D(G(3)T(4)G(3)) FORMS AN ASYMMETRIC DIAGONALLY LOOPED DIMERIC QUADRUPLEX WITH GUANOSINE 5'-SYN-SYN-ANTI AND 5'-SYN-ANTI-ANTI N-GLYCOSIDIC CONFORMATIONS

The structure formed by the DNA oligonucleotide d(G(3)T(4)G(3)) has been studied by one- and two-dimensional H-1 NMR spectroscopy. In NaCl solution, d(G(3)T(4)G(3)), like d(G(4)T(4)G(4)) (Oxy-1.5), forms a dimeric quadruplex with the thymines in loops across the diagonal of the end quartets. Unlike Oxy-1.5, the dimer is not symmetric, and both monomer strands are observed in MMR spectra. Three quartets are formed from the GGG tracts. Glycosidic conformations of the guanines are 5'-syn-syn-anti-(loop)-syn-anti-anti in one strand and 5'-syn-anti-anti-(loop)-syn-syn-anti in the other strand. Thus, the stacking of the quartets (tail-to-tail, head-to-tail) is unlike all previously described fold-back (tail-to-tail, head-to-head) and parallel-stranded (head-to-tail, head-to-tail) quadruplexes.