DENATURATION OF TYPE-I COLLAGEN FIBRILS IS AN ENDOTHERMIC PROCESS ACCOMPANIED BY A NOTICEABLE CHANGE IN THE PARTIAL HEAT-CAPACITY

Thermal transitions of typo I collagen fibrils were investigated by di fferential scanning calorimetry and spectrophotometry of turbidity wit hin a wide range of external conditions. The advanced microcalorimeter allowed us to carry out the measurements at low concentrations of col lagen (0.15-0.3 mg/mL), At these concentrations of collagen and under fibril-forming conditions, the melting curves display two pronounced h eat adsorption peaks (at 40 and 55 degrees C). The low-temperature pea k was assigned to the melting of monomeric collagen, while the high-te mperature peak was assigned to the denaturation of collagen fibrils. I t was shown that the denaturation of fibrils, in contrast to the monom eric collagen, is accompanied by a noticeable change in the partial sp ecific heat capacity. Surprisingly, comparison of the collagen calorim etric curves in the fibril-forming and nonforming conditions revealed that Delta C-p of fibril denaturation is caused by a decrease in the C -p of collagen at premelting temperatures. This suggests the existence of an intermediate structural state of collagen in a transparent solu tion preceding fibril formation. Our study also shows thar collagen fi brils formed prior to heating have thermodynamic parameters different from those of fibrils formed and denatured during heating in the calor imeter. Analysis of the data allowed us to determine the denaturation enthalpy of the mature fibrils and to conclude that the enthalpy plays a more important role in fibril stabilization than was previously ass umed. The observed large Delta C-p value of fibril denaturation as wel l as the difference between thermodynamic parameters of the mature and newly formed fibrils is readily explained by the presence of water mo lecules in the fibril structure.