Synthesis of Hydrazone-Modified Nucleotides and Their Polymerase Incorporation onto DNA for Redox Labeling

5‐(5‐Formylthiophen‐2‐yl)cytosine, 7‐(5‐formylthiophen‐2‐yl)‐7‐deazaadenosine 2′‐deoxyribonucleosides, and their 5′‐O‐triphosphates (dNTPs) were converted into the 2,4‐dinitrophenylhydrazone or nitrobenzofurazanyl. The hydrazone‐modified dNTPs were enzymatically incorporated into DNA by polymerase catalyzed primer extension (PEX). This direct incorporation of hydrazone‐linked dNTPs was compared to previously reported incorporation of aldehyde‐modified dNTPs followed by postsynthetic hydrazone formation on DNA to show that the direct incorporation can be used for incorporation of more hydrazone units, however, cleaner PEX products are formed by incorporation of aldehydes and subsequent reaction with hydrazines. Extensive study of electrochemical behavior of the nitroarylhydrazone‐linked nucleosides and DNA was performed confirming the potential utility of the hydrazone, nitroaryl, and benzofurazane groups for redox labeling of DNA. Redox labeling of DNA through nitroarylhydrazone modification has been achieved either by polymerase incorporation of hydrazone‐modified dNTPs or by incorporation of aldehydes and subsequent hydrazone formation (see figure). Electrochemical studies revealed the potential utility of different redox‐active groups for DNA labeling for bioanalysis.