Figure 10.

Schematic diagram of interconversions between the Na⁺ and K⁺ forms of telomeric G-quadruplexes. (A) For the extended four repeat telomeric sequence Tel26, the hybrid-type G-quadruplex structure is the most stable and thus the predominant form in the presence of K⁺, regardless of the presence or absence of Na⁺. Addition of K⁺ readily converts the preformed Na⁺-form G-quadruplex to the hybrid-type G-quadruplex conformation. Tel26 no longer forms a single stable intramolecular G-quadruplex structure in Na⁺ solution, likely caused by the steric interference of the flanking sequences with the diagonal loop, both of which positioned on the same side of the basket-type G-quadruplex structure. (B) The truncated Tel22 forms a single stable basket-type intramolecular G-quadruplex in Na⁺ solution. However, in the presence of K⁺ Tel22 does not form a single G-quadruplex structure and is likely to have two stable G-quadruplex conformations co-existing. A possible mechanism of the interconversion of the two G-quadruplex conformations is proposed. The exchange rate between the two stable G-quadruplexes is slow on the NMR time scale at 25°C (Figure 2C). Addition of K⁺ to the preformed Na⁺ basket-type G-quadruplex readily converts the conformation to the K⁺-form, thus the two interconvertable K⁺ G-quadruplex conformations are both more stable than the Na⁺-basket-type G-quadruplex.