The formation pathway of tetramolecular G-quadruplexes

Oligonucleotides containing guanosine stretches associate into tetrameric structures stabilized by monovalent ions. In order to describe the sequence of reactions leading to association of four identical strands, we measured by NMR the formation and dissociation rates of (TGnT)(4) quadruplexes (n = 3-6), their dissociation constants and the reaction orders for quadruplex formation. The quadruplex formation rates increase with the salt concentration but weakly depend on the nature (K+, Na+ or Li+) of the counter ions. The activation energies for quadruplex formation are negative. The quadruplex lifetimes strongly increase with the G-tract length and are much more longer in K+ solution than in Na+ or Li+ solutions. The reaction order for quadruplex formation is 3 in 0.125M KCl and 4 in LiCl solutions. The kinetics measurements suggest that quadruplex formation proceeds step by step via sequential strand association into duplex and triplex intermediate species. Triplex formation is rate limiting in 0.125M KCl solution. In LiCl, each step of the association process depends on the strand concentration. Parallel reactions to formation of the fully matched canonical quadruplex may result in kinetically trapped mismatched quadruplexes making the canonical quadruplex practically inaccessible in particular at low temperature in KCl solution.