Unusual behavior exhibited by multistranded guanine-rich DNA complexes.

The structural properties of oligonucleotides containing two different types of G-rich sequences at the 3'-ends were compared. It is shown that oligonucleotides with uninterrupted runs of guanine residues at the 3'-end, e.g., d(T15G12), form multistranded structures stabilized by guanine-guanine interactions. The chemical and physical properties of these complexes differ from those of the complexes formed by oligonucleotides with telomere-like sequences, e.g., d(T15G4T2G4). In methylation protection and methylation interference experiments, we found all the guanines in complexes formed by d(T15G15) and d(T15G12) to be accessible to methylation. Furthermore, the methylated monomers retain the ability to polymerize. This contrasts with the inaccessibility of the guanines in d(T15G4T2G4) to methylation and the inability of the methylated monomer to form supramolecular structures. The stoichiometry of the complexes arising from the two types of oligonucleotides also differs. The complexes formed by d(T15G15) consist of consecutive integer numbers of DNA strands, whereas complexes formed by telomere-like oligonucleotides contain 1, 2, 4, or multiples of four strands. Magnesium ions favor formation of high molecular weight complexes by d(T15G15) and d(T15G12), but not by d(T15G4T2G4). The d(T15G15) and d(T15G12) complexes have very high thermal stability compared with telomeric complexes. However, at low temperatures, the thymine bases within the telomeric motif, TTGGGGTTGGGG, appear to allow for the formation of stable high-molecular weight species with a longer nonguanine portion.