Ionic environment effects on collagen type II persistence length and assembly.

Collagen type II is a main structural protein in cartilage and forms fibrils. The radius of the fibrils ranges from 50 nm to a few hundred nm, and previous theoretical studies point to electrostatics and collagen elasticity (measured as the persistence length, l) as the main origin for the self-limiting size scales. In this study, we have investigated the collagen triple helical structure and fibril size scales in pH 2 solutions with varying NaCl concentrations from 10 to 100 mM, at which collagen is positively charged, and in pH 7.4 solutions, with varying ionic strengths from 100 to 250 mM, at which collagen is both positively and negatively charged. Using static and dynamic light scattering, the radius of gyration (R), hydrodynamic radius (R), and second virial coefficient (A) of collagen triple helices are determined, and l is calculated. With increasing ionic strength, triple helical l decreases in pH 2 solutions and increases in pH 7.4 solutions. The value ranges from 60 to 100 nm depending on the ionic environment, but at the salt concentration at which A is near zero, there are no net backbone interactions in solution, and the intrinsic collagen triple helix l is determined to be 90-95 nm. Electron microscopy is used to determine the diameter of fibrils assembled in pH 7.4 conditions, and we compare l of the collagen triple helices and fibril diameter using recent theory on fibril assembly. By better understanding collagen l and fibril assembly, we can further understand mechanisms of biomacromolecule self-assembly.