Triplex Formation by Oligonucleotides Containing Organomercurated Base Moieties

Homothymine oligonucleotides with a single 5-mercuricytosine or 5-mercuriuracil residue at their termini have been synthesized and their capacity to form triplexes has been examined with an extensive array of double-helical targets. UV and circular dichroism (CD) melting experiments revealed the formation and thermal denaturation of pyrimidinepurine*pyrimidine-type triple helices with all oligonucleotide combinations studied. Nearly all triplexes were destabilized upon mercuration of the 3-terminal residue of the triplex-forming oligonucleotide, in all likelihood due to competing intramolecular Hg-II-mediated base pairing. Two exceptions from this general pattern were, however, observed: 5-mercuricytosine was stabilizing when placed opposite to a TA or AT base pair. The stabilization was further amplified in the presence of 2-mercaptoethanol (but not hexanethiol, thiophenol or cysteine), suggesting a stabilizing interaction other than Hg-II-mediated base pairing.