PROTEIN METAL COMPLEXES AS REVERSIBLE SACRIFICIAL
BONDS IN SELF-HEALING BIOPOLYMERS
C.N.Z. Schmitt 1, Y. Politi1, P. Fratzl1 and M.J. Harrington1
1 Max Planck Institute of Colloids and Interfaces, Department of Biomaterials, 14424
Potsdam, Germany – e-mail: clemens.schmitt@mpikg.mpg.de, yael.politi@ mpikg.mpg.de, peter.fratzl@mpikg.mpg.de, matt.harrington@mpikg.mpg.de
Keywords: biomaterials, self-healing, metal coordination, Raman, EXAFS
ABSTRACT
Protein-metal coordination complexes often play an active role in the mechanisms of enzymatic catalysis, as well as signal transduction, gas transport, and hormones. Recent investigations reveal that these complexes can also act as load-bearing bonds in biological materials such as the jaws of Nereis worms[1] and mussel byssal threads.2 Byssal threads are protein fibers that mussels use to attach to substrates at the seashore. These threads dissipate large amounts of mechanical energy by extending up to ~100% of their initial length and subsequently self-healing to regain initial mechanical properties. Complexes between histidine residues in thread proteins and transition metal ions have been suggested as suitable candidates for reversible sacrificial bonds for this self-healing behavior [2]. However, the existence of histidine-metal coordination in mussel byssal threads and their role in thread mechanics remains to be substantiated in situ.
In this study we combine in-situ Raman spectroscopy and X-ray absorption spectroscopy (XAS) to probe the coordination environment of zinc in Mytilus californianus byssal threads during stretching and subsequent healing. Analysis of the extended X-ray absorption fine structure (EXAFS) shows stress-induced perturbations in the coordination environment of zinc and recovery to near initial coordination geometry after healing. While EXAFS sheds light on the species of the nearest neighboring atoms of zinc and the coordination geometry, Raman spectroscopy investigates transitions in histidine vibration modes due to binding of metal ions such as zinc. The combination of both in-situ techniques provides important insights into the role of histidine-zinc coordination in mussel byssal thread mechanics and healing behavior.
Extracted biochemical concepts from the byssal threads have already inspired the development of metallopolymeric hydrogels exhibiting self-repair behavior [3].
REFERENCES
[1] C.C. Broomell, M.A. Mattoni, F.W. Zok, Critical role of zinc in hardening of Nereis jaws, The Journal of Experimental Biology 209 (2006) 3219-3225.
[2] M.J. Harrington, H.S. Gupta, P. Fratzl, J.H. Waite, Collagen insulated from tensile damage by domains that unfold reversibly: In situ X-ray investigation, Journal of Structural Biology 167 (2009) 47-54.
[3] D.E. Fullenkamp, L. He, D.G. Barrett, W.R. Burghardt, P.B.D. Messersmith, Mussel-inspired histidine-based transient network metal coordination hydrogels, Macromolecules 46 (2013) 1167-1174.
