Nano-Mechanical Studies of Collagen: The Influence of Ionic Strength, pH and Collagen Sources on Molecular Flexibility

Abstract

Collagen, a key component of the extracellular matrix, is the most abundant protein in vertebrates, and is widely used in tissue engineering. Collagen is found in almost every connective tissue of the body, including skin, bone, tendon, cartilage, arteries and cornea, where it plays a crucial role in providing structural support. Collagen molecules self-assemble to form hierarchical structures, from single molecules to fibrils to fibers and tissues. Structural and mechanical changes at the molecular level may affect self-assembly of the molecules and mechanics of the resulting tissue. Despite its significance, the mechanics of collagen at the molecular level remain contentious. To rationalize the wide range of values for collagen's molecular flexibility (where literature reports span flexible to rigid rod-like descriptions), here, AFM imaging is used to study the effect of salt and pH on molecular conformations of single collagen molecules. Single molecules of collagen type I are imaged in solutions at a range of ionic strengths and different pH. Image analysis and statistical analysis of the chains are performed with our newly developed algorithm. Results show that collagen's flexibility depends strongly on ionic strength and pH. Surprisingly, collagen type and source do not influence molecular flexibility to a significant extent. These findings are interpreted in terms of polymer and biochemical models in order to shed light on the factors responsible for the stability of this fundamental triple helical protein.