  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- vs. heterotrimeric) and source (tissue-derived vs. recombinant)
influence flexibility. Using SmarTrace, a chain tracing algorithm we devised,
we performed statistical analysis of collagen conformations collected with
atomic force microscopy (AFM) to determine the protein's mechanical properties.
Our results show that types I, II and III collagens - the key fibrillar
varieties - exhibit molecular flexibilities that are very similar. However,
collagen conformations are strongly modulated by salt, transitioning from
compact to extended as KCl concentration increases, in both neutral and acidic
pH. While analysis with a standard worm-like chain model suggests that the
persistence length of collagen can attain almost any value 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 (cWLC) in two dimensions. Analysis within the cWLC model shows that
collagen's curvature depends strongly on pH and salt, while 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.
