The nacre protein perlucin nucleates growth of calcium carbonate crystals

Abstract

SummaryAtomic force microscopy (AFM) in aqueous solution was used to investigate native nacre of the marine snail Haliotis laevigata on the microscopic scale and the interaction of purified nacre proteins with calcium carbonate crystals on the nanoscopic scale. These investigations were controlled by scanning electron microscopy (SEM), light microscopy (LM) and biochemical methods. For investigations with AFM and SEM, nacre was cleaved parallel to the aragonite tablets in this biogenic polymer/mineral composite. Multilamellar organic sheets consisting of a core of chitin with layers of proteins attached on both sides lay between the aragonite layers consisting of confluent aragonite tablets. Cleavage appeared to occur between the aragonite tablet layer and the protein layer. AFM images revealed a honeycomb-like structure to the organic material with a diameter of the 'honeycombs' equalling that of the aragonite tablets. The walls of the structures consisted of filaments, which were suggested to be collagen. The flat regions of the honeycomb-like structures exhibited a hole with a diameter of more than 100 nm. When incubated in saturated calcium carbonate solution, aragonite needles with perfect vertical orientation grew on the proteinacous surface. After treatment with proteinase K, no growth of orientated aragonite needles was detected. Direct AFM measurements on dissolving and growing calcite crystals revealed a surface structure with straight steps the number of which decreased with crystal growth. When the purified nacre protein perlucin was added to the growth solution (a super-saturated calcium carbonate solution) new layers were nucleated and the number of steps increased. Anion exchange chromatography of the water-soluble proteins revealed a mixture of about 10 different proteins. When this mixture was dialysed against saturated calcium carbonate solution and sodium chloride, calcium carbonate crystals precipitated together with perlucin leaving the other proteins in the supernatant. Thus perlucin was shown to be a protein able to nucleate calcium carbonate layers on calcite surfaces, and in the presence of sodium chloride, is incorporated as an intracrystalline protein into calcium carbonate crystals.

Temporal and spatial expression patterns of biomineralization proteins during early development in the stony coral Pocillopora damicornis

“…Therefore, proteins with a basic pI (isoelectric point) are unexpected components of the coral SOM and are reported here for the first time for hexacorals. Basic proteins were reported to be a key accelerator in the control of crystal growth in mollusc nacre [38,39], and are present in echinoderm spicules [40,41]. The high content of arginine and lysine can interact with negatively charged ions (bicarbonate) or acidic matrix proteins.…”

Structure and mechanical properties of selected protective systems in marine organisms

“…Calcium carbonate (CaCO 3 ) is the biomineral phase in many of the highly rigid marine biological structural materials such as the nacre of abalone, conch and other marine mollusks as well as urchin spines and some crustacean exoskeletons [66][67][68][69]. Amorphous CaCO 3 is employed as a precursor to crystalline structures that generally take the form of aragonite (orthorhombic) or calcite (rhombohedral) depending on the growth parameters [68].…”

“…9a for the example case of abalone) [67,[93][94][95]. These shells forfeit all flexibility in favor of high (a) Cross-section of the abalone shell displaying the calcitic and aragonitic (nacre) layers; (b) the brick-and-mortar structure of nacre displaying calcium carbonate plates with mortar-like chitin layers; (c) crack deflection toughening mechanism that causes a more tortuous crack path; (d) microbuckling toughening mechanism in compression of the calcium carbonate plates that creates significant additional surface area.…”

“…strength, which is reflected in the relative proportions of rigid biomineral to ductile biopolymer found in the shells of red abalone (19:1) [96], where biomineral severely outweighs biopolymer. In spite of this, many of these shells manage to provide optimized toughness, much higher than pure calcium carbonate [7,67,93]. This is realized through the brick-and-mortar structure of nacre (Fig.…”

Unveiling the Formation Mechanism of Pseudo-Single-Crystal Aragonite Platelets in Nacre

“…2(a) Why does nature assemble aragonite nanoparticles through such a screw dislocation mechanism? Indeed, several independent studies reported screw dislocationbased spiral growth of aragonite platelets [9][10][11][12][13]. However, the assembly mechanism of the aragonite nanoparticles inside individual platelets is completely unknown.…”