Thermodynamic studies of the collagen-like region of human subcomponent C1q. A water-containing structural model.

Thermal transitions of Clq were investigated by methods of differential scanning calorimetry, circular dichroism and fluorescence. The melting curves of Clq display two pronounced heat absorption peaks with enables determination of the thermodynamic parameters characterizing each transition. The low temperature peak was assigned to melting of the Clq collagenous part. Analysis of the data has revealed unusual, as compared with the monomeric collagen molecules, thermodynamic features of the Clq collagenous part: (1) higher thermal stability strongly dependent on pH; (2) less linear co-operative regions; and (3) a noticeable change in the partial specific heat capacity (delta Cp) in contrast to both the monomeric collagen and the collagen fibrils. This unusually large delta Cp value suggested a conclusion that the fibril-like endpiece of Clq may have a cavity filled with ice-like ordered water molecules.