We have developed oligodeoxynucleotides (ODNs) that modify primary amines to produce 5,6-dimethoxy 3-methyleneisoindolin-1-one. Compared to the oxygen isosteric fluorophore, 4,5-dimethoxyphthalimide, this methyleneisoindolinone was more stable and exhibited an 85 nm blue-shifted fluorescent emission (λmax at 425 nm) with an intensity comparable to that of the phthalimide. Reaction of the DNA-binding domain of Escherichia coli DnaA protein with an ODN containing its binding sequence efficiently afforded a modified fluorescent protein at a specific lysine residue in the proximity of the ODN. A full-length DnaA protein was also successfully fluorescently labeled. These results demonstrate the potential utility of the ODNs developed in this study for the fluorescent labeling of DNA-interacting protein at the lysine residue of interest.

We evaluated the efficacy of bioconjugation of oligodeoxynucleotides (ODNs) containing 1,4-dicarbonyl groups, a C4′-oxidized abasic site (OAS), and a newly designed 2′-methoxy analogue, via reductive amination with lysine residues. Dicarbonyls, aldehyde and ketone at C1- and C4-positions of deoxyribose in the ring-opened form of OAS allowed efficient reaction with amines. Kinetic studies indicated that reductive amination of OAS-containing ODNs with a proximal amine on the complementary strand proceeded 10 times faster than the corresponding reaction of an ODN containing an abasic site with C1-aldehyde. Efficient reductive amination between the DNA-binding domain of Escherichia coli DnaA protein and ODNs carrying OAS in the DnaA-binding sequence proceeded at the lysine residue in proximity to the phosphate group at the 5′-position of the OAS, in contrast to unsuccessful conjugation with abasic site ODNs, even though they have similar aldehydes. Theoretical calculation indicated that the C1-aldehyde of OAS was more accessible to the target lysine than that of the abasic site. These results demonstrate the potential utility of cross-linking strategies that use dicarbonyl-containing ODNs for the study of protein–nucleic acid interactions. Conjugation with a lysine-containing peptide that lacked specific affinity for ODN was also successful, further highlighting the advantages of 1,4-dicarbonyls.

5-N-tert-Butylaminoxylcytidine was synthesized as a new spin-labelled cytidine to detect guanosine by EPR spectroscopy. Metal-halogen exchange reaction of TBS-protected N-benzoyl-5-iodocytidine followed by a reaction with 2-methyl-2-nitrosopropane afforded 5-N-tert-butylhydroxylaminocytidine with considerable affinity for guanosine and its oxidation gave 5-N-tert-butylaminoxylcytidine. Changes in EPR parameters monitored its selective base pairing with guanosine derivatives and the formation of a 2:2 complex with 8-oxoguanosine.

An oligodeoxynucleotide (ODN) containing a 2′,2′-difluorinated analogue of a C4′-oxidized abasic site (C4′-OAS) was designed for the amine modification of biomolecules that interact with nucleic acids. In contrast to the parent C4′-OAS, which yielded amine-modified products accompanied by DNA strand scission, the ODN containing the difluoro C4′-OAS efficiently yielded products carrying ODNs. The amine modification proceeded without additional reagents being required and might be applicable to reactions in biological systems.