Human telomeric DNA: G-quadruplex, i-motif and Watson–Crick double helix

Human telomeric DNA composed of (TTAGGG/ CCCTAA)n repeats may form a classical Watson±Crick double helix. Each individual strand is also prone to quadruplex formation: the G-rich strand may adopt a G-quadruplex conformation involving G-quartets whereas the C-rich strand may fold into an i-motif based on intercalated C´C+ base pairs. Using an equimolar mixture of the telo-meric oligonucleotides d[AGGG(TTAGGG)3] and d[(CCCTAA)3CCCT], we de®ned which structures existed and which would be the predominant species under a variety of experimental conditions. Under near-physiological conditions of pH, tempera-ture and salt concentration, telomeric DNA was predominantly in a double-helix form. However, at lower pH values or higher temperatures, the G-quad-ruplex and/or the i-motif ef®ciently competed with the duplex. We also present kinetic and thermo-dynamic data for duplex association and for G-quadruplex/i-motif unfolding