Synthesis and Properties of Triple Helix-Forming Oligodeoxyribonucleotides containing 7-Chloro-7-deaza-2′-deoxyguanosine

Multiple incorporations of 7-chloro-7-deaza-2′-deoxyguanosine in place of 2′-deoxyguanosine have been performed into a triple helix-forming oligodeoxyribonucleotide involving a run of six contiguous guanines designed to bind in a parallel orientation relative to the purine strand of the DNA target. The ability of these modified oligodeoxyribonucleotides to form triple helices in a buffer containing monovalent cations was studied by UV–melting curves analysis, gel shift assay and restriction enzyme protection assay. In the presence of Na +, the incorporation of two, three or five modified nucleosides in the third strand has improved the efficacy of formation of the triplex as compared to that formed with the unmodified oligonucleotide. The stabilities of the three modified triplexes were similar. The coupling of 6-chloro-2-methoxy-9-(ω-hexylamino)-acridine to the 5′-end of the oligonucleotides containing modified nucleosides led to an increase in triplex stability similar to that observed when the acridine was added to the 5′-end of the unmodified oligonucleotide. In the presence of K +, only the oligodeoxyribonucleotides containing modified G retained the ability to form triple helices with the same efficiency. The incorporation of the modified nucleoside has two effects: (i) it decreases TFO self-association, and (ii) it slightly increases triplex stability. The enhanced ability of the modified oligonucleotides containing 7-chloro-7-deaza-2′-deoxyguanosine over the parent oligomer to form triple helices was confirmed by inhibition of restriction enzyme cleavage using a circular plasmid containing the target sequence. We report multiple incorporations of 7-chloro-7-deaza-2′-deoxyguanosine G ∗ in place of 2′-deoxyguanosine into a triple helix-forming oligodeoxyribonucleotide (TFO) involving a run of six contiguous G. The incorporation of the modified nucleoside slightly increases triplex stability and decreases the TFO auto-association.