Solid-Phase Synthesis of Hybrid Urea Oligomers Containing Conservative Thiourea Mutations

Urea based foldamers

N,N'-linked oligoureas are a class of enantiopure, sequence-defined peptidomimetic oligomers without amino acids that form well-defined and predictable helical structures akin to the peptide α-helix. Oligourea-based foldamers combine a number of features-such as synthetic accessibility, sequence modularity, and folding fidelity-that bode well for their use in a range of applications from medicinal chemistry to catalysis. Moreover, it was recently recognized that this synthetic helical backbone can be combined with regular peptides to generate helically folded peptide-oligourea hybrids that display additional features in terms of helix mimicry and protein-surface recognition properties. Here we provide detailed protocols for the preparation of requested monomers and for the synthesis and purification of homo-oligoureas and peptide-oligourea hybrids.

Hybrid Cell-Penetrating Foldamer with Superior Intracellular Delivery Properties and Serum Stability

Sequence specific molecules with high folding ability (i.e., foldamers) can be used to precisely control the distribution and projection of side chains in space and have recently been introduced as tailored systems for delivering nucleic acids into cells. Designed oligourea sequences with an amphipathic distribution of Arg- and His-type residues were shown to form tight complexes with plasmid DNA, and to effectively promote the release of DNA from the endosomes. Herein, we report the synthesis of new cell-penetrating foldamer sequences in which the foldamer segment is conjugated via a reducible disulfide bond to a ligand that binds cell-surface expressed nucleoproteins with the idea that this system could facilitate both assemblies with nucleic acids and cell entry. This new system was evaluated for delivery of DNA in several cell lines and was found to compare favorably with all comparators tested (DOTAP and b-PEI as well as a number of known cell penetrating peptides) in various cell lines and particularly in culture medium containing up to 50% of serum. These results suggest that this dual molecular platform which is long lasting and noncytotoxic could be of practical use for in vivo applications.

Postelongation Strategy for the Introduction of Guanidinium Units in the Main Chain of Helical Oligourea Foldamers

The synthesis of hybrid urea-based foldamers containing isosteric guanidinium linkages at selected positions in the sequence is described. We used a postelongation approach whereby the guanidinium moiety is introduced by direct transformation of a parent oligo(urea/thiourea) foldamer precursor. The method involves activation of the thiourea by treatment with methyl iodide and subsequent reaction with amines. To avoid undesired cyclization with the preceding urea moiety, resulting in heterocyclic guanidinium formation in the main chain, the urea unit preceding the thiourea unit in the sequence was replaced by an isoatomic and isostructural γ-amino acid. The approach was extended to solid-phase techniques to accelerate the synthesis of longer and more functionalized sequences. Under optimized conditions, an octamer hybrid oligomer incorporating a central guanidinium linkage was obtained in good overall yield and purity. This work also reports data related to the structural consequences of urea by guanidinium replacements in solution and reveals that helical folding is substantially reduced in oligomers containing a guanidinium group.

Effect of replacing main-chain ureas with thiourea and guanidinium surrogates on the bactericidal activity of membrane active oligourea foldamers

Membrane-active foldamers have recently emerged as potential mimics of antimicrobial peptides (AMPs). Amphiphilic cationic helical N,N'-linked oligoureas are one such class of AMP mimics with activities in vitro against a broad range of bacteria including Bacillus anthracis, a Gram-positive sporulating bacillus and causing agent of anthrax. Here we have used site-selective chemical modifications of the oligourea backbone to gain additional insight into the relationship between structure and function and modulate anthracidal activity. A series of analogues in which urea linkages at selected positions are replaced by thiourea and guanidium surrogates have been prepared on solid support and tested against different bacterial forms of B. anthracis (germinated spores and encapsulated bacilli). Urea→thiourea and urea→guanidinium replacements close to the negative end of the helix dipole led to analogues with increased potency and selectivity for B. anthracis versus mammalian cells.