Influence of saline and pH on collagen type I fibrillogenesis in vitro: fibril polymorphism and colloidal gold labelling.

We have produced different collagen type I fibrils by in vitro fibrillogenesis of acetic acid-soluble collagen within the pH range 2.5-9.0, in the presence and absence of 150 mM NaCl. The varying relatively stable molecular assemblies and polymorphic fibrillar end-products produced after 24 h incubation have been assessed and compared by the TEM study of specimens negatively stained with uranyl acetate. In the presence of 150 mM NaCl, the assembly of collagen at low pH (2.5) leads to the formation of initial molecular aggregates that progressively link together at slightly higher pH (5.0) to form sub-fibrils and spindle-shaped D-banded bundles of sub-fibrils. At pH 6.0 these D-banded bundles aggregate into larger spindle-shaped fibrils with lateral misalignment of the D-banding across the bundle. However, at pH 7.0 and 8.0, in the presence of 150 mM NaCl, the characteristic parallel-sided mature D-banded collagen type I fibres are formed. At pH 9.0 more loosely formed parallel-sided D-banded collagen fibrils are present, within which the spindle-shaped sub-fibrils can be defined by negative staining more convincingly than at pH 7-8. In the presence of 50 mM buffer at pH 2.5, but absence of 150 mM NaCl, collagen type I forms disorganized periodic initial molecular aggregates, which have a tendency to link together to form sub-fibrils. Flexuous collagen type I sub-fibrils predominate at pH 5.0, alongside large spindle-shaped fibrils that possess a regular transverse approximately 10 nm periodicity, with an oblique approximately 67 nm periodicity, significantly different to the D-banding periodicity. At pH 7.0 and pH 8 in the absence of saline loosely-formed flexuous and spindle-shaped fibres co-exist, with underlying sub-fibrils visible, but at pH 9.0 only disorganized flexuous fibrillar aggregates are present. Colloidal gold labelling of the characteristic D-banded collagen type I fibrils with 5 nm and 2 nm chemically reactive gold particles reveals a periodic labelling pattern, which is not apparent with 10 nm and 15 nm gold particles, due to steric hindrance. The flexuous and spindle-shaped collagen fibrils also bind 2 nm gold particles in a specific manner. In all cases, the specific chemisorption of gold onto the collagen fibrils is probably determined by the availability of repeating amino acid side chains of the collagen molecules along the fibril surface. The controlled production of varying stable collagen type I fibrillogenesis products is likely to be of value within numerous areas of biotechnology, biology and medicine, including experimental biomineralization.