Resilient Intracrystalline Occlusions: A Solid-State NMR View of Local Structure as It Tunes Bulk Lattice Properties.

In many marine organisms, biomineralization-the crystallization of calcium-based ionic lattices-demonstrates how regulated processes optimize for diverse functions, often via incorporation of agents from the precipitation medium. We study a model system consisting of l-aspartic acid (Asp) which when added to the precipitation solution of calcium carbonate crystallizes the thermodynamically disfavored polymorph vaterite. Though vaterite is at best only kinetically stable, that stability is tunable, as vaterite grown with Asp at high concentration is both thermally and temporally stable, while vaterite grown at 10-fold lower Asp concentration, yet 2-fold less in the crystal, spontaneously transforms to calcite. Solid-state NMR shows that Asp is sparsely occluded within vaterite and calcite. CP-REDOR NMR reveals that each Asp is embedded in a perturbed occlusion shell of ∼8 disordered carbonates which bridge to the bulk. In both the as-deposited vaterites and the evolved calcite, the perturbed shell contains two sets of carbonate species distinguished by their proximity to the amine and identifiable based on 13C chemical shifts. The embedding shell and the occluded Asp act as an integral until which minimally rearranges even as the bulk undergoes extensive reorganization. The resilience of these occlusion units suggests that large Asp-free domains drive the vaterite to calcite transformation-which are retarded by the occlusion units, resulting in concentration-dependent lattice stability. Understanding the structure and properties of the occlusion unit, uniquely amenable to ssNMR, thus appears to be a key to explaining other macroscopic properties, such as hardness.

Biomacromolecules within bivalve shells: Is chitin abundant?

Bivalve shells are inorganic-organic nanocomposites whose material properties outperform their purely inorganic mineral counterparts. Most typically the inorganic phase is a polymorph of CaCO3, while the organic phase contains biopolymers which have been presumed to be chitin and/or proteins. Identifying the biopolymer phase is therefore a crucial step in improving our understanding of design principles relevant to biominerals. In this work we study seven shells; four are examples of nacroprismatic shells (Alathyria jacksoni, Pinctada maxima, Hyriopsis cumingii and Cucumerunio novaehollandiae), one homogeneous (Arctica islandica), and two are crossed lamellar (Callista kingii, Tridacna gigas). Both intact shells, their organic extracts as isolated after decalcification in acid, and the periostracum overlay have been studied by solid-state CP-MAS NMR, FTIR, SEM and chemical analysis. In none of the shells examined in this work do we find a significant contribution to the organic fraction from chitin or its derivatives despite popular models of bivalve biomineralization which assume abundant chitin in the organic fraction of mollusk bivalve shells. In each of the nacroprismatic extracts the 13C NMR spectra represent similar proteinaceous material, Ala and Gly-rich and primarily organized as ß-sheets. A different, yet highly conserved protein was found in the periostracum covering each of the three nacreous shells studied. The Arctica islandica shells with homogeneous microstructure contained proteins which do not appear to be silk-like, while in the crossed lamellar shells we extracted too little organic matter to characterize. STATEMENT OF SIGNIFICANCE: Hydrophobic macromolecules are structural components within the calcareous inorganic matrix of bivalve shells and are responsible for enhanced materials properties of the biominerals. Prevalent models suggest that chitin is such major hydrophobic component. Contrary to that we show that chitin is rare within the hydrophobic biopolymers which primarily consist of proteinaceous matter with structural motifs as silk-like ß-sheets, or others yet to be determined. Recognizing that diverse proteinaceous motifs, devoid of abundant chitin, can yield the optimized mechanical properties of bivalve shells is critical both to understand the mechanistic pathways by which they regulate biomineralization and for the design of novel bioinspired materials.

Direct characterization of cotton fabrics treated with di-epoxide by nuclear magnetic resonance.

A non-acid-based, di-functional epoxide, neopentyl glycol diglycidyl ether (NPGDGE), was used to modify cotton fabrics. Direct characterization of the modified cotton was conducted by Nuclear Magnetic Resonance (NMR) without grinding the fabric into a fine powder. NaOH and MgBr2 were compared in catalyzing the reaction between the epoxide groups of NPGDGE and the hydroxyl groups of cellulose. Possible reaction routes were discussed. Scanning electron microscopy (SEM) images showed that while the MgBr2-catalyzed reaction resulted in self-polymerization of NPGDGE, the NaOH-catalyzed reaction did not. Fourier transform infrared spectroscopy (FTIR) showed that at high NaOH concentration cellulose restructures from allomorph I to II. NMR studies verified the incorporation of NPGDGE into cotton fabrics with a clear NMR signal, and confirmed that at higher NaOH concentration the efficiency of grafting of NPGDGE was increased. This demonstrates that use of solid state NMR directly on woven fabric samples can simultaneously characterize chemical modification and crystalline polymorph of cotton. No loss of tensile strength was observed for cotton fabrics modified with NPGDGE.

Variation of mechanical properties with amino acid content in the silk of Nephila clavipes.

In this paper, we explore the impact of dietary deprivation, where spiders are provided diets missing one or more of the amino acids, on the properties of the spider dragline silk spun after one month on the diet. Cohorts of female N. clavipes spiders were selected for diets deprived of alanine (Ala) and glycine (Gly), arginine (Arg), leucine (Leu), or tyrosine (Tyr), and their silk was harvested twice weekly during the one-month course of the diet. Significant mechanical differences are observed after as little as 6 days on the diet. Utilizing conventional tensile testing methods, single fibers were strained to break so as to study the influence of diet on the stress/strain properties. Diets deprived of Ala and Gly appear to most directly impact the load-bearing foundation of dragline silk. Diets deprived of Arg, Tyr, and possibly Leu reduce the strength of the silk, and diets missing Tyr and Leu reduce the strain-to-failure. Observations obtained from ESEM photos of the fracture interfaces after tensile testing illustrate the fracture mechanics of spider silk. Both solid-state NMR and amino acid analysis of the digested protein suggest, however, that the relationship between diet and amino acid incorporation into the silk fiber is not straightforward.

Ca6[Cr2N6]H, the first quaternary nitride-hydride.

The novel quaternary nitride-hydride Ca(6)[Cr(2)N(6)]H was synthesized at 1000 degrees C in sealed niobium or stainless steel tubes. It crystallizes in the space group R3 (No. 148, Z = 3) with lattice constants (A) a = 9.0042(2) and c = 9.1898(3) and contains the complex anion [Cr(2)N(6)](11)(-) with a short chromium-chromium bond length of 2.26 A. To our knowledge, this is the first example of a non-nitrogen-bridged chromium-chromium dimer in an extended structure compound. Magnetic susceptibility measurements reveal the compound to be paramagnetic at room temperature and with a broad antiferromagnetic ordering centered around 55 K.