Correlation between protein secondary structure and mechanical performance for the ultra-tough dragline silk of Darwin's bark spider

The spider major ampullate (MA) silk exhibits high tensile strength and extensibility and is typically a blend of MaSp1 and MaSp2 proteins with the latter comprising glycine–proline–glycine–glycine-X repeating motifs that promote extensibility and supercontraction. The MA silk from Darwin''s bark spider (Caerostris darwini) is estimated to be two to three times tougher than the MA silk from other spider species. Previous research suggests that a unique MaSp4 protein incorporates proline into a novel glycine–proline–glycine–proline motif and may explain C. darwini MA silk''s extraordinary toughness. However, no direct correlation has been made between the silk''s molecular structure and its mechanical properties for C. darwini. Here, we correlate the relative protein secondary structure composition of MA silk from C. darwini and four other spider species with mechanical properties before and after supercontraction to understand the effect of the additional MaSp4 protein. Our results demonstrate that C. darwini MA silk possesses a unique protein composition with a lower ratio of helices (31%) and β-sheets (20%) than other species. Before supercontraction, toughness, modulus and tensile strength correlate with percentages of β-sheets, unordered or random coiled regions and β-turns. However, after supercontraction, only modulus and strain at break correlate with percentages of β-sheets and β-turns. Our study highlights that additional information including crystal size and crystal and chain orientation is necessary to build a complete structure–property correlation model.     

The toughening mechanism of nacre and structural materials inspired by nacre

Abstract

The structure and the toughening mechanism of nacre have been the subject of intensive research over the last 30 years. This interest originates from nacre’s excellent combination of strength, stiffness and toughness, despite its high, for a biological material, volume fraction of inorganic phase, typically 95%. Owing to the improvement of nanoscale measurement and observation techniques, significant progress has been made during the last decade in understanding the mechanical properties of nacre. The structure, microscopic deformation behavior and toughening mechanism on the order of nanometers have been investigated, and the importance of hierarchical structure in nacre has been recognized. This research has led to the fabrication of multilayer composites and films inspired by nacre with a layer thickness below 1 μm. Some of these materials reproduce the inorganic/organic interaction and hierarchical structure beyond mere morphology mimicking. In the first part of this review, we focus on the hierarchical architecture, macroscopic and microscopic deformation and fracture behavior, as well as toughening mechanisms in nacre. Then we summarize recent progress in the fabrication of materials inspired by nacre taking into consideration its mechanical properties.     

The structure of latherin,a surfactant allergen protein from horse sweat and saliva

Latherin is a highly surface-active allergen protein found in the sweat and saliva of horses and other equids. Its surfactant activity is intrinsic to the protein in its native form, and is manifest without associated lipids or glycosylation. Latherin probably functions as a wetting agent in evaporative cooling in horses, but it may also assist in mastication of fibrous food as well as inhibition of microbial biofilms. It is a member of the PLUNC family of proteins abundant in the oral cavity and saliva of mammals, one of which has also been shown to be a surfactant and capable of disrupting microbial biofilms. How these proteins work as surfactants while remaining soluble and cell membrane-compatible is not known. Nor have their structures previously been reported. We have used protein nuclear magnetic resonance spectroscopy to determine the conformation and dynamics of latherin in aqueous solution. The protein is a monomer in solution with a slightly curved cylindrical structure exhibiting a ‘super-roll’ motif comprising a four-stranded anti-parallel β-sheet and two opposing α-helices which twist along the long axis of the cylinder. One end of the molecule has prominent, flexible loops that contain a number of apolar amino acid side chains. This, together with previous biophysical observations, leads us to a plausible mechanism for surfactant activity in which the molecule is first localized to the non-polar interface via these loops, and then unfolds and flattens to expose its hydrophobic interior to the air or non-polar surface. Intrinsically surface-active proteins are relatively rare in nature, and this is the first structure of such a protein from mammals to be reported. Both its conformation and proposed method of action are different from other, non-mammalian surfactant proteins investigated so far.     

Dislocation core structures in B2 NiAl alloys

Single crystals of Ni-48Al, Ni-50Al and Ni-52Al have been deformed in the [101] orientation. Slip trace and diffraction contrast Burger's vector analyses have been used to characterize the deformation characteristics of these alloys. Ni-50Al was found to display predominantly 001{100} slip with some limited 001{100} slip while the non-stoichiometric Ni-48Al and Ni-52Al exhibited a predominance of 001{100} slip with limited 001{110} dislocation motion. Direct imaging of the 001{100} edge dislocation cores by high resolution electron microscopy revealed differences between Ni-50Al and Ni-48Al. Cores in the Ni-48Al were found to display significantly greater spreading of the cores, both within and normal to the slip plane, than for Ni-50Al. The mechanical behavior of these alloys is rationalized in terms of these observations. Comparison of the experimental core images with simulated images based on embedded atom models of the cores indicates that the models are able accurately to predict core structures in stoichiometric alloys, but may have some limitations in predicting structures in non-stoichiometric alloys.     

Study of transformation behavior in a Ti–4.4 Ta–1.9 Nb alloy

An alloy of composition Ti–4.4 wt.% Ta–1.9 wt.% Nb is being developed as a structural material for corrosion applications, as titanium and its alloys possess excellent corrosion resistance in many oxidizing media. The primary physical metallurgy database for the Ti–4.4 wt.% Ta–1.9 wt.% Nb alloy is being presented for the first time. Determination of the β transus, M_s temperature and classification of the alloy have been carried out, employing a variety of microscopy techniques, X-ray diffraction (XRD), micro-hardness and differential scanning calorimetry (DSC). The β transition temperature or β transus determined using different experimental techniques was found to agree very well with evaluations based on empirical calculations. Based on chemistry and observed room temperature microstructure, the alloy has been classified as an + β titanium alloy. The high temperature β transforms to either ′ or + β by a martensitic or Widmanstatten transformation. The mechanisms of transformation of β under different conditions and characteristics of different types of have been studied and discussed in this paper.     

Change in anisotropy of mechanical properties with β-phase stability in high Zr-containing Ti-based alloys

High Zr-containing β-type Ti-based alloys were designed using electronic parameters to investigate experimentally the effect of β-phase stability on their elastic and plastic properties. Texture structures formed by cold rolling or recrystallization were related closely to the β-phase stability and hence affected the mechanical properties. In tensile tests, as the β-phase stability decreased, non-linearity in the elastic zone was enhanced and the work hardening tended to be diminished. Also, it was found that the lower β-phase stability led to the weaker anisotropy of plastic properties, but to the stronger anisotropy of elastic properties.     

Sample preparation and microstructural characterization of the gamma titanium aluminide Ti-48Al-2W-0.5Si

Preparing samples that faithfully reveal all microstructural features in γ-TiAl-base alloys for both optical and scanning electron microscope studies is fraught with difficulties. This study demonstrates that satisfactory results can be obtained through mechanical grinding, polishing, and proper etching. A preparation recipe is presented. Nine lots of investmentcast γ-TiAl material with the nominal composition of Ti-48Al-2W-0.5Si have been characterized through optical and scanning electron microscope examinations. The study shows that a small depletion in Al content has a large effect on the microstructure. The duplex microstructure with a lamellar ₂/γ colony size of about 500μm and a large percentage of singlephase γ grains as large as 200μm is altered to a coarse, nearly lamellar microstructure with a colony size as large as 5000μm and a small percentage of small single-phase γ grains in the colony boundaries.     

Microstructural influence on the intrinsic fatigue properties of Al---Li 8090 alloy

A study has been made to investigate the influence of microstructure on the extrinsic and intrinsic fatigue properties of the Al---Li alloy, 8090. Two types of microstructure have been produced to compare the relative fatigue properties, one with a δ′ phase dominant microstructure and the other with a S′ + δ′ microstructure. Crack closure loads measured by the crack-opening displacement method have been used to obtain intrinsic fatigue resistance of the δ′ and S′ + δ′ microstructures. Results have shown that the extrinsic fatigue resistance of the δ′ microstructure was considerably higher than that of the S′ + δ′ microstructure, especially at lower growth rate, which was mainly due to the more severe crack path tortuosity and associated high levels of crack closure. In addition, the intrinsic fatigue resistance of the δ′ microstructure was also observed to be higher than that of the S′ + δ′ microstructure, presumably due to greater slip reversibility in the δ′ microstructure.     

25th Anniversary Article: Artificial Carbonate Nanocrystals and Layered Structural Nanocomposites Inspired by Nacre: Synthesis,Fabrication and Applications

Rigid biological systems are increasingly becoming a source of inspiration for the fabrication of next generation advanced functional materials due to their diverse hierarchical structures and remarkable engineering properties. Among these rigid biomaterials, nacre, as the main constituent of the armor system of seashells, exhibiting a well‐defined ‘brick‐and‐mortar’ architecture, excellent mechanical properties, and interesting iridescence, has become one of the most attractive models for novel artificial materials design. In this review, recent advances in nacre‐inspired artificial carbonate nanocrystals and layered structural nanocomposites are presented. To clearly illustrate the inspiration of nacre, the basic principles relating to plate‐like aragonite single‐crystal growth and the contribution of hierarchical structure to outstanding properties in nacre are discussed. The inspiration of nacre for the synthesis of carbonate nanocrystals and the fabrication of layered structural nanocomposites is also discussed. Furthermore, the broad applications of these nacre inspired materials are emphasized. Finally, a brief summary of present nacre‐inspired materials and challenges for the next generation of nacre‐inspired materials is given.     

Physical and chemical characterization of adsorbed protein onto gold electrode functionalized with Tunisian coral and nacre

Bone substitutes are more and more used in bone surgery because of their biologic safety, clinic efficiency and facility to synthesize. Bone substitutes with active osteogenic properties, associating biomaterials with organic macromolecule components of the extracellular matrix (protein, GAG) are recommended. Nevertheless, we should have a simple technique to control interactions between proteins and the material. Natural coral and nacre have been found to be impressive bone graft substitutes. In this work, we characterize nacre and coral powder using energy dispersive X-ray analysis (EDX). We used electrochemical impedance spectroscopy (EIS) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy to evaluate bovine serum albumin (BSA) as model protein, adsorbed to these biomaterial surfaces. In order to understand the nacre/coral-protein interfacial compatibility, it is necessary to investigate the wettability.     

Modification of CaCO3 precipitation rates by water-soluble nacre proteins

Mineral growth in nacre and other CaCO₃-containing biominerals is controlled by biopolymers. Water-soluble proteins were extracted from nacre of the sea snail Haliotis laevigata by dissolving the mineral phase with 6% acetic acid. The influence of this protein mixture on CaCO₃ precipitation rates was investigated at different concentrations. A well-established assay for measuring the pH-value during CaCO₃ precipitation with and without protein additives was extended by calculating maximum precipitation rates from the pH-values. It could be shown that precipitation rates are greatly influenced by the mixtures of water-soluble nacre proteins. At very low protein concentrations (0.02 μg/ml) a rate enhancement in comparison to the pure supersaturated calcium carbonate solution by a factor of 1.4 was observed. At higher protein concentrations, a strong inhibitory effect occurred, with total inhibition at concentrations of 1.0 μg/ml and higher. Two unspecific proteins (bovine serum albumin and lysozyme) showed little or no modification of precipitation rates. In vivo, the function of the strong inhibition of CaCO₃ precipitation by nacre proteins at higher concentrations is presumably to prevent uncontrolled crystallization in the extrapallial fluid. The rate-enhancing capability of proteins at low concentrations may be explained by the presence of acidic and/or hydrophilic moieties.     

Computer simulation of martensitic textures

We consider a Ginzburg-Landau model free energy F(ε, e₁, e₂) for a (2D) martensitic transition, that provides a unified understanding of varied twin/tweed textures. Here F is a triple well potential in the rectangular strain (ε) order parameter and quadratic e₁², e₂² in the compressional and shear strains, respectively. Random compositional fluctuations η(r) (e.g. in an alloy) are gradient-coupled to ε, ˜ − ∑_rε(r)[(Δ_x² − Δ_y²)η(r)] in a “local-stress” model. We find that the compatibility condition (linking tensor components ε(r) and e₁(r), e₂(r)), together with local variations such as interfaces or η(r) fluctuations, can drive the formation of global elastic textures, through long-range and anisotropic effective ε-ε interactions. We have carried out extensive relaxational computer simulations using the time-dependent Ginzburg-Landau (TDGL) equation that supports our analytic work and shows the spontaneous formation of parallel twins, and chequer-board tweed. The observed microstructure in NiAl and Fe_xPd_{1 − x} alloys can be explained on the basis of our analysis and simulations.     

Molecular encapsulation of thalidomide with sulfobutyl ether-7 beta-cyclodextrin for immediate release property: enhanced in vivo antitumor and antiangiogenesis efficacy in mice

Thalidomide's reported ability to inhibit tumor angiogenesis has led to clinical trials determining its effectiveness in combating various types of cancer. Since thalidomide exhibits low oral bioavailability due to limitations in solubility, inclusion complexation using sulfobutyl ether-7 β-cyclodextrin was used to improve the delivery of thalidomide. Our main goals were to increase the solubility, bioavailability as well as chemical stability of thalidomide through complexation with anionic β-cyclodextrin, to characterize the complex in solid state using differential scanning calorimetry, X-ray powder diffractometry, and to explore thalidomide's antitumorigenic and antiangiogenesis potential when administered orally as free and in combination with cyclodextrin to experimental animals. The aqueous solubility and aqueous alkaline stability of thalidomide was markedly increased by the SBE7βCD complexation. Thalidomide administered orally in combination with SBE7βCD, led to a significant delay in tumor formation as a result of improved cellular drug absorption, distribution through solubilization in experimental animals. The improved pharmacological efficacy of the thalidomide-cyclodextrin complex compared to free thalidomide in mouse melanoma model suggest that such a delivery system may be useful for the improved therapeutics of thalidomide, in vivo.     

Characterization of long-periodic layered structures by X-ray diffraction I: A system for small angle and intermediate angle X-ray diffraction using a reflection kratky camera

A high resolution camera has been developed for the small angle X-ray diffraction measurement of long-periodic layered structures such as Langmuir-Blodgett multilayers, superlattices and liquid crystals. A block collimation system known as a Kratky camera is used to produce a very narrow incident beam. The camera is mounted on a computer-controlled goniometer whose sample holder is rotated around a vertical Θ axis by a pulse motor. Measurements can be carried out in a θ-2θ scan, and also in θ or 2θ scans. Processing of the collected data includes smoothing, and correction for absorption, polarization factor and instrumental broadening. The performance of the present system has been demonstrated by observation of diffraction patterns of a Langmuir-Blodgett film of cadmium arachidate and a GaAs/AlAs superlattice.     

Effect of Dosage on the Conduction of Electron Beam Cross-Linked Thermoplastic Elastomeric Films from Blend of LDPE and EVA Copolymer

The effect of electron beam irradiation at different radiation doses (50, 100, 150, 200, and 300 kGy) on Low-density polyethylene/ethylene-vinyl acetate copolymer (LDPE/EVA) blend was investigated. Measurements of I-V characteristics have been carried out at room temperature. It was found that the β parameter increases with the doses. This feature was attributed to the differences in electrical conductivity of the materials. The conduction mechanism is discussed qualitatively on the basis of Poole-Frenkel conduction mechanisms (enhanced conductivity at high electric fields due to the lowering of the potential barrier), also, at low-voltage region conduction was discussed in terms of the Schottky conduction mechanism namely β_s (Schottky field-lowering coefficient). The value of the Poole-Frenkel high field-lowering coefficient β_PF increases with increasing voltage range. Both mechanisms are related and display a highly consistent response to electron beam exposure.     

Thermal and mechanical properties of simultaneous and sequential full-interpenetrating polymer networks

An investigation of the thermal and mechanical characteristics of new full-interpenetrating polymer networks (full-IPNs), prepared by simultaneous and sequential synthesis paths, has been performed in the temperature regions above and below the glass transition. It has been observed that simultaneous full-IPNs exhibit higher values of density than sequential full-IPNs, as a consequence of an enhanced intermolecular packing. This peculiarity leads to an increase of the glass transition temperature and to a larger “γ-suppression” effect for the γ₂-relaxation characterizing the polycyanurate phase of simultaneous full-IPNs, which has been ascribed to reduction of the free-volume available for the molecular group rearrangements.     

An accurate redetermination of the Sn binding energy

The energy of well-known strong γ line from ¹⁹⁸Au, the “gold standard”, has been modified in the light of new adjustments in the fundamental constants and the value of 411.80176(12) keV was determined, which is 0.29 eV lower than the latest 1999 value. An energy calibration procedure for determining the neutron binding energy, B_n, from complicated (n, γ) spectra has been developed. A mathematically simple minimization function consisting only of terms having as parameters the coefficients of the energy calibration curve (polynomial) is used. A priori information about the relationships among the energies of different peaks on the spectrum is taken into account by a Monte-Carlo simulation. The procedure was used in obtaining B_n for ¹¹⁸Sn. The γ-ray spectrum from thermal neutron radiative capture by ¹¹⁷Sn has been measured on the IBR-2 pulsed reactor. γ-rays were detected by a 72 cm³ HPGe detector. For a better determination of B_n it was important to determine B_n for ⁶⁴Cu. This value was obtained from two γ-spectra. One spectrum was measured on the IBR-2 by the same detector. The other spectrum was measured with a pair spectrometer at the Brookhaven High Flux Beam Reactor. From these two spectra, B_n for ⁶⁴Cu was determined to be equal to 7915.52(8) keV. This result essentially differs from the previous value of 7915.96(11) keV. The mean value of the two most precise results of the B_n for ¹¹⁸Sn, was determined to be 9326.35(9) keV. The B_n for ⁵⁷Fe was determined to be 7646.08(9) keV.     

The dynamics of nacre self-assembly

We show how nacre and pearl construction in bivalve and gastropod molluscs can be understood in terms of successive processes of controlled self-assembly from the molecular- to the macro-scale. This dynamics involves the physics of the formation of both solid and liquid crystals and of membranes and fluids to produce a nanostructured hierarchically constructed biological composite of polysaccharides, proteins and mineral, whose mechanical properties far surpass those of its component parts.     

Molecular origin of the sawtooth behavior and the toughness of nacre

We present in this paper a method to build a computer model that mimic the mineral–protein composite structure of a nacre tablet. Motivated by the interesting observations in AFM experiments of nacre, protein chains stretching out from grain boundaries are simulated by steered molecular dynamics (SMD) to gain an insight into the effect of protein–aragonite interaction on the mechanical properties of nacre and the molecular mechanisms of the sawtooth behavior. Force-extension curves are obtained and the key characteristics of sawtooth behavior are observed in SMD simulations in agreement with existing AFM experiments of nacre. The effect of water on protein–mineral interaction is investigated through including and excluding water molecules in the grain boundaries of the models. Different from the existing belief that protein unfolding is the origin of the “sawtooth” behavior, we have found that the electrostatic interactions between the protein and aragonite mineral are responsible for the sawtooth behavior and hence the high toughness of nacre.     

Why is nacre so tough and strong?

In this paper we describe the details of simulations conducted on three-dimensional finite element models of nacre integrated with experiments. This work gives an overview of modeling mechanical behavior in nacre and quantitatively elucidates the specific role of many details of structure in nacre on the stress–strain response. We describe the role of each of the details of nanostructure on the mechanics and deformation behavior of nacre as well as identify the key mechanisms responsible for the unique mechanical behavior of nacre. Nanoscale asperities and mineral contacts have marginal role on mechanical response of nacre and platelet interlocks have a significant role on deformation in nacre. We describe the key strengthening and toughening mechanisms in nacre as:

• 1.Material properties of aragonite and organic matrix, especially the unique properties of the organic phase in the confined space between platelets.
• 2.Structure at micro scale: size, shape of platelets etc.
• 3.Interlocking of aragonite platelets: progressive failure of interlocks guides the fracture path.
• 4.Molecular interactions at the organic–inorganic interface.