Abstract
All living things use DNA and RNA to store, retrieve, and transmit their genetic information.
The complementary Watson–Crick nucleobase-pairs (A/T and G/C base-pairs), have been
documented for years as being essential for the integrity of the DNA double helix
and also for replication and transcription. With only four poorly fluorescent naturally
occurring nucleic acid bases (namely A, G, T/U, and C), the extraction of genetic
information is difficult. Further, the chemical diversity of DNA and RNA is severely
limited. Deoxyribose/ribose-phosphate backbones also constrain DNA and RNA characteristics
and have poor chemical and physiological stability, which significantly restricts
the practical applications of DNA and RNA. Over the years, extensively modified nucleobase
pairs with novel base-pairing properties have been synthesized. Such designer nucleobases,
serving as an expanded genetic alphabet, have been used for the design and synthesis
of DNA and RNA analogues with tailored informational/functional properties. Recent
developments in the production of synthetic unnatural base pairs pave the way for
xenobiology research and genetic alphabet expansion technology. In this review, we
present a brief history of the development of several hydrogen- and non-hydrogen-bonded
unnatural base pairs and their applications. We also highlight our work in designing
and synthesizing a new class of triazolyl unnatural nucleosides that offer a unique
charge-transfer (CT) complexation force towards stabilizing DNA-duplexes when incorporated
into short oligonucleotide sequences.
Key words
genetic alphabet expansion - unnatural nucleobase pairs - triazolyl nucleosides -
tetrazolyl nucleosides - fluorescent nucleobases - nucleic acids