A DNA-triplex stabilizing purine (8-aminoguanine) is designed based on molecular modeling and synthesized. The substitution of guanine by 8-aminoguanine largely stabilizes the triplex both at neutral and acidic pH, as suggested by molecular dynamics and thermodynamic integration calculations, and demonstrated by melting experiments. NMR experiments confirm the triplex-stabilizing properties of 8-aminoguanine and demonstrate that few changes are found in the structure of the triplex due to the presence of this modified base.