Type I collagen in solution. Structure and properties of fibril fragments.

We have measured the diffusion coefficient and weight average molecular weight of type I collagen fibril fragments obtained by acid extraction of rat tail tendons and neutral extraction of lathyritic chick skin, using laser light scatttering techniques. The molecular weight and translational diffusion coefficient were found to be 8.05 +/- 0.40 x 10(5) and 0.450 +/- 0.04 x 10 (-7) cm2/s, respectively, for preparations which contain aggregates in 0.01 M HCl, and 2.82 +/- 0.20 x 10 (5) and 0.780 +/- 0.04 x 10 (-7) cm(2)/s for collagen single molecules. Using these data, as well as the theoretical diffusion coefficient for a single collagen molecule, models for various staggering modes and aggregate mixtures were developed in an attempt to understand the structure of the fibril components present in solution. It was found that several models with 1 to 4 D (D = 67 nm) staggers fit the experimental data for the aggregated preparations. Analysis of the end-to-end distance of negatively stained segment long spacing crystallites prepared from solutions containing fibril fragments and the banding pattern of positively stained segment long spacing crystallites suggest that collagen solutions contain linear aggregates with 4 D and possibly 4.4 D staggers in agreement with light scattering data. Thermal denaturation studies demonstrate that the apparent melting point of cross-linked linear aggregates, 33.5 +/- 0.5 degrees C, is identical with that of single molecules at pH 2.0. We conclude that a linear filament with predominant 4 D stagger is a basic unit of type I collagen fibrillar structure.