Figure 6.

Top: CD spectra of 50 mM G₃(TTAG₃)₃ in the presence of 100 and 500 mM of Na⁺ or K⁺. Positive bands at ∼245 and ∼293 nm and negative band at ∼265 nm affirm antiparallel foldback of the Na⁺-stabilized G₃(TTAG₃)₃, regardless of the Na⁺ concentration. Even though without negative band at ∼265 nm, the CD spectrum of 100 mM K⁺-stabilized G₃(TTAG₃)₃ was recently proven to be indicative for essentially the same antiparallel topology as adopted by Na⁺-stabilized quadruplex (30), contradicting the previous interpretation attributing CD spectra of this shape to hybrid (3 + 1) form (18,19,22). Transition to the parallel structure via hybrid (3 + 1) forms is accompanied by appearance of a strong positive band at ∼263 nm, decrease of the positive band at ∼290 nm and the presence of negative band at ∼240 nm (30), the CD features visible in the presence of 500 mM K⁺. Bottom: native PAGE of 50 mM G₃(TTAG₃)₃ in 100–500 mM of K⁺ before heating (N) and after slow annealing (A). The PAGE was performed at 20°C in 30 mM PBS containing 300 mM K⁺ in total, pH 6.7.