Studies of the Collagen-like Peptide (Pro-Pro-Gly)10 Confirm that the Shape and Position of the Type I Collagen Denaturation Endotherm is Governed by the Rate of Helix Unfolding

The kinetics of unfolding of a collagen-like peptide, (Pro-Pro-Gly)10, has been studied under isothermal conditions to gain a better understanding of the stabilization of the collagen triple helix. The formation process was third-order and relatively insensitive to temperature at concentrations of 1 mg/ml and below, while the unfolding process was first-order and highly temperature-dependent. The helix–coil transition was studied over a range of scanning rates and polymer concentrations, using differential scanning calorimetry and the observations were compared with solutions of an approximate differential equation governing the process. At high concentrations (24 mg/ml) and very low scanning rates (0.025 °C min−1), the helicity, F, approached a quasistatic state in which it reached its equilibrium value at all temperatures. Under these conditions, the temperature at which the endotherm peaked, Tmax, increased with chain concentration but was independent of scanning rate, while (dF/dT)max was dependent on the vanʹt Hoff enthalpy and on the order of the formation process. On scanning from a low to a high temperature (up-scanning) at low concentrations (0.25–1.0 mg/ml) and higher scanning rates (0.1 °C min−1 and above), the peak in dF/dT was taller and narrower than for slow quasistatic scanning. Tmax increased linearly with the logarithm of the scanning rate, and was independent of concentration, while (dF/dT)max was governed by the temperature-dependence of the rate of unfolding. At intermediate scanning rates, two peaks in dF/dT were apparent. One peak was a nascent “quasistatic peak”; the other was a nascent “rate peak”. Comparison of this peptide data with the properties of the collagen denaturation endotherm showed that the collagen denaturation endotherm was determined only by the rate of unfolding, and not by an unobserved equilibrium.