Triple helix–coil transition of covalently bridged collagenlike peptides

The thermal triple helix–coil transition of covalently bridged collagenlike peptides with repeating sequences of (Ala‐Gly‐Pro)n, n = 5–15, was studied optically. The peptides were soluble in water/acetic acid (99:1) and were found to form triple‐helical structures in this solvent system beginning with n = 8. The thermodynamic analysis of the transition equilibrium curves for n = 9–13 yielded the parameters ΔH°s = −7.0 kJ per tripeptide unit, ΔS°s = −23.1 J deg−1 mol−1 per tripeptide unit for the coil‐to‐helix transition, and the apparent nucleation parameter σ ≃ 5 × 10−2. It was suggested through double‐jump temperature experiments that the rate‐limiting step during refolding is not only influenced by the difficulties of nucleation, but also by cis–trans isomerization of the Gly‐Pro peptide bond.