Optimization of acetonitrile co-solvent and copper stoichiometry for pseudo-ligandless click chemistry with nucleic acids.

The copper(I) catalyzed azide-alkyne cycloaddition 'click' reaction yields a specific product under mild conditions and in some of the most chemically complex environments. This reaction has been used extensively to tag DNA, proteins, glycans and only recently RNA. Click reactions in aqueous buffer typically include a ligand for Cu(I), however we find that acetonitrile as a minor co-solvent can serve this role. Here we investigate the click labeling of RNA and DNA in aqueous buffer to determine the relationship between the stoichoimetry of Cu(I) and the acetonitrile co-solvent that affects nucleic acid stability. We find that very low concentrations of acetonitrile perform equally well and obviate the need for any additional Cu(I) stabilizing ligand. These pseudo-ligandless reaction conditions are optimal for nucleic acids click conjugations.