Modified Oligonucleotides in Triple Helix Formation

Tendency towards triple helix formation strongly depends on nucleotide structures. Thus, a decrease in tendency is observed with a change in the sugar moiety with deoxyribose ribose 2 -0-methylribose [249], With phosphorothioates, triple helix formation depends strongly on the sequence. A modification next to the 5 -position in presence of a purine base increases stability of the triple helix, while a modification next to the 5 -position in presence of a pyrimidine base decreases stability [250], 

With the N-3 atom of cytidine unprotonated under physiological conditions, bases have been proposed to circumvent this disadvantage 87 [254], 88 [255], 89 and 90 [256].The use of artificial nucleosides with bases such as 4-(3-benzamidophenyl)imidazole, which binds to T A and C G, has also been proposed in triple helix antisense concepts [257]. 

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