Abstract
Conformationally constrained nucleotides, LNA or α- l -LNA, at the 5′ terminus of the antisense strand impeded gene silencing of small interfering RNA (siRNA) by hindering phosphorylation, thereby deterring loading into the RNA-induced silencing complex. Installation of a phosphate mimic, ( E )-vinyl phosphonate (VP), improved activity considerably. Gene silencing was more efficient when the antisense strand of the siRNA was modified with 5′-VP-α- l -LNA, which adopts a C3′- exo (south) conformation, than when the antisense strand was modified with 5′-VP-LNA, which adopts a C3′- endo (north) pucker. These data underscore the critical role of conformation of nucleotides in RNA interference.