Polysaccharide chemistry regulates kinetics of calcite nucleation through competition of interfacial energies

Abstract

Calcified skeletons are produced within complex assemblages of proteins and polysaccharides whose roles in mineralization are not well understood. Here we quantify the kinetics of calcite nucleation onto a suite of high-purity polysaccharide (PS) substrates under controlled conditions. The energy barriers to nucleation are PS-specific by a systematic relationship to PS charge density and substrate structure that is rooted in minimization of the competing substrate-crystal and substrate-liquid interfacial energies. Chitosan presents a low-energy barrier to nucleation because its near-neutral charge favors formation of a substrate-crystal interface, thus reducing substrate interactions with water. Progressively higher barriers are measured for negatively charged alginates and heparin that favor contact with the solution over the formation of new substrate-crystal interfaces. The findings support a directing role for PS in biomineral formation and demonstrate that substrate-crystal interactions are one end-member in a larger continuum of competing forces that regulate heterogeneous crystal nucleation.biomineralization | calcium carbonate | free energy | algae | crustacean E fforts to decipher patterns of biomineralization have identified proteins and polysaccharides (PSs) as major components of the organic matrices associated with sites of calcification. In mollusks and other organisms including the red algae, coccolithophores, and foraminifera (1, 2) (Table 1), calcifying macromolecules are dominated by functional groups with an acidic character-proteins that are rich in carboxylated amino acids (14) and PSs that are highly sulfated and carboxylated (15,16). Although a physical picture of these interactions is not well developed, this recurring affiliation of carboxylate and sulfate groups with zones of mineralization in organisms suggests specific roles for macromolecules in nucleation and growth. Early studies have led the biomineralization community to generally assume that charged proteins actively regulate mineralization. In contrast, PSs, such as the chitin found in the insoluble fraction of the mollusk shell, are thought to provide an inert scaffolding to support these proteins (17,18).Recent studies challenge this assumption with qualitative evidence that PSs can also promote calcium carbonate (CaCO 3 ) mineralization (19,20) (Table 1). For example, specific orientations of chitin fibers with neutral functional groups promote the templating of CaCO 3 in the lobster carapace (13, 21), crab cuticle (21), and nautilus shell (22). More generally, the monosaccharide sequences along PS chains can influence biological function and their interactions with proteins (23). This suggests PS chemistry and interactions with proteins could regulate patterns of mineralization.Anecdotal observations from in vitro studies also support the thinking that PSs influence mineral formation, but their specific effects are unclear. For example, PSs with higher carboxyl and sulfate content promote either faster (24, 25) or slower (2...

Reconciling disparate views of template-directed nucleation through measurement of calcite nucleation kinetics and binding energies

“…Second, in agreement with previous findings for odd and even COOH SAMs (16) and polysaccharide films (17), the kinetics of crystal nucleation can be understood for all of these surfaces through a classical approach. As seen in Fig.…”

“…Recent studies of calcite nucleation on self-assembled monolayers (16) or polysaccharide films (17) and calcium phosphate nucleation on collagen (18) show that Eqs. 1 and 3 give a reasonable description of template-directed nucleation and that the interfacial energy is indeed the governing parameter.…”

“…Using established methods (16,17), the crystallite number density was measured over time. The data show a linear relationship with slopes that increase with σ.…”

A unifying model for Neoproterozoic–Palaeozoic exceptional fossil preservation through pyritization and carbonaceous compression

“…For instance, NH 3 þ terminated surfaces can be completely passive to the formation of amorphous SiO 2 ; however, under identical conditions, NH 3 þ /COO -surfaces promote SiO 2 deposition to the extent that the organic may be coated by a nearly-coherent nanoscale layer of amorphous material within a few hours 34 . Moreover, as evidenced by calcite nucleation on polysaccharide matrices 35 , significant variations in the surface nucleation rate may occur between biopolymers with similar functionalities, with the specific order of substrate preference depending chiefly on supersaturation.…”

A classical view on nonclassical nucleation

“…CNT describes the formation of nuclei from the dynamic and stochastic association of monomeric units (e.g., ions, atoms, or molecules) that overcome a free-energy barrier at a critical nucleus size and grow out to a mature bulk phase. Calcium carbonate (CaCO 3 ) is a frequently used model system to study nucleation (3)(4)(5); however, despite the many years of effort, there are still phenomena associated with CaCO 3 crystal formation where the applicability of classical nucleation concepts have been questioned (6). These include certain microstructures and habits of biominerals formed by organisms (7), or geological mineral deposits with unusual mineralogical and textural patterns (8).…”