Making the negative positive – fluorination of indole as an efficient strategy to improve guanidinium-containing gene carriers

The balance between hydrophilic and hydrophobic components plays an important role in polymeric delivery of nucleic acids. Besides using hydrophobic moieties in the polymer design, fluorination is a promising method to increase the hydrophobicity of polymers. To systematically investigate this effect, <i>N</i>-(2-(1<i>H</i>-indol-3-yl)ethyl)methacrylamide and three fluorinated analogues have been synthesized and copolymerized with 3-guanidinopropyl methacrylamide and 2-hydroxypropyl methacrylamide <i>via</i> an aqueous reversible addition-fragmentation chain transfer (aRAFT) polymerization. A library of eight terpolymers with 5 to 23 mol% of an indole analogue and molar mass about 20 kg mol^-1 showed comparably strong DNA binding starting at N/P 2 and formed polyplexes with hydrodynamic diameters around 100 nm. Additionally, no negative impact on biocompatibility was observed. Heparin release studies showed increased DNA binding strength with higher amounts of hydrophobic moieties, while fluorination exhibited similar effects as increasing the indole content. This was also important for pDNA transfection efficiency, where an optimum for DNA binding strength was unveiled. The rapid release and the excessive binding of DNA were identified as factors that negatively impacted transfection efficiency, both influenced by the amount of indole moieties and fluorination. On the other hand, the right degree of hydrophobicity was able to increase the transfection efficiency of the modified polymer by more than threefold. These findings highlight the role of hydrophobic moieties in nucleic acid delivery and provide valuable insights for future polymer design, suggesting that the strategic incorporation of fluorinated monomers can effectively fine-tune DNA interactions.