The effect of different ions on the formation and behavior of quadruplex structures of the human telomere sequence d(TTAGGG)(4) has been studied by photon correlation spectroscopy (PCS) and circular dichroism (CD). The saturation and melting curves obtained in the presence of K+, Na+, Rb+, Li+, Cs+, and Sr2+ ions were recorded by CD spectroscopy and indicated the formation of monomeric quadruplexes. Analysis of the saturation curves obtained at 2degreesC has shown that the presence of a single Sr2+ ion per oligomer is sufficient for the formation of a monomeric quadruplex of the DNA sequence studied. In the presence of SrCl2 at a concentration of 50 mM, the formation of tetrameric quadruplexes has been detected. The effect of Sr2+ ions on the formation of quadruplex structures by the human telomere sequence d(TTAGGG)(4) is stronger and different from that of the other ions tested. The paper also presents results of a study of electrostatic interactions in solution. The translation diffusion coefficients D-T of the structures present in solution have been determined by photon correlation spectroscopy and the effective charges on the structures have been calculated by combining the experimental data with the results based on the coupled mode theory. Analysis of the melting points monitored by the CD method has permitted a determination of An, the number of ions released in the process of thermal denaturation. All the results are in good agreement with the predictions based on the theory of polyelectrolytes. The effect of ions on the formation and behavior of quadruplex structures of the human telomere sequence d(TTAGGG)(4) has been studied by photon correlation spectroscopy and circular dichroism.