Triple helices of optimally capped duplex DNA (sigma-DNA) with homopyrimidine DNA and RNA at neutral pH

An optimally capped duplex DNA (sigma-DNA) was synthesized in which the cyt osines in the homopyrimidine strand of sigma-DNA 1 were replaced by 5-methy lcytosine leading to sigma-DNA 2. Although only involving 13 base pairs, th is modification resulted in very high melting temperatures above 90 degrees . In addition, sigma-DNA 2 was able to form triple helices with the corresp onding homopyrimidine DNA or RNA even at neutral pH. This opens up the poss ibility to use sigma-DNA in a triple-helix approach to modulate gene expres sions on the level of the translation process.