Luminescence study of G-quadruplex formation in the presence of Tb3+ ion

The interactions of Tb3+ with the quadruplex-forming oligonucleotide bearing human telomeric repeat sequence d(G3T2AG3T2AG3T2AG3), (htel21), have been studied using luminescence spectroscopy and circular dichroism (CD). Enhanced luminescence of Tb3+, resulting from energy transfer from guanines, indicated encapsulation of Tb3+ ion in the central cavity of quadruplex core. The ability of lanthanide ions (Eu3+ and Tb3+) to mediate formation of quadruplex structure has been further evidenced by the fluorescence energy transfer measurements with the use of oligonucleotide probe labeled with fluorescein and rhodamine FRET partners, FAM-htel21-TAMRA. The CD spectra revealed that Tb3+/htel21 quadruplex possesses antiparallel strand orientation, similarly as sodium quadruplex. Tb3+ binding equilibria have been investigated in the absence and the presence of competing metal cations. At low Tb3+ concentration (8 μM) Tb3+/htel21 quadruplex stability is very high (5 × 106 M−1) and stoichiometry of 5–7 Tb3+ ions per one quadruplex molecule is observed. Luminescence and CD titration experiments suggested that the cavity of quadruplex accommodates two Tb3+ ions and the remaining Tb3+ ions bind probably to TTA loops of quadruplex. Higher concentration of Tb3+ (above 10 μM) results in the excessive binding of Tb3+ ions that finally destabilizes quadruplex, which undergoes transformation into differently organized assemblies. Such assemblies (probably possessing multiple positive charge) exhibit kinetic stability, which is manifested by a very slow kinetics of displacement of Tb3+ ion by competing cations (Li+, Na+, K+).