Synthesis and Characterization of Methylated Poly(L-histidine) To Control the Stability of Its siRNA Polyion Complexes for RNAi

Poly(L-histidine) (PLH) with dimethylimidazole groups has been synthesized as a pH-sensitive polypeptide to control the stability of its small interfering RNA (siRNA) polyion complexes for RNA interference (RNA. The resulting methylated PLH (PLH-Me) was water-soluble despite deprotonation of the imidazole groups at physiological pH, as determined by acid base titration and solution turbidity measurement. Agarose gel retardation assay proved that the quaternary dimethylimidazole groups worked as cationic groups to retain siRNA. The stability of the PLH-Me/siRNA complexes has depended on the content of hydrophobic groups, that is, tau/pi-methylirnidazole groups as well as deprotonated imidazole groups. PLH-Me exhibited no significant cytotoxicity despite the existence of cationic dimethylimidazole groups. By use of PLH-Me as a pH sensitive siRNA carrier, the PLH-Me/siRNA complexes mediated efficient siRNA delivery attributed to the dimethylimidazole groups, and the gene silencing depended on the content balance among dimethyl, tau/pi-methyl, and unmodified imidazole groups. These results suggest that PLH-Me controls the stability of siRNA polyion complexes by enhancing noncytotoxic siRNA delivery by optimizing the content balance of dimethyl, tau/pi-methyl, and unmodified imidazole groups.