G-quadruplex structures formed by oligodeoxyribonucleotides TGGU^NH₂GGT (AM, U^NH₂ = 5-amino-2′-deoxyuridine), TGGU^BrGGT (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.