Evidence for the tetraplex structure of the d(GT)n repetitive sequences in solution

The ability of oligonucleotides 3'-d(GT)5pO(CH2)6Opd(GT)5-5' (anti[d(GT)]) and 3'-d(GT)5pO(CH2)6Opd(GT)5-3' (par[par[d(GT)]) to form tertiary structures has been studied. Circular dichroism (CD) as well as the fluorescence of the ethidium bromide (EtBr) complexes with oligonucleotides and hydrodynamic volume measurements in solutions containing 0.01 M phosphate buffer, pH 7 and NaCl in concentrations from 0.1 M to 1 M, have been used. The data obtained in the temperature interval from 3 degrees C to 10 degrees C are in good agreement with the structure suggested earlier where the par[d(GT)] and anti[d(GT)] form structures with four parallel strands in which layers of four G-residues alternate with unpaired bulged-out T-residues. Ethidium bromide interacts with the structure in a cooperative manner. Two ethidium bromide molecules intercalate between two layers of four G-residues.