Abstract
The predominant structural protein in vertebrates is collagen, which plays a key role in extracellular matrix and connective tissue mechanics. Despite its prevalence and physical importance in biology, the mechanical properties of molecular collagen are far from established. The flexibility of its triple helix is unresolved, with descriptions from different experimental techniques ranging from flexible to semirigid. Furthermore, it is unknown how collagen type (homo- versus heterotrimeric) and source (tissue derived versus recombinant) influence flexibility. Using SmarTrace, a chain-tracing algorithm we devised, we performed statistical analysis of collagen conformations collected with atomic force microscopy to determine the protein's mechanical properties. Our results show that types I, II, and III collagens-the key fibrillar varieties-exhibit similar molecular flexibilities. However, collagen conformations are strongly modulated by salt, transitioning from compact to extended as KCl concentration increases in both neutral and acidic pH. Although analysis with a standard worm-like chain model suggests that the persistence length of collagen can attain a wide range of values within the literature range, closer inspection reveals that this modulation of collagen's conformational behavior is not due to changes in flexibility but rather arises from the induction of curvature (either intrinsic or induced by interactions with the mica surface). By modifying standard polymer theory to include innate curvature, we show that collagen behaves as an equilibrated curved worm-like chain in two dimensions. Analysis within the curved worm-like chain model shows that collagen's curvature depends strongly on pH and salt, whereas its persistence length does not. Thus, we find that triple-helical collagen is well described as semiflexible irrespective of source, type, pH, and salt environment. These results demonstrate that collagen is more flexible than its conventional description as a rigid rod, which may have implications for its cellular processing and secretion.