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  • Title: Synthesis of conformationally locked Carba-LNAs through intramolecular free-radical addition to C=N. Electrostatic and steric implication of the carba-LNA substituents in the modified oligos for nuclease and thermodynamic stabilities.
    Author: Xu J, Liu Y, Dupouy C, Chattopadhyaya J.
    Journal: J Org Chem; 2009 Sep 04; 74(17):6534-54. PubMed ID: 19670835.
    Abstract:
    The syntheses of the hitherto unavailable parent fully unsubstituted carba-LNA and its C7'-amino and/or C6'-hydroxyl substituted derivatives, have been accomplished by the intramolecular 5-exo free-radical addition to a C4'-tethered C=N to give carba-LNAs with variable hydrophilic substituents at C6'/C7' (amino and/or hydroxyl). They have been introduced into isosequential antisense oligonucleotides (AONs) to examine how their relative electrostatic and steric effects in the minor groove of a putative AON-RNA duplex affect the target affinity, nuclease resistance, and RNase H elicitation. We show that 2'-oxygen in LNA is important in stabilizing the DNA/DNA and DNA/RNA duplexes vis-a-vis the unsubstituted carba-LNA and its other derivatives and that hydrophobic groups at C6'/C7' in both carba-LNA and carba-ENA relatively destabilize the AON/DNA duplex more profoundly than those in the AON/RNA duplexes. Two main factors affect the relative stabilities of AON/DNA versus AON/RNA duplexes: (i) hydration in the minor groove depending upon hydrophilicity vis-a-vis hydrophobicity of the substituents, and (ii) the relative size of the minor groove in the AON/DNA versus AON/RNA duplexes dictates the steric clash with the substituents depending upon their relative chiralities. We also show how the chirality and chemical nature of the C6'/C7' substituents affect the nuclease stability as well as the thermal stability and the RNase recruitment by AON/RNA duplexes.
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