A topological transition from bimolecular quadruplex to G-triplex/tri-G-quadruplex exhibited by truncated double repeats of human telomere

Fractionation of Single-stranded DNAs with/without Stable Preorganized Structures Using Capillary Sieving Electrophoresis for Aptamer Selection

Aptamers, single-stranded DNAs/RNAs with a strong and specific interaction towards a target molecule, have wide applications in the fields of medicine and biosensors. In conventional aptamer selection methods, it is difficult to obtain "preorganized" and/or "induced-fit" type of aptamers selectively. In this study, separation and fractionation of single-stranded DNAs with/without stable preorganized structures were carried out using capillary sieving electrophoresis. The fractionated DNAs showed different mobilities and thermodynamic stabilities of their secondary structures; this outcome is deemed to be necessary for the synthesis of novel aptasensors with a desirable sensing mechanism.

NMR structural study on the self-trimerization of d(GTTAGG) into a dynamic trimolecular G-quadruplex assembly preferentially in Na+ solution with a moderate K+ tolerance

Vast G-quadruplexes (GQs) are primarily folded by one, two, or four G-rich oligomers, rarely with an exception. Here, we present the first NMR solution structure of a trimolecular GQ (tri-GQ) that is solely assembled by the self-trimerization of d(GTTAGG), preferentially in Na⁺ solution tolerant to an equal amount of K⁺ cation. Eight guanines from three asymmetrically folded strands of d(GTTAGG) are organized into a two-tetrad core, which features a broken G-column and two width-irregular grooves. Fast strand exchanges on a timescale of second at 17°C spontaneously occur between folded tri-GQ and unfolded single-strand of d(GTTAGG) that both species coexist in dynamic equilibrium. Thus, this tri-GQ is not just simply a static assembly but rather a dynamic assembly. Moreover, another minor tetra-GQ that has putatively tetrameric (2+2) antiparallel topology becomes noticeable only at an extremely high strand concentration above 18 mM. The major tri-GQ and minor tetra-GQ are considered to be mutually related, and their reversible interconversion pathways are proposed accordingly. The sequence d(GTTAGG) could be regarded as either a reading frame shifted single repeat of human telomeric DNA or a 1.5 repeat of Bombyx mori telomeric DNA. Overall, our findings provide new insight into GQs and expect more functional applications.