to monitor bimolecular G-quadruplex formation from 12-mer telomeric strands, in the presence of common organic co-solvents (methanol, ethanol, isopropanol and acetonitrile). Based on the ammonium ion distribution, the total dimer signal was decomposed into contributions from the parallel and antiparallel structures, to obtain individual reaction rates, and the antiparallel G-quadruplex structure was found to form faster than the parallel one. A dimeric reaction intermediate, in rapid equilibrium with the single strands, was also identified. Organic co-solvents increase the stability of the final structures mainly by increasing the folding rates. Our quantitative analysis of reaction rate dependence on co-solvent percentage shows that organic co-solvent molecules can be captured or released upon G-quadruplex formation, highlighting that they are not inert with DNA. In contrast to the folding rates, the G-quadruplex unfolding rates are almost insensitive to solvation effects, but are instead governed by the sequence and by the final structure: parallel dimers dissociate slower than antiparallel dimers only when thymine bases are present at the 5’-end. These results contribute unraveling the folding pathways of telomeric G-quadruplexes. The solvent effects revealed here enlighten that G-quadruplex structure in dehydrated and molecularly crowded environments is modulated by the nature of co-solvent (e.g., methanol favors antiparallel structures) due to direct interactions, and by the time scale of the reaction, with > 200-fold acceleration of bimolecular G-quadruplex formation in the presence of 60% co-solven