Synthesis and Properties of the Simplified Nucleic Acid Glycol Nucleic Acid

The nucleosides of glycol nucleic acid (GNA), with the backbone comprising just the three carbons and one stereocenter of propylene glycol (1,2-propanediol), probably constitute the simplest possible building blocks for a chemically stable nucleic acid that contains phosphodiester bonds. However, it was not until 2005 that the astonishing duplex formation properties of GNA homoduplexes were discovered in our laboratory. The R- and S-enantiomers of GNA, (R)-GNA and (S)-GNA, pair in like-symmetric combinations to form highly stable antiparallel duplexes in a Watson−Crick fashion, with thermal and thermodynamic stabilities exceeding those of analogous duplexes of DNA and RNA. Interestingly, (R)-GNA and (S)-GNA do not significantly cross-pair with each other, either in a parallel or antiparallel fashion. GNA discriminates strongly in favor of the Watson−Crick base-pairing scheme, with only slightly lower fidelity than DNA. Two (S)-GNA homoduplex structures recently determined by X-ray crystallography, one a brominated 6-mer duplex and the other an 8-mer duplex containing two copper(II) ions, reveal that the overall GNA double helix is distinct from canonical A- and B-form nucleic acids. The structure is perhaps best described as a helical ribbon loosely wrapped around the helix axis. Within the backbone, the propylene glycol nucleotides adopt two different conformations, gauche and anti, with respect to the torsional angles between the vicinal C3′−O and C2′−O bonds. A strikingly large backbone−base inclination results in extensive zipper-like interstrand and reduced intrastrand base−base interactions. This strong backbone−base inclination might explain the observation that neither the R- nor S-enantiomer of GNA cross-pairs with DNA, whereas (S)-GNA can interact with RNA strands that are devoid of G:C base pairs. Given the combination of structural simplicity, straightforward synthetic accessibility, and high duplex stability of GNA duplexes, GNA affords a promising nucleic acid scaffold for biotechnology and nanotechnology. Along these lines, we describe the functionalization of GNA duplexes through the incorporation of metal-ion-mediated base pairs. Finally, the properties of GNA discussed here reinforce its candidacy as one of the initial genetic molecules formed during the origins of life on Earth.