Base-boronated dinucleotides: synthesis and effect of N7-cyanoborane substitution on the base protons

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Abstract
Boron-modified nucleic acids comprise a new set of DNA mimics that have potential biological and therapeutic applications. A series of nine dinucleotides containing N7-cyanoborane-2′-deoxyguanosine (7bdG) at the 3′, 5′ or both positions of the phosphodiester linkage have been synthesized using solution phase phosphoramidite chemistry. Fmoc was used as the 5′-protecting group because of incompatibility of the cyanoborane moiety with 5′-DMT cations generated during the deprotection step. The presence of the cyanoborane group was confirmed on the basis of Fab-MS and 1H NMR spectroscopy. The H-8 proton of 7bdG in the dinucleotides shifted 0.35-0.80 p.p.m. downfield relative to that of unmodified dG. A comparison of the D2O exchange kinetics of the H-8 proton at 60°C showed that H-8 of 7bdG is very labile relative to unmodified dG, indicating that the N7-cyanoborane modification increases the acidity of the H-8 proton of 7bdG. These studies illustrate the feasibility of synthesizing boron-containing oligonucleotides which are modified at the N7-guanine to block Hoogsteen pairing in the DNA major groove.