Effect of hydration upon the thermal stability of tropocollagen and its dependence on the presence of neutral salts

TLDR: The endotherm enthalpy changes ΔHD and temperatures TD of thermal denaturation of tropocollagen fibers were measured by DSC calorimetry as functions of water content and the effect of water uptake on theEnthalpy term is explained by water bridge formation within the collagen triple helix.

Abstract: The endotherm enthalpy changes ΔHD and temperatures TD of thermal denaturation of tropocollagen fibers were measured by DSC calorimetry as functions of water content. The denaturation temperatures decrease with increasing water content. The enthalpy change values increase sharply in the range 0–28% of water content, where a maximum of 14.3 cal g−1 is reached. The effect of water uptake on the enthalpy term is explained by water bridge formation within the collagen triple helix. Evidence is given for the existence of approximately three intercatenary water bridges per triplet at the enthalpy maximum, their H-bond energy amounting to approximately 4000 kcal/mol of protein. In the 30–60% range of water content, ΔHD decreases by 2 cal−1 probably due to interactions between secondary water structures and the stabilizing intrahelical water bonds. The influence of two neutral potassium salts, with a structure-stabilizing and a structure-breaking anion (F− and I−), on the hydration dependence of ΔHD and TD was also studied. It was shown that the primary hydration is not influenced by these ions, but that TD and ΔHD are altered in an ion specific way in the presence of interface and bulk water. Hydrophobic interactions do not explain the experimental results. A reaction mechanism of the effects of ions upon the structural stability of collagen is proposed and discussed in terms of interactions of the medium water molecules with the intrahelical water bonds, and in terms of proton-donor/proton-acceptor equilibria between peptide groups, hydrated ions, and intrahelical water molecules.