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 Δ H ° s = −7.0 kJ per tripeptide unit, Δ S ° 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.