(38) Altogether, a diverse toolbox of TLs has been reported over the past two decades, all having distinct triazole orientations and linker lengths as indicated in Figure 1. (9) In search of the ideal nucleic acid triazole linkage, we recently developed a 1,5-disubstituted 1,2,3-triazole internucleoside linkage which was prepared by Ru II-catalyzed azide–alkyne cycloaddition (RuAAC) (TL 3 Figure 1). (8,9,19,29) However, TL 6 suffers from reduced binding affinity to a complementary DNA or RNA target (35−38) and induces a TL 6-dependent slowdown in PCR replications. Furthermore, click ligation has been used to assemble long DNA templates with isolated TL 6 modifications which can be replicated (9,29) or transcribed (8,9) by polymerases in bacterial (9) or mammalian (8) cells while retaining high fidelity read-through.

(17) However, the authors reported low replication efficiencies by several polymerases through TL 1. For example, the CuAAC click ligation of 3′-azido ONs with 5′-alkyne adaptor ONs to form TL 1 was recently described for the preparation of next-generation sequencing libraries. Many applications of such triazole backbones exist. (33,34) Further modification can be achieved by alkylation of the triazole as exemplified by cationic TL 7, (19) which is made by methylation of TL 6. (8,9,19,27−29) Among these, the triazole linkage (TL) represents a powerful and versatile chemical moiety that can be readily formed by the Cu I-catalyzed azide–alkyne cycloaddition (CuAAC) reaction, resulting in 1,4-disubstituted 1,2,3-triazoles. (19) Such polymerase compatible artificial backbones comprise 5′- S-phosphorothioesters, (20) phosphorothioates, (21) disulfides, (22) boranophosphates, (23) phosphoramidates, (10,24,25) amides, (19,26) ureas, (18) squaramides, (18) and triazoles. (18) A detailed study revealed several molecular characteristics of artificial backbones that are required for compatibility with DNA polymerases during replication.

In general, replication-competent artificial DNA backbones can be used for gene synthesis, (8,9) sequencing, (17) or nucleic acid detection.

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