The enthalpy of interaction between collagen fibres and denaturing solvents.

It is well known that both urea and acid cause collagen fibres to swell and at high conentrations to denature (Rigby, 1961). The mechanisms by which these reactions occur are not well understood. Paz Andrade et al. (1975) have used microcalorimetry to study the reaction between urea and some globular proteins, and have interpreted the results in terms of the binding of urea to the peptide units. The present work was undertaken to extend these observations to a non-globular protein, collagen, and to compare the results with those obtained for the interaction of collagen with dilute acid (HCI). The collagen used was a bovine achilles-tendon preparation obtained from Calbiochem (Los Angeles, CA, U.S.A.). An LKB Sorption microcalorimeter (Kusano et al., 1971), which directly measures the heat of interaction between solids and liquids, was used to determine the enthalpy (AH) of swelling of a known weight of collagen (5-10mg) with about 0.4 cm3 of the appropriate solution at 37°C. Fig. 1 shows the enthalpy of interaction of urea with collagen as a function of the molarity of urea. The reaction of insoluble collagen fibres with water is exothermic and the mean enthalpy for this swelling reaction is -3730 k 240 J/mol(-868 k 58 cal/mol) (where a mol is one amino acid residue of mean mol.wt. 91). The effect becomes more exothermic with increasing urea concentrations, reaching a maximum at about 4-5 mol/dm3. Above this concentration, LU€ decreases (absolute value) to a minimum at 6-7 mol/dm3, and then increases again. We believe these results support the following mechanism for the interaction between collagen and urea. The increase in the exothermal effect observed is due, as postulated by Robinson & Jencks (1965), to the binding of the amine group of urea to the accessible peptide bonds via hydrogen bonds. The initial