G-quadruplex structures formed by oligodeoxyribonucleotides TGGUNH2GGT (AM, UNH2 = 5-amino-2'-deoxyuridine), TGGUBrGGT (BR, UBr = 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 UNH2-tetrad characterized by eight hydrogen bonds and stabilizing the G-quadruplex structure more efficiently than UBr- and T-tetrads.