The triple-helical structure of collagen, responsible for collagen's remarkable biological and mechanical properties, has inspired both basic and applied research in synthetic peptide mimetics for decades. Since non-proline amino acids weaken the triple helix, the cyclic structure of proline has been considered necessary, and functional collagen mimetic peptides (CMPs) with diverse sidechains have been difficult to produce. Here we show that N-substituted glycines (N-glys), also known as peptoid residues, exhibit a general triple-helical propensity similar to or greater than proline, allowing synthesis of thermally stable triple-helical CMPs with unprecedented sidechain diversity. We found that the N-glys stabilize the triple helix by sterically promoting the preorganization of individual CMP chains into the polyproline-II helix conformation. Our findings were supported by the crystal structures of two atomic-resolution N-gly-containing CMPs, as well as experimental and computational studies spanning more than 30 N-gly-containing peptides. We demonstrated that N-gly sidechains with diverse exotic moieties including a 'click'-able alkyne and a photo-sensitive sidechain can be incorporated into stable triple helices, enabling functional applications such spatio-temporal control of cell adhesion and migration on a gelatin matrix. The folding principles discovered in this study open up opportunities for a new generation of collagen mimetic therapeutics and materials with extraordinary properties. design principles uncovered in this study drastically expand the library of residues with high triple-helical propensity, which has immense implications for a new generation of collagenmimetic therapeutics and materials.
Results and DiscussionTriple-helical stability of CMPs with N-glys. To investigate the triple-helical folding propensity of peptoids residues, we inserted a series of N-gly guests into the central X position of a conventional CMP host peptide with the sequence: Ac-(GlyProHyp)3-Gly-X-Hyp-(GlyProHyp)3-NH2 (designated as X-CMP, Fig. 1b, and supplementary Materials and Methods) 34 . We measured the X-CMPs' triple-helical stability via thermal unfolding experiments monitored under circular dichroism (CD, Fig. 1c, Supplementary Section 3 and Table S1), first in a group of N-glys with sidechains selected from the canonical amino acids (Fig. 1d). It is known that Pro is the most stabilizing amino acid at position X, and substitution from Pro to another canonical amino acid clearly reduces Tm by 4-17 °C (Fig. 1d, column AA) 14 . Surprisingly, we found that many of the N-glys with canonical sidechains were at least as stable as Pro, and almost all N-gly residues were more stable than their amino acid counterparts (Fig. 1d, exception: Nval), with the biggest Tm difference seen between Nphe-and Phe-CMP (Fig. 1c). These results demonstrate that, in the X position of the CMP, shifting the sidechain from the Cα carbon to the nitrogen (i.e., transforming a canonical amino acid to its peptoid analogue) may improve triple-he...