Reducing product inhibition in nucleic acid-templated ligation reactions: DNA-templated cycligation.

Programmable interactions allow nucleic acid molecules to template chemical reactions by increasing the effective molarities of appended reactive groups. DNA/RNA-triggered reactions can proceed, in principle, with turnover in the template. The amplification provided by the formation of many product molecules per template is a valuable asset when the availability of the DNA or RNA target is limited. However, turnover is usually impeded by reaction products that block access to the template. Product inhibition is most severe in ligation reactions, where products after ligation have dramatically increased template affinities. We introduce a potentially generic approach to reduce product inhibition in nucleic acid-programmed ligation reactions. A DNA-triggered ligation-cyclization sequence ("cycligation") of bifunctional peptide nucleic acid (PNA) conjugates affords cyclic ligation products. Melting experiments revealed that product cyclization is accompanied by a pronounced decrease in template affinity compared to linear ligation products. The reaction system relies upon haloacetylated PNA-thioesters and isocysteinyl-PNA-cysteine conjugates, which were ligated on a DNA template according to a native chemical ligation mechanism. Dissociation of the resulting linear product-template duplex (induced by, for example, thermal cycling) enabled product cyclization through sulfur-halide substitution. Both ligation and cyclization are fast reactions (ligation: 86 % yield after 20 min, cyclization: quantitative after 5 min). Under thermocycling conditions, the DNA template was able to trigger the formation of new product molecules when fresh reactants were added. Furthermore, cycligation produced 2-3 times more product than a conventional ligation reaction with substoichiometric template loads (0.25-0.01 equiv). We believe that cyclization of products from DNA-templated reactions could ultimately afford systems that completely overcome product inhibition.