Effects of Loop Nucleobase Substitution on G-Quadruplex Thermal Stability in Aqueous Glycine Betaine, Proline, TMAO,...

G-quadruplexes are guanine-rich DNA or RNA structures comprising two or more stacked guanine tetrads (G-quartet) with nucleobases in the loops linking the G-quartets. The thermal stability of the thrombin-binding aptamer G2 d(G₂T₂G₂TGTG₂T₂G₂) G-quadruplex was quantified using m-values in aqueous glycine betaine (GB), proline, TMAO, and urea solutions using UV-absorbance spectroscopy. The thermal stability of the G2 variants was also explored with nucleobase substitutions in the TGT loop (TAT, TCT, TTT, UGU, and UUU), in the T₂ loops (T₄, U₂), or in both loops (UGU and U₂, UUU and U₂). GB and TMAO strongly stabilized G2 and its variants. Urea weakly destabilized G2 and its variants, while proline acted as a weak stabilizer or destabilizer depending on G2 variant nucleobase sequence. Both the unfolding enthalpy and entropy were positively correlated with cosolute molality for all cosolute and G-quadruplex combinations. Analysis of the change in solvent-accessible surface area (ΔASA) after unfolding G2 suggested the stability of G2 and its variants in aqueous cosolute solutions was driven by favorable interaction of cosolutes with G-quartet ΔASA and net-favorable or unfavorable interactions with the loop ΔASA. Our results reinforce a general mechanism of folded DNA and RNA stability modulation in cosolute solutions.