5-Mercuricytosine: An Organometallic Janus Nucleobase

The base-pairing properties of 5-mercuricytosine have been explored at the monomer level by NMR titrations and at the oligonucleotide level by melting temperature measurements. The NMR studies revealed a relatively high affinity for guanine, hypoxanthine, and uridine, that is, bases that are deprotonated upon coordination of Hg-II. Within an oligonucleotide duplex, 5-mercuricytosine formed Hg-II-mediated base pairs with thymine and guanine. In the former case, the duplex formed was as stable as the respective duplex comprising solely Watson-Crick base pairs. Based on detailed thermodynamic analysis of the melting curves, the stabilization by the Hg-II-mediated base pairs may be attributed to a comparatively low entropic penalty of hybridization.