Formation and structural determinants of multi-stranded guanine-rich DNA complexes

We have investigated the complexes formed by oligonucleotides with the gene ral sequence d(T(15)G(n)), where n = 4-15. Two distinct classes of structur es are formed, namely, the four-stranded tetraplex and frayed wires. Frayed wires differ from four-stranded tetraplexes in both strand association sto ichiometry and the ability of dimethyl sulfate to methylate the N7 position of guanine. Thus, it appears that these two guanine-rich multistranded ass emblies are stabilised by different guanine-guanine interactions. The numbe r of contiguous guanine residues determines which of the complexes is favou red. Based on the stoichiometry of the associated species and the accessibi lity of the N7 position of guanine to methylation we have found that oligon ucleotides with smaller number of contiguous guanines; n = 5-8 form primari ly four-stranded tetraplex. Oligonucleotides with larger numbers of contigu ous guanines adapt primarily the frayed wire structure. The stability of th e complexes formed by this series of oligonucleotides is determined by the number and arrangement of the guanines within the sequences. We propose tha t the formation of the two types of complex proceed by a parallel reaction pathways that may share common intermediates. (C) 2000 Elsevier Science B.V . All rights reserved.