In vitro collagen fibril assembly: thermodynamic studies

The in vitro fibril assembly of calf skin collagen was examined as a function of ionic strength and temperature. In a 0.03 M NaPi, pH 7.0, buffer, fibril assembly required a minimum critical concentration of collagen. At nearly physiological ionic strengths and temperatures, the critical concentration was less than 1 microgram/mL and required a very sensitive method for measurement. Raising the ionic strength of the buffer resulted first in higher and then lower critical concentrations. Raising the temperature led to lower critical concentrations. A van't Hoff plot of the fibril growth constant calculated from the critical concentration gave positive enthalpy changes and positive heat capacity changes which indicate that the fibril growth is driven by both hydrophobic and ionic inter-collagen interactions. Sedimentation equilibrium studies showed the collagen to be monomeric at subcritical concentrations. Differential scanning microcalorimetric studies showed only one very sharp heat absorption peak for the fibril assembly which coincided with the appearance of solution turbidity. Within experimental error, the enthalpy changes of the fibril assembly measured with the microcalorimeter were of the same magnitude as the van't Hoff enthalpy changes. These results are discussed in light of a cooperative nucleation-growth mechanism of collagen fibril assembly proposed earlier.