The Role of pH, Temperature, Salt Type, and Salt Concentration on the Stability of the Crystalline, Helical, and Randomly Coiled Forms of Collagen

The role of pH, temperature, salt type, and salt concentration on the three possible transformations involving the crystalline, dispersed helical, and randomly coiled forms of collagen has been investigated. The results are given in terms of pseudo phase diagrams where the field of stability of each form is delimited by the curves representing the variation of the three transformation temperatures with salt concentration for a given salt type and pH. The data are adequate to characterize the behavior of the systems corresponding to situations where the solute is an isoelectric protein or a cationic polyelectrolyte. It is shown that a general mechanism for the interaction between salts and proteins based on the previously defined specific (i.e. binding) and diluent effects, is adequate to describe the transformations crystalline form → randomly coiled form, crystalline form → helical form, helical form → randomly coiled form under isoelectric conditions. The polyelectrolyte behavior included the reversal of the orders of effectiveness of anions with respect to the orders observed under isoelectric conditions. This is explained on the basis of an order for binding of the anions to the solute which remains the same irrespective of whether the protein is iso-electric or cationic.