The structure of the Tetrahymena thermophila telomeric sequence d(TGGGGT)4 was studied by photon correlation spectroscopy (PCS) in aqueous solution in the presence of NaCl, KCl and SrCl2. The sample studied was polydisperse in all conditions studied. Translational diffusion coefficients DT describing the diffusion modes observed were determined. On the basis of a comparison between the experimental DT values with those calculated assuming the bead model, two forms were identified as telomeric quadruplex structures: monomer and tetramer. In the presence of SrCl2 formation of aggregates was observed, with a size that reached several micrometres. The relative weighted concentrations of the structures observed for different concentrations of a salt and DNA were determined. The results obtained in the presence of monovalent ions were qualitatively similar and could be presented in a coherent plot in which the concentration of salt was expressed by the number of ions per DNA molecule. A large number of ions per DNA molecule favoured tetramer formation while a small number favoured the monomer form. A structural phase transition from the monomer to the tetramer induced by a change in the number of ions per DNA molecule was observed. The main difference between the results for Na+ and K+ was a greater effectiveness of the K+ ions in formation of tetramers. The effect of Sr2+ ions on the structures formed was different than that of the monovalent ions. The results obtained in the presence of Sr2+ could not be described as a function of the number of ions per DNA molecule.
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