Sequence-dependent Stability and the Apparent Two-state Thermal Transition of Extended Collagen Triple Helices

The thermal stability of collagen triple helices is strongly influenced by the amino acid sequence of the repeating Gly-X-Y tripeptides, yet how these residue-specific interactions are integrated within an extended triple helix to determine thermal behavior remains poorly understood. Here, we addressed this question using recombinant collagen mimetic peptides (rCMPs) containing extended native sequences from the α ₁ (I) and α ₂ (I) chains of human type I collagen. Triple-helix formation was nucleated by a C-terminal foldon domain and further stabilized by interchain disulfide crosslinking, allowing the apparent melting temperature ( Tₘ ) to reflect interactions within the triple-helical domain independent of nucleation. The stabilizing effects of Pro and Y-position Arg identified in host-guest peptides were largely preserved in extended triple helices, whereas the proposed Lys-Gly-Glu (KGE) interchain salt bridge produced little measurable stabilization, demonstrating the influence of sequence context. Remarkably, identical triple-helical sequences exhibited markedly different thermal behavior when unfolding was initiated under different conditions. Nevertheless, extended triple helices differing substantially in sequence and length retained an apparently two-state thermal transition. These findings support a mechanism in which unfolding is preferentially initiated within regions of lower intrinsic stability, while the continuity of the triple helix couples neighboring regions into a cooperative unfolding process throughout the helix. This mechanism provides a plausible explanation for the longstanding paradox that extended collagen triple helices exhibit persistent sequence-dependent thermodynamic heterogeneity despite a two-state thermal transition, and a framework for investigating how sequence-dependent stability contributes to the structure and function of collagen molecules.