The molecular properties of the phosphodiester backbone that made it the evolutionary choice for the enzymatic replication of genetic information are not well understood. To address this, and to develop new chemical ligation strategies for assembly of biocompatible modified DNA, we have synthesized oligonucleotides containing several structurally and electronically varied artificial linkages. This has yielded a new highly promising ligation method based on amide backbone formation that is chemically orthogonal to CuAAC “click” ligation. A study of kinetics and fidelity of replication through these artificial linkages by primer extension, PCR, and deep sequencing reveals that a subtle interplay between backbone flexibility, steric factors, and ability to hydrogen bond to the polymerase modulates rapid and accurate information decoding. Even minor phosphorothioate modifications can impair the copying process, yet some radical triazole and amide DNA backbones perform surprisingly well, indicating that the phosphate group is not essential. These findings have implications in the field of synthetic biology.

Templated chemical ligation of 5′-amino and 3′-phosphate oligonucleotides was used to synthesise a 762 base pair gene for green fluorescent protein. The phosphoramidate linkage can be read by DNA polymerase and transcribed to make RNA. We also show that phosphoramidate ligation and orthogonal CuAAC-mediated DNA ligation can be used simultaneously.

Oligonucleotides containing internal triazole–3′-LNA linkages bind to complementary RNA with similar affinity and specificity to unmodified oligonucleotides, and significantly better than oligonucleotides containing triazole alone. In contrast LNA on the 5′-side of the triazole does not stabilise duplexes. Triazole–LNA confers great resistance towards enzymatic degradation relative to LNA alone.

Oligonucleotides containing internal triazole–3′-LNA linkages bind to complementary RNA with similar affinity and specificity to unmodified oligonucleotides, and significantly better than oligonucleotides containing triazole alone. In contrast LNA on the 5′-side of the triazole does not stabilise duplexes. Triazole–LNA confers great resistance towards enzymatic degradation relative to LNA alone.

Electron paramagnetic resonance (EPR) spectroscopy is a powerful method to elucidate molecular structure through the measurement of distances between conformationally well-defined spin labels. Here we report a sequence-flexible approach to the synthesis of double spin-labeled DNA duplexes, where 2′-alkynylnucleosides are incorporated at terminal and internal positions on complementary strands. Post-DNA synthesis copper-catalyzed azide–alkyne cycloaddition (CuAAC) reactions with a variety of spin labels enable the use of double electron–electron resonance experiments to measure a number of distances on the duplex, affording a high level of detailed structural information.

We demonstrate a new method to reversibly cross-link DNA-nanoparticle dimers, trimers, and tetramers using light as an external stimulus. A DNA interstrand photo-cross-linking reaction is possible via ligation of a cyano-vinyl carbazole nucleoside with an opposite thymine when irradiated at 365 nm. This reaction results in nanoparticle assemblies that are not susceptible to DNA dehybridization conditions. The chemical bond between the two complementary DNA strands can be reversibly broken upon light irradiation at 312 nm. This is the first example of reversible ligation in DNA-nanoparticle assemblies using light and enables new developments in the field of programmed nanoparticle organization.

5-Azidomethyl dUTP and two 5-trans-cyclooctene dUTPs with different linkers between the TCO and the uracil base have been incorporated into DNA by primer extension, reverse-transcription and PCR amplification. For azidomethyl dUTP the PCR reaction was successful even when the modified dUTP was not supplemented with dTTP. In one case 335 azidomethyl dU residues were incorporated into the 523 base pair amplicon using this methodology. 5-Azidomethyl dUTP was found to be a better substrate for DNA polymerases than the trans-cyclooctene dUTPs. However, the inverse electron demand Diels–Alder reaction between cyclooctene DNA and a tetrazine Cy3-dye was more efficient than the strain-promoted reaction between azide DNA and a bicyclo [6.1.0] non-4-yne Cy3 dye.

X-pyrene is a new nucleic acid duplex stabilizing cytosine analogue that combines enhanced π-stacking, hydrogen bonding and electrostatic interactions to greatly increase the stability of bulged DNA duplexes and DNA/RNA hybrids. X-pyrene is highly selective for guanine as a partner and duplex stability is reduced dramatically when X-pyrene or a neighboring base is mismatched. An NMR study indicates that the pyrene moiety stacks within the helix, and large changes in fluorescence emission on duplex formation are consistent with this. These favorable properties make X-pyrene a promising cytosine analogue for use in a variety of biological applications.

Transcription factor binding and high resolution crystallographic studies (1.3 Å) of Dickerson–Drew duplexes with cytosine, methylcytosine and hydroxymethylcytosine bases provide evidence that C-5 cytosine modifications could regulate transcription by context dependent effects on DNA transcription factor interactions.

Modified triplex-forming oligonucleotides distinguish 5-methyl cytosine from unmethylated cytosine in DNA duplexes by differences in triplex melting temperatures. The discrimination is sequence-specific; dramatic differences in stabilisation are seen for CpA methylation, whereas CpG methylation is not detected. This direct detection of DNA methylation constitutes a new approach for epigenetic analysis.

A triazole linkage is formed in RNA by untemplated strain-promoted or CuAAC chemical ligation of 3′-azide and 5′-cyclooctyne oligonucleotides under denaturing conditions. Reverse transcriptase reads through these artificial linkages with omission of one nucleotide. These surprising results have implications for RNA isolation, amplification, sequencing and a variety of biological applications.

Modified triplex-forming oligonucleotides distinguish 5-methyl cytosine from unmethylated cytosine in DNA duplexes by differences in triplex melting temperatures. The discrimination is sequence-specific; dramatic differences in stabilisation are seen for CpA methylation, whereas CpG methylation is not detected. This direct detection of DNA methylation constitutes a new approach for epigenetic analysis.

X-pyrene is a new nucleic acid duplex stabilizing cytosine analogue that combines enhanced π-stacking, hydrogen bonding and electrostatic interactions to greatly increase the stability of bulged DNA duplexes and DNA/RNA hybrids. X-pyrene is highly selective for guanine as a partner and duplex stability is reduced dramatically when X-pyrene or a neighboring base is mismatched. An NMR study indicates that the pyrene moiety stacks within the helix, and large changes in fluorescence emission on duplex formation are consistent with this. These favorable properties make X-pyrene a promising cytosine analogue for use in a variety of biological applications.

A triazole linkage is formed in RNA by untemplated strain-promoted or CuAAC chemical ligation of 3′-azide and 5′-cyclooctyne oligonucleotides under denaturing conditions. Reverse transcriptase reads through these artificial linkages with omission of one nucleotide. These surprising results have implications for RNA isolation, amplification, sequencing and a variety of biological applications.

Transcription factor binding and high resolution crystallographic studies (1.3 Å) of Dickerson–Drew duplexes with cytosine, methylcytosine and hydroxymethylcytosine bases provide evidence that C-5 cytosine modifications could regulate transcription by context dependent effects on DNA transcription factor interactions.