The Transition from Stiff to Compliant Materials in Squid Beaks

Abstract

The beak of the Humboldt squid Dosidicus gigas represents one of the hardest and stiffest wholly organic materials known. As it is deeply embedded within the soft buccal envelope, the manner in which impact forces are transmitted between beak and envelope is a matter of considerable scientific interest. Here, we show that the hydrated beak exhibits a large stiffness gradient, spanning two orders of magnitude from the tip to the base. This gradient is correlated with a chemical gradient involving mixtures of chitin, water, and His-rich proteins that contain 3,4-dihydroxyphenyl-L-alanine (dopa) and undergo extensive stabilization by histidyl-dopa cross-link formation. These findings may serve as a foundation for identifying design principles for attaching mechanically mismatched materials in engineering and biological applications.Living organisms are functional assemblages of different interconnected tissues. Not infrequently, tissues with highly disparate mechanical properties (e.g., bone and cartilage, shell and adductor muscle, nail and skin) are joined together (1). In practice, the joining of dissimilar materials can lead to high interfacial stresses and contact damage (2,3). In apparent contradiction to this, the contacts between mechanically mismatched biomolecular tissues are remarkably robust. Mechanical-property gradients are increasingly invoked as the principal reason for their mechanical performance. The dentino-enamel junction (4), arthropod exoskeleton (5), polychaete jaws, and mussel byssal threads (6) all exhibit such gradients. Optical properties in squid eyes have also been correlated to a protein-density gradient (7). Although much is known about the mechanical and biochemical properties of the separate tissues, surprisingly little has been done to explain how mixtures of macromolecules are adapted for incremental mechanical effects at interfaces.The beak of the Humboldt squid Dosidicus gigas is an example of a system with grossly mismatched tissues. It is composed of slightly offset apposing upper and lower parts that make no hard pivotal contact with one another and are set into a muscular buccal mass that controls

Infiltration of chitin by protein coacervates defines the squid beak mechanical gradient

“…Comparing the amino acid compositions of the DgCBPs with those previously reported for the beak 6 (Table 1) shows a clear trend: DgCBP amino acid profiles are similar to those of the untanned portions of the beak, but very different from those of the tanned regions, which include the rostrum. It is noteworthy that unlike the tanned beak portions, DgCBPs have low levels of His, Gly and alanine (Ala) residues.…”

“…Instead, an alternative method was developed in which peptide bond-cleaving reagents were used to chemically cleave the constituent proteins so that proteins and peptide fragments were released from the beak. Hydroxylamine (NH 2 OH), which cleaves between asparaginyl-glycyl peptide bonds 19 , was used since D. gigas beak proteins were previously reported to be rich in asparagine and aspartic acid (Asx) and glycine (Gly) amino acid residues 6,20 . NH 2 OH extraction resulted in distinct beak proteins and/or protein fragments, ranging in size from 5 kDa to more than 250 kDa, that could be visualized easily by SDS-PAGE analysis (Fig.…”

“…The mechanical gradients identified in tissue transitions such as skin-fingernail 1 , bone-tendon 2,3 , tendon-muscle 4 , and distal-proximal transitions in mussel byssal threads 5 and squid beak 6 are efficient at mitigating mechanical damage at interfaces [7][8][9] . Conversely, failures in engineering multicomponent systems are often due to mechanical property mismatches between various components 7 .…”

“…This design principle is key to mitigating the interfacial mechanical damage at the junction between the base of the beak and the buccal mass. The relative proportions of beak components indirectly define the stiffness and hardness gradient in the mature beak structure by controlling hydration levels 6,14 . A fundamental understanding of how a mechanical gradient forms in squid beak necessitates the identification and characterization of the beak proteins and their post-secretion processing.…”

“…1a) has been proposed as a model system for the synthesis of bio-inspired functionally graded materials, with particular relevance to engineering and medical applications 6,12,13 . The squid beak is fully organic and composed of chitin, water, proteins and chemical crosslinks, the relative abundance of which varies spatially, from the soft base embedded within the buccal mass (Fig.…”