Collagen fibril formation in vitro. A quasielastic light-scattering study of early stages

Purified rat tail tendon collagen used in our in vitro assembly studies has been characterized by quasielastic light scattering. At 4 degrees C and neutral pH it behaves as a single monomeric species with a translational diffusion coefficient, D20,w, at infinite dilution of 0.78 X 10(-7) cm2/s. This value is consistent with a somewhat flexible rod (300 X 1.2 nm( having 0.5 to 1.0 g of associated water/g of protein. When the temperature of a neutral solution of this collagen (0.1 mg/ml) is raised to 26 degrees C to initiate assembly, D20,w decreases in 10 min or less to 0.15 X 10(-7) cm2/s (Step 1) and remains constant for about 50 min (Step 2). The material present during Step 2 behaves as if it were a single high molecular weight species. These results together with earlier turbidity studies are consistent with an assembly mechanism whereby during Step 1 monomer is rapidly converted to aggregates > 1500 nm long and < 8 mm in diameter. The minimal number of collagen molecules in these aggregates is between 5 and 100, and they could be polydisperse with monomer still present. Linear growth continues during Step 2 until a critical length is reached. The constant value of D20,w may be explained by its insensitivity to length at these high asymmetries. Step 3, during which the turbidity increases and D20,w cannot be measured, consists of lateral association of the product of Step 2 to form native fibrils.