Twice tied tight: Enforcing conformational order in bicyclic peptoid oligomers

Certain proteins contain subunits that enable their active translocation across the plasma membrane into cells. In the specific case of HIV-1, this subunit is the basic domain Tat(49-57) (RKKRRQRRR). To establish the optimal structural requirements for this translocation process, and thereby to develop improved molecular transporters that could deliver agents into cells, a series of analogues of Tat(49-57) were prepared and their cellular uptake into Jurkat cells was determined by flow cytometry. All truncated and alanine-substituted analogues exhibited diminished cellular uptake, suggesting that the cationic residues of Tat(49-57) play a principal role in its uptake. Charge alone, however, is insufficient for transport as oligomers of several cationic amino acids (histidine, lysine, and ornithine) are less effective than Tat(49-57) in cellular uptake. In contrast, a 9-mer of l-arginine (R9) was 20-fold more efficient than Tat(49-57) at cellular uptake as determined by Michaelis-Menton kinetic analysis. The d-arginine oligomer (r9) exhibited an even greater uptake rate enhancement (>100-fold). Collectively, these studies suggest that the guanidinium groups of Tat(49-57) play a greater role in facilitating cellular uptake than either charge or backbone structure. Based on this analysis, we designed and synthesized a class of polyguanidine peptoid derivatives. Remarkably, the subset of peptoid analogues containing a six-methylene spacer between the guanidine head group and backbone (N-hxg), exhibited significantly enhanced cellular uptake compared to Tat(49-57) and even to r9. Overall, a transporter has been developed that is superior to Tat(49-57), protease resistant, and more readily and economically prepared.

Antimicrobial peptides (AMPs) and their mimics are emerging as promising antibiotic agents. We present a library of "ampetoids" (antimicrobial peptoid oligomers) with helical structures and biomimetic sequences, several members of which have low-micromolar antimicrobial activities, similar to cationic AMPs like pexiganan. Broad-spectrum activity against six clinically relevant BSL2 pathogens is also shown. This comprehensive structure-activity relationship study, including circular dichroism spectroscopy, minimum inhibitory concentration assays, hemolysis and mammalian cell toxicity studies, and specular x-ray reflectivity measurements shows that the in vitro activities of ampetoids are strikingly similar to those of AMPs themselves, suggesting a strong mechanistic analogy. The ampetoids' antibacterial activity, coupled with their low cytotoxicity against mammalian cells, make them a promising class of antimicrobials for biomedical applications. Peptoids are biostable, with a protease-resistant N-substituted glycine backbone, and their sequences are highly tunable, because an extensive diversity of side chains can be incorporated via facile solid-phase synthesis. Our findings add to the growing evidence that nonnatural foldamers will emerge as an important class of therapeutics.