A two step model for lateral growth of collagen fibrils.

A two step model of lateral growth of type I collagen fibrils is presented which is an extension of a previous model. Step I of lateral growth involves interactions between pairs of opposite charges which form dipoles perpendicular to the molecular axis and results in removal of loosely bound water between collagen molecules in solution. During step II of lateral aggregation, rearrangement of the inner hydration layer occurs as a result of intermolecular interactions between dipoles parallel to the molecular axis, unpaired charges and hydrophobic residues. It is proposed that differences in fibril diameters between types I, II and III collagen reflect differences in the extent of step II lateral aggregation. Type III collagen which forms thinner fibrils than type I, is shown to have fewer theoretical step II charge-charge interactions consistent with the model that fibril diameters are dependent on secondary lateral growth. Based on this model the fibril forming ability of collagen is in part a result of interactions between dipoles oriented perpendicular to the molecular axis whereas the extent of fibril diameter growth reflects secondary charge-charge and hydrophobic interactions.