The formation of collagen fibrils from soluble monomers and aggregates by thermal gelation at neutral pH can be divided into two distinct stages: a nucleation phase and a growth phase. Turbidity studies of the kinetics of the precipitation reaction show that the lag‐phase time or nucleation reaction time, t′l, is markedly temperature dependent while the growth reaction time is temperature independent. The activation energy of the nucleation reaction is essentially constant over the temperature range studied. In monitoring the nucleation‐phase reaction by various physicochemical techniques, including viscosity, sedimentation equilibrium, and light scattering, no evidence for the formation of aggregates was observed. Enrichment of the initial collagen solution with aggregates accelerates nucleation, but de novo nuclei formation is still required even in highly aggregated collagen preparations. Removal of pepsin and pronase susceptible peptides lengthens the nucleation reaction time and increases the sensitivity of the rate of nuclei formation to changes in ionic strength. Electron microscope studies show the fibrils formed from the protease‐treated collagen to be less well organized. With pepsin‐treated collagen, subfibrils and obliquely striated fibrils are seen, showing that while microfibrils are formed interactions between them are modulated by the enzyme susceptible peptides in the same way that these regions modulate nuclei assembly. It appears that pepsin and pronase susceptible peptide regions of collagen play a more prominent role in the in vitro assembly of collagen molecules to form D‐stagger nuclei and fibrils than do ionic interactions between helical molecular regions. A mechanism of nucleation of collagen fibrillogenesis is discussed.
ASJC Scopus subject areas
- Organic Chemistry