Solid-phase synthesis of positively charged deoxynucleic guanidine (DNG) oligonucleotide incorporating 7-deazaguanine bases

DNG nucleotides represent a positively charged DNA analog in which the negatively charged phosphodiester linkages of DNA are replaced by positively charged guanidinium linkages. We report herein the synthesis of 3′-end, middle, and 5′-end monomers required for the synthesis of a DNG sequence in which the natural guanine base is replaced by 7-deazaguanine (c7G). 7-Deazaguanine nucleobase was chosen because of their unique glycoside bond stability and their ability to prevent G-quartet formation. A facile and high yield two-step synthesis of xylo-7-deazaguanine 7, a key intermediate for introducing 3′-amino functionality, is carried out under Mitsunobu conditions. Subsequently, the 3′-Fmoc-protected thiourea monomers 13 and 19 were prepared from 7 via their corresponding 3′-amino-7-deazaguanines 11 and 18, respectively. The smooth coupling of these thiourea monomers with monomethoxytrityl (MMTr)-protected 3′-end monomer 25, prepared from 5, occurred on solid phase in 3′ → 5′ direction. The resultant trimeric HO-c7Ggc7Ggc7G-OH (1) has been designed to be included into DNA using standard DNA synthesis technology. The combination of C-c7G base pairing and electrostatic association of phosphodiester and guanidinium backbone allows the small synthesized DNG trimer 1 to form 1:1 complex with DNA-C pentamer.

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