The role of extrahelical peptides in stabilization of collagen fibrils.

A quantitative model for fibril assembly of type I collagen was extended to include the explicit effect of extrahelical peptides. The collagen molecule was simulated by rod-like sequences to which short, rigid tails were connected by "nondimensional" flexible joints. Three collagen structures were studied: (1) intact collagen, simulated by a rod of axial ratio 200 (The axial ratio x was taken as a segment length) with two tails of length x = 1 and x = 2, respectively, appended to each end; (2) pepsin-digested collagen, simulated by one rigid segment of length 200 and one tail of length 1; and (3) pronase-digested collagen, by a single rigid segment of length x = 200. Phase equilibria of such structures were calculated, using a lattice theory of Matheson and Flory, and the relation of the polymer-solvent interaction parameter chi to the equilibrium solubility was determined. The chi for each collagen species was then related to temperature (T) and ionic strength (I), based on the approximation that local (per segment) stabilization of collagen fibrils was due to hydrophobic and electrostatic forces only. Solubility vs temperature curves for all three collagen species were computed and compared to published experimental data. From the chi factors for each species, the composite chi was resolved into components representing energetic contributions of the extrahelical peptides relative to the helix, which were interpreted in terms of hydrophobic or electrostatic interactions stabilizing the collagen fibril.