End-Group Engineering in Amphiphilic Segmented Polyurethanes: Strong Impact on Hierarchical Self-Assembled Structure and Antibacterial Activity

A series of amphiphilic polyurethanes were synthesized by condensation polymerization between hexyl-diisocyanate and Boc-protected serinol in the presence of a chain-stopper R-OH (R = short hydrocarbon chain). Deprotection of the Boc group produced amphiphilic polymers with pendant amine groups, which, at pH ∼5, adopted an intrachain H-bonding-stabilized pleated structure. Hierarchical assembly of such folded chains produced unilamellar vesicles with excellent surface display of the amine groups. During this hierarchical assembly, presumably through hydrogen bonding between the terminal urethanes, terminal hydrocarbon chains remained secluded and were compelled to be exposed at the polymersome surface. This resulted in an unexpectedly strong impact of hydrophobicity on antibacterial activity, as the surface-protruded hydrocarbons could very effectively engage in bacterial membrane disruption. Consequently, an increase in the hydrocarbon chain length from C5 to C8 caused a significant change in the minimum inhibitory concentration (MIC) from 7.8 to 3.9 μg/mL and 500 to 31.2 μg/mL, respectively, for E. coli and S. aureus. In contrast, these polymers showed negligible toxicity toward mammalian cells (HC50 in the range of 300 μg/mL). Instead of a hydrocarbon, when a hydrophilic pyridine-containing chain-stopper was used, the resulting polymer, despite having an otherwise identical structure, showed drastically lower antibacterial activity, and the HC50/MIC (E. coli) ratio reduced by about 70 times compared to the most optimized polymer having a 2-ethyl-hexyl chain-stopper. On the other hand, a longer (C16) hydrophobic chain-stopper blocked the hierarchical assembly pathway and produced micelle-like spherical aggregates, which, upon aging, formed elongated structures with significantly reduced antibacterial activity. The lead candidate revealed highly effective biofilm eradication.