5056 Nucleic Acids Research, 2008, Vol. 36, No. 15 Figure 6. Comparison between the tetra- and mono-molecular G-quadruplexes. AFM images of tetra-molecular (Inset: enlargement of a tetra- molecular complex) (A) and monomolecular (B) G-quadruplexes. Tetra-molecular G-quadruplexes were prepared, as described in ‘Materials and methods’ section, using complex of four 1.4 kb 50biotin–poly(dG)-poly(dC) molecules connected to avidin as starting material. Monomolecular G-quadruplexes were prepared as described in ‘Introduction’ section using 5.5 kb G-strand. Both types of molecules were deposited on mica under the same conditions. Statistical contour length analysis of tetra-molecular (C) and monomolecular (D) G-quadruplexes. More than 300 single-well separated molecules of each type were subjected to the statistic analysis. Inset: Enlargement of one of the tetra-molecular G4-DNA molecules. Figure 7. The height proﬁles of tetra-molecular G-quadruplexes. AFM images of tetra-molecular G4-wires (A) and their height proﬁles (B and C). Tetra-molecular G-quadruplexes were prepared, as described in ‘Materials and methods’ section, using complex of four 1.4 kb 50biotin–poly(dG)- poly(dC) molecules connected to avidin as starting material. 260 nm (Figure 8, lines 1 and 2, respectively). The base- induced unfolding of the tetra-molecular structures results in the complete disappearance of the original positive CD band at 260 nm (Figure 8, line 4). We have shown earlier (18) that long mono-molecular structures are stable without K+ or Na+ in contrast to short tetraplexes, which formation takes place only in the presence of these cations. Here we show that the same is true for tetra- molecular quadruplexes; these structures are very stable in the absence of K+(Na+) and do not undergo any noticeable changes during weeks. Addition of K+ to tetra-molecular G-quadruplexes prepared in the absence