Creating a three-dimensional fold and biological function in a non-biological polymer

The only molecules that are currently known to fold into unique three-dimensional conformations and perform sophisticated functions are biological polymers-proteins and some RNA molecules. However, the statistical mechanics of model polymers has shown that polymers having particular sequences of hydrophobic and polar monomers should be able to fold uniquely. This opens up the possibility of generating a well-defined tertiary structure in non-biological polymers. Such molecules may ultimately provide a platform for designing specific functions.; Our aim is to create a non-biological sequence-specific polymer that folds in aqueous solution and performs a biological function. Toward that end, we synthesized sequence-specific 30mer, 45mer and 60mer peptoid oligomers (N-substituted glycine polymers) consisting of amphiphilic 15mer units we chained together by disulfide and oxime linkages in order to mimic the helical bundle structures commonly found in proteins. To probe whether they folded, we used fluorescence resonance energy transfer (FRET) reporter groups. We found that certain constructs fold up with a hydrophobic core and have cooperative folding transitions, resembling those of proteins.; To generate a biological function with a well-defined tertiary structure, we introduced a high-affinity zinc-binding function into a peptoid two-helix bundle. Borrowing from well-understood zinc-binding motifs in proteins, thiol and imidazole moieties were incorporated into the peptoid such that both helices must align in close proximity to form a binding site. We used FRET reporter groups to measure the change of the distance between the two helical segments as well as probe the binding of zinc. We systematically varied the location and number of zinc-binding residues, and the sequence and size of the loop that connects the two helical segments. We found that certain peptoid two-helix bundles bind zinc with subnanomolar affinities and high selectivity over other divalent metal ions. Our work provides a significant step toward generating biomimetic polymers with enzyme-like functions.