Stable and Selective Antiparallel Type Triplex DNA Formation by Targeting a GC Base Pair with the TFO Containing One <i>N</i><sup>2</sup>-Phenyl-2′-deoxyguanosine

The antiparallel triplex DNA is formed by the interaction between purine-rich triplex forming oligonucleotides (TFOs) and the homo-purine region within a duplex DNA. The formation of such a structure with the genome DNA promises to control the gene expression in a living cell. In this study, in an attempt to enhance the stability of the triplex DNAs, we have designed the N2-arylated deoxyguanosine derivatives. Among these analogues, we found that the TFOs containing N2-phenyl-2′-deoxyguanosine (PhdG) showed a stable and selective triplex DNA formation with the GC base pair as compared to the natural dG/GC triplet. However, the multiple incorporation of PhdG into the TFOs hampered the stable triplex DNA, instead, showed a tendency to form a higher order structure. Therefore, we concluded that the stable and selective triplex DNA formation is expected by the replacement of dG by PhdG in the purine-rich TFO sequence.