THERMAL-STABILITY OF CALF SKIN COLLAGEN TYPE-I IN SALT-SOLUTIONS

The thermal stability of acid-soluble collagen type I from calf skin i n salt solutions is studied by high-sensitivity differential scanning calorimetry. Three concentration ranges have been clearly distinguishe d in the dependence of collagen thermal stability on ion concentration . At concentrations below 20 mM, all studied salts reduce the temperat ure of collagen denaturation with a factor of about 0.2 degrees C per 1 mM. This effect is attributed to: screening of electrostatic interac tions leading to collagen stabilisation. At higher concentrations, rou ghly in the range 20-500 mM, the different salts either slightly stabi lise or further destabilise the collagen molecule in salt-specific way that correlates with their position in the lyotropic series. The effe ct of anions is dominating and follows the order H2PO4- greater than o r equal to SO42- > Cl- > SCN-, with sign inversion at about SQ(4)(2-). This effect, generally known as the Hofmeister effect, is associated with indirect protein-salt interactions exerted via competition for wa ter molecules between ions and the protein surface. At still higher sa lt concentrations (onset concentrations between 200 and 800 mM for the different salts), the temperature of collagen denaturation and soluti on opacity markedly increase for all studied salts due to protein salt ing out and aggregation. The ability of salts to salt out collagen als o correlates with their position in the lyotropic series and increases for chaotropic ions, The SO42- anions interact specifically with coll agen - they induce splitting of the protein denaturation peak into two components in the range 100-150 mM Na2SO4 and 300-750 mM Li2SO4. The variations of the collagen denaturation enthalpy at low and intermedia te salt concentrations are consistent with a weak linear increase of t he enthalpy with denaturation temperature. Its derivative, d(Delta H)/ dT, is approximately equal to the independently measured difference in the heat capacities of the denatured and native states, Delta C-p = C -p(D) - C-p(N) approximate to 0.1 cal . g(-1) K-1.