Synthesis and redox-active base-pairing properties of DNA incorporating mercapto C-nucleosides

Here, we describe the synthesis and incorporation of the nucleoside base analogue C-deoxyribonucleoside 3 carrying thiophenol into DNA. The 1′-β compound 3 was synthesized by Friedel-Crafts alkylation, followed by deprotection. The coupling reaction with 3,5-ditoluoyl-1-α/ β-methoxy-2-deoxy-d-ribose and diphenyldisulfide in the presence of SnCl₄ afforded the α/β mixture 2 (β/α=2.8), and the β-form was separated by silica gel chromatography. After formation of the phosphoramidite derivative, the C-nucleoside 3 was incorporated into DNA. When the mercapto-bases were incorporated into complementary singled-stranded (ss) DNAs, the resulting duplex displayed high thermal stabilization on treatment with bubbling O₂ (T_m 73°C), but was destabilized in the presence of mercaptoethanol (T_m 33°C). CD spectra showed that the duplex had a right-handed double-stranded structure. Imino proton NMR studies of temperature stability suggested that the strength of hydrogen bonding around the mercapto C-nucleoside was larger when treated with bubbling O₂ than when in treated with reducing agent. Thus, formation of the base-to-base disulfide bond increased the stability of the duplex; correspondingly, reduction of the disulfide to two thiol bases destabilized the DNA reversibly. The duplex-forming disulfide base pair showed resistance to exonulease III. The present strategy could be used to introduce new functionalities into cells and novel biomaterials.