Oxidative stress and the mechanical properties of naturally occurring chimeric collagen-containing fibers

The byssal threads of marine mussels are a fiber-reinforced composite mater ial. Fibers are continuous, separated by matrix, and consist of chimeric co llagens that encompass within the same primary protein structure domains co rresponding to collagen, polyhistidine, and either elastin or dragline spid er silk. The elastic modulus (stiffness) of the proximal portion of byssal threads was measured by cyclic stress-strain analysis at 50% extension. Bef ore measurement, the threads were conditioned by various treatments, partic ularly agitation in aerated or nitrogen-sparged seawater. Stiffness can be permanently increased by more than two times, e.g., from 25 MPa to a maximu m of 65 MPa, by simple agitation in aerated seawater. Much but not all of t his stiffening can be prevented by agitation under nitrogen. Reversible str ain stiffening would seem to be a useful adaptation to lower residual stres ses arising from the deformation of two joined materials, i.e., distal and proximal portions with rather different elastic moduli. The permanent strai n stiffening that characterizes proximal byssal threads subjected to oxidat ive stress is probably due to protein cross-linking. In the short term, thi s results in a stronger thread but at the expense of dynamic interactions b etween the molecules in the structure.