Unusual signal broadening in NMR of oligonucleotides

Solution structure of single-stranded small oligonucleotides (up to pentamer) has been studied by 1H-NMR (600 MHz) and it has been found that when those having some specific base sequences produce unusual broadening of the lines attributed to the protons of guanine residues. These broadened signals did not become sharp even at elevated temperature. This extremely unusual behavior has been attributed to the interaction between the guanine residues.