Fig. 4. (a) Schematic representation of the competition experiment between GdC/AdU dinucleosides and C/U mononucleosides. Cyclic tetramers are in equilibrium with monomers (KT) and, under the conditions at which the competition experiments were carried out, such equilibrium is strongly (GdC) or moderately (AdU) shifted to the macrocycle side. Upon addition of the complementary C/U mononucleoside, a competing equilibrium is established in which the free dinucleoside monomer binds to the mononucleoside stopper (Ka), which shifts the cyclotetramerization equilibrium toward the formation of GdC·C/AdU·U pairs. (b and c) Normalized fluorescence emission changes (λexc = 385 nm, T = 298 K, toluene) observed in the titration of (b) GdC with increasing amounts of C ([GdC] = 1.0 × 10–4 M, [C] = 1.4 × 10–2 M) and (c) AdU with increasing amounts of U ([AdU] = 1.4 × 10–4 M, [U] = 3.0 × 10–3 M). (d) Representation of the degree of cyclic tetramer association (χT) or emission changes (I/I0) as a function of the equivalents of complementary pyrimidine mononucleoside added for GdC + C ([GdC] = 1.0 × 10–4 M (red circles) or 1.0 × 10–5 M (red squares; see Fig. S3A†)) and for AdU + U ([AdU] = 1.4 × 10–4 M (blue circles)). For the sake of comparison, we also include previous results obtained in competition experiments between GC/AU and C/U measured by 1H NMR at 10–2 M concentration in CDCl3 (red and blue squares).21c Speciation curves were simulated using the previously calculated Ka and KT values (Table 1).
