Oligonucleotides triplex structures

Polynucleotides were reported to form triple helices as early as 1957. Triple strands can form by non-Watson-Crick hydrogen bonds between the third strand and purines involved in Watson-Crick hydrogen bonding with the complementary strand of the duplex (for review, see Ref 34). Thus, triple-stranded structures can be formed between a third strand composed of pyrimidines or purines that interact with a homopurine strand in a homopurine-homopyrimidine strand in a duplex DNA. With the demonstration that homopyrimidine oligonucleotides could indeed form triplex structures (35-37), interest in triple-strand approaches to inhibit transcription heightened. 

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