Characterization of nuclei in in vitro collagen fibril formation

Heat precipitation fibril formation in collagen solutions depends upon the prior thermal history of the solution. Collagen solutions were heat precipitated to various extents at 30°C, cooled, and then brought to a second precipitation. Kinetic analysis of the secondary precipitation demonstrated that only the nucleation phase of the precipitation was affected, not the fibril growth phase. Thermal history, or memory, is thus related to the formation of low‐temperature‐stable nuclei. A range of nuclei sizes is evident, supporting the concept of a homogeneous nucleation process. Schiffs base formation and establishment of cross‐linkages play no role in the in vitro nucleation: thiosemicarbazide treated collagen behaves identically to untreated collagen in kinetics of assembly to fibrils. Low‐temperature‐stable nuclei formed at neutral pH are dissociated in the cold in acetic acid at pH 4. Pronase and pepsin susceptible molecular end regions are important in establishing the low‐temperature‐stable nuclei. Pronase treatment completely abolishes the acquisition of memory of prior thermal history in collagen solutions. We speculate that biological control mechanisms for fibril formation in vivo relate to specific interactions between non‐helical, enzyme susceptible regions on collagen molecules.