A Competing C-Terminal β-Turn Controls Frame-Dependent Stability of Collagen Triple Helices

Frame-dependent stability of collagen triple helices is well documented, but its structural origin remained unresolved. Here we identify a C-terminal β-turn-like conformational state with intramolecular CO···HN contacts as the competing motif governing this behavior. In POG-frame collagen mimetic peptides, a C-terminal primary amide promotes this β-turn-like state via transient intramolecular CO···HN contacts involving the proline carbonyl, reducing its availability for interstrand hydrogen bonding and perturbing terminal polyproline II geometry, directly opposing triple-helix assembly. N-methylation or esterification suppresses β-turn formation, enhancing stability by up to 7.5 °C. Conversely, β-turn formation is disfavored in the GPO frame, where the same terminal amide reinforces interstrand hydrogen bonding. Density-functional theory, NMR, X-ray crystallography, circular dichroism, and molecular dynamics establish this structural basis. Trends across 20−22 residue collagen-mimetic peptides confirm the mechanism arises from local C-terminal interactions, independent of length. These findings reveal how terminal hydrogen-bonding interactions encode competing conformations modulating peptide folding and supramolecular assembly.