Yuji Higaki,* Kazutaka Kamitani, Takuji Ohigashi, Teruaki Hayakawa, Atsushi Takahara*
"Exploring the Mesoscopic Morphology in Mussel Adhesive Proteins by Soft X‑ray Spectromicroscopy"
Biomacromolecules, in press (2021) DOI: 10.1021/acs.biomac.0c01746
“How marine mussels achieve adhesion to wave-swept reefs?”
Scientists have tackled this simple issue for centuries. Mussels attach to marine reefs through holdfasts comprising a spatulate adhesive plaque and a byssal thread. The plaque is cured products of mussel foot proteins. The protein compositions have been identified, all of which are positively charged proteins with a high content of post-translationally modified residues, including 3,4-dihydroxyphenylalanine (Dopa). Mussel-inspired artificial adhesives have been proposed by simply adapting the Dopa and charged groups into synthetic polymers. However, underwater adhesion is often inferior to natural mussel plaque because mussels employ hierarchical structure control at multiple length and time scales by sequential Mfp secretion under precise redox control for adhesion under wet conditions. The well-organized reaction injection-molded adhesive processing in mussel plaque involves coacervation, which is fluid-fluid phase separation in a colloidal system triggered by aggregation of polyions.
We explored the local chemical speciation of the mussel plaque mesoscale structure employing scanning transmission soft X-ray spectromicroscopy installed in the UVSOR synchrotron facility at the Institute for Molecular Science. This instrument was applied to identify the chemical species in the rocks picked up from “Ryugu” last year. The high spatial resolution STXM chemical imaging with C 1s Near Edge X-ray Absorption Fine Structure yields the distribution of the hydroxy-substituted aromatic residues in sub-micron scale non-uniform mussel plaque morphology. The matrix consists of a high protein density cured product containing a large number of hydroxy-substituted aromatic carbons, including tyrosine and 3,4-dihydroxyphenylalanine (Dopa), whereas the microdomains are poor protein density regions with low aromatic residue relative content. The adhesive interface was covered with the matrix phase to ensure adhesion. The cuticle layer involves moderate Dopa content, which appears to be optimized for the mechanical performance of the skin. It is my greatest pleasure to unravel what humans never know. I was so impressed by the fantastic molecular system adopting controlled liquid-liquid phase separation of plaque proteins. This finding would be of great help to our future research project.
I appreciate all coauthors who participated in this project. I would like to express my sincere gratitude to Prof. Takahara for giving me a valuable opportunity to tackle this exiting research project. Also, the experiments could be never done without the cooperation of Dr. K. Kamiya (Sessile Research Corp.) who conduct the mussel attachment sample preparation, and R. Kikuchi (TIT) who support thin film sample preparation using an ultra-microtome. I sincerely appreciate their kind contribution to our project.
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