To determine the relative contributions of interchain hydrogen bonds and covalent cross-linkages to the stability of collagen, the solubilization process was compared with the thermal shrinkage in a variety of solvents, some of which compete with internal protein hydrogen bonds (e.g., 2.0 M KCNS, 6.0 M urea). The solubilization process in all solvents clearly occurred in two stages showing the existence of at least two species of molecules in the intact structure. This was confirmed by coacervation-fractionation of the soluble gelatins. The thermodynamics of the activation step in solubilization were determined by rate studies. In water, at TExtraction < TShrinkage, ΔF50 ≠ = 27.7 kcal., ΔH50 ≠ = 42.5 kcal., ΔS50 ≠ = 45.6 e.u.; at TE > TS, ΔF70 ≠ = 27.5, ΔH70 ≠ = 5.9, ΔS70 ≠ = -63. In 2.0 M KCNS ΔF50 ≠ = 25.9, ΔH50 ≠ = 13.3, ΔS50 ≠ = -38.9 while ΔF70 ≠ = 27.0, ΔH70 ≠ = 6.7, ΔS70 ≠ = -59.2. Since ΔS60 ≠ = +361 e.u. and ΔH60 ≠ 141 for shrinkage, it is evident that imbibition of solvent rather than thermal shrinkage is essential for extraction. The discontinuity in ΔS ≠ occurred when the molecular species being extracted was changed rather than at TS as would have been expected if H-bonds were the primary cause of the insolubility of collagen. The origin and distribution of the cross-linkages is discussed.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry