Elastic and Ionic Environment Effects on Collagen Fibril Assembly*

Collagen type II is an abundant structural protein found in extracellular matrix (ECM) of cartilage and self assembles into fibrils to provide this tissue structural integrity. However, the origin of the fibril radial length scales is not fully understood and is hypothesized to be related to the flexibility of the protofibril and environmental effects. We have determined the elasticity of collagen type II as a function of the ionic strength, [I]. We then correlate the flexibility to the structure of collagen fibrils in dried and hydrated states in solution and in polyacrylamide (PAM) gels to mimic the elastic environment of the ECM. Determined by light scattering, persistence length, a measure of backbone flexibility, of the protofibrils increases from about 60 to 90 nm with increasing [I] in physiological pH solutions. Interestingly, fibril diameter, as determined by scanning and transmission electron microscopy (TEM), increases similarly in ionic strength solutions ranging from 100 mM to 167 mM. We also find, using cryogenic TEM, that the structure contains little to no water. When the fibrils are formed in a PAM gel, the diameter decreases as a function of the network strength, but fibrils still form even in gels with 9 kPa modulus. Collagen’s triple helical persistence length, solution [I], and surrounding network play significant roles in fibril formation, and these results could lead to further understanding of biomacromolecular assemblies.