Electrostatic Persistence Length of Collagen Type II Protofibrils and Fibrils

Collagen type II is a main structural component of the extracellular matrix and self-assembles into rigid fibrils. The origin of the fibril radial lengthscales is hypothesized to be related to the flexibility, measured by the persistence length, l_p, of protofibrils. We have investigated the electrostatic contribution to l_p of collagen type II by changing the solution ionic strength (I) of solutions at pH 2, where the collagen is positively charged, and pH 7.4, where collagen has both charges. Using static and dynamic light scattering, l_p is determined from the radius of gyration, R_g. In addition to R_g, the hydrodynamic radius R_h, shape factor R_g/R_h, and second virial coefficient A₂ of the soluble collagen are reported. In pH 2 solutions, collagen does not form fibrils because of electrostatic repulsion, determined from A₂. As electrostatics are screened, both R_g/R_h and l_p decrease with I. In pH 7.4 solutions, in part due to electrostatic attraction, collagen forms fibrils. The fibril radii are reported using Scanning Electron Microscopy imaging and are correlated to the size scales, and l_p determined from solution scattering. By understanding how collagen type II l_p and interactions influence self-assembly, we can better understand the origin of the fibril lengthscales.