A novel pyrimidine tetrad contributing to stabilize tetramolecular G-quadruplex structures.

G-quadruplex structures formed by oligodeoxyribonucleotides TGGU(NH2)GGT (AM, U(NH2) = 5-amino-2'-deoxyuridine), TGGU(Br)GGT (BR, U(Br) = 5-bromo-2'-deoxyuridine) and TGGTGGT (TH) have been investigated through circular dichroism, nuclear magnetic resonance, gel electrophoresis and molecular modeling techniques. Collected data indicate that all 7-mer oligonucleotides form tetramolecular parallel G-quadruplex structures with all residues adopting anti glycosidic bonds. In the case of AM, data suggest the occurrence of a novel U(NH2)-tetrad characterized by eight hydrogen bonds that stabilizes the G-quadruplex structure more efficiently than U(Br)- and T-tetrads.