Previous optimisation studies of peptide/peptoid hybrids typically comprise comparison of structurally related analogues displaying different oligomer length and diverse side chains. The present work concerns a systematically constructed series of 16 closely related 12-mer oligomers with an alternating cationic/hydrophobic design, representing a wide range of hydrophobicity and differences in relative side-chain lengths. The aim was to explore and rationalise the structure–activity relationships within a subclass of oligomers displaying variation of three structural features: (i) cationic side-chain length, (ii) hydrophobic side-chain length, and (iii) type of residue that is of a flexible peptoid nature. Increased side-chain length of cationic residues led to reduced hydrophobicity till the side chains became more extended than the aromatic/hydrophobic side chains, at which point hydrophobicity increased slightly. Evaluation of antibacterial activity revealed that analogues with lowest hydrophobicity exhibited reduced activity against E. coli, while oligomers with the shortest cationic side chains were most potent against P. aeruginosa. Thus, membrane-disruptive interaction with P. aeruginosa appears to be promoted by a hydrophobic surface of the oligomers (comprised of the aromatic groups shielding the cationic side chains). Peptidomimetics with short cationic side chains exhibit increased hemolytic properties as well as give rise to decreased HepG2 (hepatoblastoma G2 cell line) cell viability. An optimal hydrophobicity window could be defined by a threshold of minimal hydrophobicity conferring activity toward E. coli and a threshold for maximal hydrophobicity, beyond which cell selectivity was lost.

PEGylation of antimicrobial peptides as a shielding tool that increases stability toward proteolytic degradation typically leads to concomitant loss of activity, whereas incorporation of ultrashort PEG-like amino acids (sPEGs) remains essentially unexplored. Here, modification of a peptide/β-peptoid hybrid with sPEGs was examined with respect to influence on hydrophobicity, antibacterial activity and effect on viability of mammalian cells for a set of 18 oligomers. Intriguingly, the degree of sPEG modification did not significantly affect hydrophobicity as measured by retention in reverse-phase HPLC. Antibacterial activity against both wild-type and drug-resistant strains of Escherichia coli and Acinetobacter baumannii (both Gram-negative pathogens) was retained or slightly improved (MICs in the range 2–16 µg/mL equal to 0.7–5.2 µM). All compounds in the series exhibited less than 10% hemolysis at 400 µg/mL. While the number of sPEG moieties appeared not to be clearly correlated with hemolytic activity, a trend toward slightly increased hemolytic activity was observed for analogues displaying the longest sPEGs. In contrast, within a subseries the viability of HepG2 liver cells was least affected by analogues displaying the longer sPEGs (with IC₅₀ values of ~1280 µg/mL) as compared to most other analogues and the parent peptidomimetic (IC₅₀ values in the range 330–800 µg/mL).

In response to the growing threat of antimicrobial resistance, scaffold-based peptidomimetics were explored as potential antibacterial agents.