Figure 3. (A) NOESY spectrum of 22CT at 50°C (mixing time, 50 ms) showing exchange cross peaks between folded and unstructured conformations. Exchange cross peaks are labeled with stars. Experimental conditions: strand concentration, 0.2 mM; pH 5; 10 mM NaCl. (B) Lifetime of the folded 22AG and 22CT, and association time of the duplex (22AG + 22CT). Strand concentration, 0.2 mM. Squares represent the lifetime of folded 22AG at pH 7 in 100 mM KCl (filled) and in 10 mM NaCl (open). Circles represent the lifetime of folded 22CT in 100 mM KCl at pH 6 (filled) and in 10 mM NaCl at pH 5 (open). Crosses represent the association time of the duplex measured in a real-time experiment at pH 5, 100 mM NaCl. The lines are interpolations. In the case of 22AG, multiple structures were present in 100 mM KCl (the sample was first prepared with sodium counterions, then KCl was added) and only the lifetime of the major form was measured. (C) Real-time mixing experiments at 0°C: 22AG and 22CT were mixed and followed with 1D 1H NMR spectra, obtained at different times as indicated. The reference spectrum, labeled ‘ref.’, was obtained at equilibrium (after 1 month). G-quadruplex, i-motif and Watson–Crick duplex structures were followed by imino protons at 10–12, 15–16 and 12–14 p.p.m., respectively. Experimental conditions: strand concentration, 0.2 mM; pH 5; 100 mM NaCl.