Site-specific N-alkylation of DNA Oligonucleotide Nucleobases by DNAzyme-catalyzed Reductive Amination

DNA and RNA nucleobase modifications are biologically relevant and valuable in fundamental biochemical and biophysical investigations of nucleic acids. However, directly introducing site-specific nucleobase modifications into long unprotected oligonucleotides is a substantial challenge. In this study, we used in vitro selection to identify DNAzymes that site-specifically N-alkylate the exocyclic nucleobase amines of particular cytidine, guanosine, and adenosine (C, G and A) nucleotides in DNA substrates, by reductive amination using a 5'-benzaldehyde oligonucleotide as the reaction partner. The new DNAzymes each require one or more of Mg2+, Mn2+, and Zn2+ as metal ion cofactors and have kobs from 0.04 to 0.3 h-1, with rate enhancement as high as ∼104 above the splinted background reaction. Several of the new DNAzymes are catalytically active when an RNA substrate is provided in place of DNA. Similarly, several new DNAzymes function when a small-molecule benzaldehyde compound replaces the 5'-benzaldehyde oligonucleotide. These findings expand the scope of DNAzyme catalysis to include nucleobase N-alkylation by reductive amination. Further development of this new class of DNAzymes is anticipated to facilitate practical covalent modification and labeling of DNA and RNA substrates.