贝壳珍珠层不同取向弹性模量的研究

研究天然生物材料的组织结构特征与其性能之间的关系对于材料的仿生有重要意义．在自制的激光测试设备上用三点弯曲法对贝壳珍珠层不同取向的弹性模量进行了研究,报道了不同取向和加载方式条件下弹性模量的变化规律。结果表明,在平行和倾斜于生长纹路方向上弹性模量的平均值分别为60．3GPa和56．7GPa,而垂直于纹路方向的为48GPa,呈现出各向异性。弹性模量的各向异性主要来自于珍珠层微观组织结构和贝壳生长纹结构的特点．     

Novel CaO/polylactic acid nanoscaffold as dental resin nanocomposites and the investigation of physicochemical properties

In this study, for the first time, calcium oxide (CaO)/polylactic acid nanoscaffolds were synthesized by co‐precipitation assistant reverse micelles method. The physical and chemical (physicochemical) properties of the structures as dental resin composites were also studied. Nanocomposite materials as primary and basic dental compounds can be conveniently applied as dental filling materials with a high esthetic quality. In this research nanoscaffolds act as a bed for nanoparticles and improve the mechanical and chemical (mechanochemical) properties, CaO nanoparticles were loading in polylactic acid nanoscaffold as a bioactivity polymer for usage in the dental resin composites. Mechanical properties of the dental resin composite containing CaO/polylactic acid nanoscaffold were calculated: the flexural strength (137.2 MPa), modulus (12.9GPa) and compressive strength (344.2 MPa). Potential of the basic nanoparticle and the products were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), dynamic light scattering (DLS), ultraviolet‐visible spectroscopy (UV‐visible) and atomic force microscopy (AFM) showed the size of the optimized nanostructures was about 85 to 120 nm. According to TGA results of polylactic acid nanofibers with thermal stability below 300°C these high thermal stability materials can be used as dental resin composites.     

Organization pattern of nacre in Pteriidae (Bivalvia: Mollusca) explained by crystal competition

Bivalve nacre is a brick-wall-patterned biocomposite of aragonite platelets surrounded by organic matter. SEM-electron back scatter diffraction analysis of nacre of the bivalve family Pteriidae reveals that early aragonite crystals grow with their c-axes oriented perpendicular to the growth surface but have their a- and b-axes disoriented. With the accumulation of successive lamellae, crystals progressively orient themselves with their b-axes mutually parallel and towards the growth direction. We propose that progressive orientation is a result of competition between nacre crystals at the growth front of lamellae, which favours selection of crystals whose fastest growth axis (b-axis) is oriented parallel to the direction of propagation of the lamella. A theoretical model has been developed, which simulates competition of rhombic plates at the lamellar growth front as well as epitaxial growth of crystals onto those of the preceding lamella. The model predicts that disordered nacre progressively produces bivalve-like oriented nacre. As growth fronts become diffuse (as is the common case in bivalves) it takes longer for nacre to become organized. Formation of microdomains of nacre platelets with different orientations is also reproduced. In conclusion, not only the organic matrix component, but also the mineral phase plays an active role in organizing the final microstructure.     

Determination of the substrate specificity of the phospholipase D from Streptomyces chromofuscus via an inorganic phosphate quantitation assay

The substrate specificity for phospholipase D from Streptomyces chromofuscus (PLD(Sc)) has been determined utilizing an assay based on the quantitation of inorganic phosphate. 1,2-Di-n-hexanoyl phosphatidylcholine (C6PC), phosphatidylethanolamine (C6PE), phosphatidylserine (C6PS), phosphatidylglycerol (C6PG), and an unnatural phospholipid bearing a neohexyl headgroup (C6PDB) were examined as substrates. The assay relies on the quenching of the PLD(Sc)-catalyzed hydrolysis of the phospholipid substrates with EDTA followed by the hydrolysis of the phosphatidic acid product with alkaline phosphatase. The inorganic phosphate thus released is quantitated through the formation of a complex with ammonium molybdate, which has an absorbance maximum at 700 nm. To minimize the time involved and the reagents consumed, the assay is conducted in 96-well plates. The results of this study indicate that the catalytic efficiency for PLD(Sc) on the substrates is C6PC > C6PS approximately C6PE > C6PG > C6PDB.     

Mineral bridges in nacre

We confirm with high-resolution techniques the existence of mineral bridges between superposed nacre tablets. In the towered nacre of both gastropods and the cephalopod Nautilus there are large bridges aligned along the tower axes, corresponding to gaps (150–200 nm) in the interlamellar membranes. Gaps are produced by the interaction of the nascent tablets with a surface membrane that covers the nacre compartment. In the terraced nacre of bivalves bridges associated with elongated gaps in the interlamellar membrane (>100 nm) have mainly been found at or close to the edges of superposed parental tablets. To explain this placement, we hypothesize that the interlamellar membrane breaks due to differences in osmotic pressure across it when the interlamellar space below becomes reduced at an advanced stage of calcification. In no cases are the minor connections between superimposed tablets (<60 nm), earlier reported to be mineral bridges, found to be such.     

A scanning electron microscopic study of the inorganic and organic matrices comprising the mature shell of Amblema, a fresh-water mollusc

The scanning electron microscope has been used to describe the morphology of the mature shell in a fresh-water bivalve. The structure of the organic and inorganic components within the nacre, the myostracum, and the prismatic layer is described. A transitional or intermediate zone, interposed between the prismatic layer and the nacre, was identified. In demineralized samples, the organic component of the nacre was found to consist of parallel matricial sheets interconnected by irregular transverse bridges. The structure of the mineral component of the nacre was found to vary with the method of specimen preparation. With polished-etched samples, brick-like units were seen. When shells were simply broken and fixed in osmium, the layers of nacreous material consisted of fusing rhomboidal crystals of aragonite which demonstrated subconchoidal fractures. On the inner surface of the shell, the rhomboidal crystals showed an apparent spiral growth pattern. The myostracum was characterized by regions of modified nacreous structure consisting of enlarged aragonite crystals with a pyramidal morphology. The peripheral aspect of the muscle scars was characterized by rhomboidal crystals, the latter fusing to form the typical nacreous laminae. The uniqueness of the anterior adductor scar is exemplified by the presence of pores, each pore walled by pyramidal units, for the insertion of adductor fibres. In most regions of the shell, the prismatic layer consisted of one prism unit thickness with a height of approximately 225–250 μm. However, in two specialized regions of the shell, this layer was seen to consist of multiple layers of stacked prisms. The organic matrices of the prismatic layer are arranged in a honeycomb-like arrangement and packed with mineralized spherical subunits.     

Measurement of inorganic orthophosphate in biological materials: Extraction properties of butyl acetate

The properties of several organic solvents were investigated to determine their suitability for use in phosphomolybdic acid (PMA) extraction procedures for the measurement of inorganic orthophosphate (P_i). Butyl acetate exhibited the most satisfactory properties for measurements at 310 nm, which is the absorption peak for unreduced PMA. Butyl acetate exhibits essentially no absorption at 310 nm, is highly selective for PMA over molybdate and silicomolybdate, does not extract molybdate in the presence of trichloroacetic acid (TCA), exhibits negligible volume changes when equilibrated with equal volumes of aqueous phase, and is among the least toxic of organic solvents. Optimal conditions of acidity, molybdate concentration, and reaction and extraction times were determined for the formation of PMA and extraction into butyl acetate. A procedure for measurement of P_i in biological material employing 8% TCA for precipitation of organic material at 0°C, reaction of the supernatant with acid molybdate, extraction of the PMA with butyl acetate and reading of unreduced PMA at 310 nm is described. The procedure is simple, rapid, accurate, selective, and has high sensitivity. Because of the short (20 sec) sample exposure to acid molybdate, it is suggested that the procedure may also be useful in the measurement of P_i in the presence of adenosine triphosphate such as for the assay of adenosinetriphosphatase activity.     

Derivation of the Stress-Strain Behavior of the constituents of Bio-Inspired Layered TiO₂/PE-Nanocomposites by Inverse Modeling Based on FE-Simulations of Nanoindentation Test

Owing to the apparent simple morphology and peculiar properties, nacre, an iridescent layer, coating of the inner part of mollusk shells, has attracted considerable attention of biologists, material scientists and engineers. The basic structural motif in nacre is the assembly of oriented plate-like aragonite crystals with a ’brick’ (CaCO₃ crystals) and ’mortar’ (macromolecular components like proteins) organization. Many scientific researchers recognize that such structures are associated with the excellent mechanical properties of nacre and biomimetic strategies have been proposed to produce new layered nanocomposites. During the past years, increasing efforts have been devoted towards exploiting nacre’s structural design principle in the synthesis of novel nanocomposites. However, the direct transfer of nacre’s architecture to an artificial inorganic material has not been achieved yet. In the present contribution we report on laminated architecture, composed of the inorganic oxide (TiO₂) and organic polyelectrolyte (PE) layers which fulfill this task.
To get a better insight and understanding concerning the mechanical behaviour of bio-inspired layered materials consisting of oxide ceramics and organic layers, the elastic-plastic properties of titanium dioxide and organic polyelectrolyte phase are determined via FE-modelling of the nanoindentation process. With the use of inverse modeling and based on numerical models which are applied on the microscopic scale, the material properties of the constituents are derived.     

不同pH值对三角帆蚌珍珠质分泌的影响

运用多种组织化学方法和透射电镜技术,研究了５种ｐＨ水环境（ｐＨ５、６、７、８、９）对三角帆砷外套膜珍珠质分泌的影响机制,结果表明,在中性水环境中,贝体能积极地从外界水环境中吸收钙,并能旺盛地合成和分泌贝壳珍珠层及珍珠有机基质前体物质,持续的酸性水环境导致贝体的钙严重丢失,并引起珍珠质分泌细胞对有机基质前体物质的合成和分泌能力减弱,持续的碱性水环境虽能导致贝体对钙的积累,但珍珠质分泌细胞合成和分泌珍     

Antioxidant protection against organic and inorganic oxygen radicals by normal human plasma: the important primary role for iron-binding and iron-oxidising proteins.

Normal human plasma contains numerous high- and low-molecular-mass redox-active molecules that are able to react rapidly with organic and inorganic oxygen radicals. The ability of such plasma molecules to substantially inhibit, or delay, free-radical mediated oxidation of added substrates has led to their classification as important biological antioxidants. Using phospholipids to detect organic oxygen radicals and deoxyribose to detect inorganic oxygen radicals, we here show that the primary antioxidants of normal human plasma reside mainly in two plasma proteins representing no more than 4% of the total proteins present. The iron-binding properties of transferrin and the iron-oxidising properties of caeruloplasmin, at a reaction dilution of 1:50, offer considerable protection against organic and inorganic oxygen radicals generated by iron and ascorbate. Plasma thiol-group-containing molecules, at concentrations well below those that would be required to compete with the detector molecule (based on known second-order rate constants for reaction with hydroxyl radicals) inhibited damage to deoxyribose, but stimulated damage to phospholipids.     

Phytase activity in tobacco (Nicotiana tabacum) root exudates is exhibited by a purple acid phosphatase

Phytases are enzymes that catalyze liberation of inorganic phosphates from phytate, the major organic phosphorus in soil. Tobacco (Nicotiana tabacum) responds to phosphorus starvation with an increase in extracellular phytase activity. By a three-step purification scheme, a phosphatase with phytase activity was purified 486-fold from tobacco root exudates to a specific activity of 6,028 nkat mg(-1) and an overall yield of 3%. SDS-PAGE revealed a single polypeptide of 64 kDa, thus indicating apparent homogeneity of the final enzyme preparation. Gel filtration chromatography suggested that the enzyme was a ca. 56 kDa monomeric protein. De novo sequencing by tandem mass spectrometry resulted in a tryptic peptide sequence that shares high homology with several plant purple acid phosphatases. The identity of the enzyme was further confirmed by molybdate-inhibition assay and cDNA cloning. The purified enzyme exhibited pH and temperature optima at 5.0-5.5 and 45 degrees C, respectively, and were found to have high affinities for both p-nitrophenyl phosphate (pNPP; K(m)=13.9 microM) and phytate (K(m)=14.7 microM), but a higher kcat for pNPP (2,056 s(-1)) than phytate (908 s(-1)). Although a broad specificity of the enzyme was observed for a range of physiological substrates in soil, maximum activity was achieved using mononucleotides as substrates. We conclude that the phytase activity in tobacco root exudates is exhibited by a purple acid phosphatase and its catalytic properties are pertinent to its role in mobilizing organic P in soil.     

Evolutionary engineering reveals divergent paths when yeast is adapted to different acidic environments

Tolerance of yeast to acid stress is important for many industrial processes including organic acid production. Therefore, elucidating the molecular basis of long term adaptation to acidic environments will be beneficial for engineering production strains to thrive under such harsh conditions. Previous studies using gene expression analysis have suggested that both organic and inorganic acids display similar responses during short term exposure to acidic conditions. However, biological mechanisms that will lead to long term adaptation of yeast to acidic conditions remains unknown and whether these mechanisms will be similar for tolerance to both organic and inorganic acids is yet to be explored. We therefore evolved Saccharomyces cerevisiae to acquire tolerance to HCl (inorganic acid) and to 0.3 M L-lactic acid (organic acid) at pH 2.8 and then isolated several low pH tolerant strains. Whole genome sequencing and RNA-seq analysis of the evolved strains revealed different sets of genome alterations suggesting a divergence in adaptation to these two acids. An altered sterol composition and impaired iron uptake contributed to HCl tolerance whereas the formation of a multicellular morphology and rapid lactate degradation was crucial for tolerance to high concentrations of lactic acid. Our findings highlight the contribution of both the selection pressure and nature of the acid as a driver for directing the evolutionary path towards tolerance to low pH. The choice of carbon source was also an important factor in the evolutionary process since cells evolved on two different carbon sources (raffinose and glucose) generated a different set of mutations in response to the presence of lactic acid. Therefore, different strategies are required for a rational design of low pH tolerant strains depending on the acid of interest.     

Biomineralization: functions of calmodulin-like protein in the shell formation of pearl oyster

Calmodulin-like protein (CaLP) was believed to be involved in the shell formation of pearl oyster. However, no further study of this protein was ever performed. In this study, the in vitro crystallization experiment showed that CaLP can modify the morphology of calcite. In addition, aragonite crystals can be induced in the mixture of CaLP and a nacre protein (at 16 kDa), which was detected and purified from the EDTA-soluble matrix of nacre. These results agreed with that of immunohistological staining in which CaLP was detected not only in the organic layer sandwiched between nacre (aragonite) and the prismatic layer (calcite), but also around the prisms of the prismatic layer. Take together, we concluded that (1) CaLP, as a component of the organic layer, can induce the nucleation of aragonite through binding with the 16-kDa protein, and (2) CaLP may regulate the growth of calcite in the prismatic layer.     

Flexible nacre in the nautiloid Isorthoceras, with remarks on the evolution of cephalopod nacre

Secondarily phosphatized shells of Isorthoceras sociale (Hall) from the Upper Ordovician Maquokcta shale, Iowa, U.S.A., often have their original ultrastructurc preserved. Scanning Electron Microscope study reveals that the nacreous layer is similar to that in the shells of Recent Nautilus , except that the nacreous tablets usually have a much larger diameter, and that numerous vertical interspaces arc left between adjacent stacks of tablets. Because of the latter condition, the nacre in A sociale is porous, whereas it is compact in Nautilus. The pores in the nacre were probably filled with a non-mincralizcd organic matrix. The high porosity and high content of organic matrix made the nacreous layer of I. sociale much more flexible and considerably less strong than that in Nautilus. As a consequence, the shells of I. sociale , and probably also of other orthoconic cephalopods, were more flexible and could not withstand such high hydrostatic pressures as the shells of Nautilus. During evolution, the porosity of the nacreous layer became completely reduced. As a result, the mechanical properties of the nacre changed towards a lower flexibility and a higher tensile strength. Cephalopoda, Nautiloidea, nacre, ultrastructurc, phosphatization, Upper Ordovician.     

Interaction of lactoperoxidase with hydrogen peroxide. Formation of enzyme intermediates and generation of free radicals

Peroxidases belong to a group of enzymes which catalyze the oxidation of numerous organic and inorganic substrates by hydrogen peroxide. Most peroxidases, including lactoperoxidase (LPO), contain ferriprotoporphyrin IX as a prosthetic group. A characteristic feature of hemoprotein peroxidases is their ability to exist in various oxidation states. There are five known enzyme intermediates. In increasing order of their oxidative equivalents these are ferrous enzyme, ferric or native enzyme, Compound II, Compound I, and Compound III (sections 5, 7). They are readily distinguished from each other by their absorbance in the Soret region (380-450 nm) and visible range (450-650 nm). In the course of Compound III and Compound II conversion back to the native peroxidase, oxygen derived free radicals such as O2-, HO.2, and .OH are generated. Simultaneously the enzyme is irreversibly damaged. In the presence of an exogenous electron donor, such as iodide, the interconversion between the various oxidation states of the peroxidase is markedly affected. Compound II and/or Compound III formation is inhibited, depending on the H2O2 concentration. In addition, the enzyme is largely protected from irreversible inactivation. These effects of iodide are readily explained by 1) the two-electron oxidation of iodide to Iox by Compound I, which bypasses Compound II as an intermediate, and 2) the rapid oxidation of H2O2 to O2 by the oxidized species of iodide which prevents the generation of oxygen derived free radicals.     

Aptamer-graphene oxide for highly sensitive dual electrochemical detection of Plasmodium lactate dehydrogenase

A 90 mer ssDNA aptamer (P38) enriched against Plasmodium falciparum lactate dehydrogenase (PfLDH) through SELEX process was immobilized over glassy carbon electrode (GCE) using graphene oxide (GO) as an immobilization matrix, and the modified electrode was investigated for detection of PfLDH. The GO was synthesized from powdered pencil graphite and characterized by XRD based on the increased interlayer distance between graphitic layers from 0.345 nm for graphite to 0.829 nm for GO. The immobilization of P38 on GO was confirmed by I_D/I_G intensity ratio in Raman spectra where, the ratio were 0.67, 0.915, and 1.35 for graphite, GO and P38-GO, respectively. The formation of the P38 layer over GO-GCE was evident from an increase in the surface height in AFM analysis of the electrode from ∼3.5 nm for GO-GCE to ∼27 nm for P38-GO-GCE. The developed aptasensor when challenged with the target, a detection of as low as 0.5 fM of PfLDH was demonstrated. The specificity of the aptasensor was confirmed through a voltametric measurement at 0.65 V of the reduced co-factor generated from the PfLDH catalysis. Studies on interference from some common proteins, storage stability, repeatability and analysis of real samples demonstrated the practical application potential of the aptasensor.     

Gastropod nacre: structure, properties and growth--biological, chemical and physical basics

The biogenic polymer/mineral composite nacre is a non-brittle biological ceramic, which self-organizes in aqueous environment and under ambient conditions. It is therefore an important model for new sustainable materials. Its highly controlled structural organization of mineral and organic components at all scales down to the nano- and molecular scales is guided by organic molecules. These molecules then get incorporated into the material to be responsible for properties like fracture mechanics, beauty and corrosion resistance. We report here on structure, properties and growth of columnar (gastropod) nacre with emphasis on the genus Haliotis in contrast to sheet nacre of many bivalves.     

Characterization of a mammalian homolog of the Escherichia coli MutY mismatch repair protein.

A protein homologous to the Escherichia coli MutY protein, referred to as MYH, has been identified in nuclear extracts of calf thymus and human HeLa cells. Western blot (immunoblot) analysis using polyclonal antibodies to the E. coli MutY protein detected a protein of 65 kDa in both extracts. Partial purification of MYH from calf thymus cells revealed a 65-kDa protein as well as a functional but apparently degraded form of 36 kDa, as determined by glycerol gradient centrifugation and immunoblotting with anti-MutY antibodies. Calf MYH is a DNA glycosylase that specifically removes mispaired adenines from A/G, A/7,8-dihydro-8-oxodeoxyguanine (8-oxoG or GO), and A/C mismatches (mismatches indicated by slashes). A nicking activity that is either associated with or copurified with MYH was also detected. Nicking was observed at the first phosphodiester bond 3' to the apurinic or apyrimidinic (AP) site generated by the glycosylase activity. The nicking activity on A/C mismatches was 30-fold lower and the activity on A/GO mismatches was twofold lower than that on A/G mismatches. No nicking activity was detected on substrates containing other selected mismatches or homoduplexes. Nicking activity on DNA containing A/G mismatches was inhibited in the presence of anti-MutY antibodies or upon treatment with potassium ferricyanide, which oxidizes iron-sulfur clusters. Gel shift analysis showed specific binding complex formation with A/G and A/GO substrates, but not with A/A, C.GO, and C.G substrates. Binding is sevenfold greater on A/GO substrates than on A/G substrates. The eukaryotic MYH may be involved in the major repair of both replication errors and oxidative damage to DNA, the same functions as those of the E. coli MutY protein.     

pH-Conditional, Ammonia Assimilation-Deficient Mutants of Hydrogenomonas eutropha: Isolation and Growth Characteristics

Two mutants of the facultative autotroph Hydrogenomonas eutropha were isolated by using a modified penicillin selection method. The mutation involved was unusual in that its effect on cellular growth was conditional with regard to extracellular pH and the type of substrate employed. Growth of both mutants was abnormal under autotrophic conditions and during heterotrophic cultivation in the presence of organic substrates which lacked an amino group. Abnormal growth was characterized by linear growth rates which were low at pH 6.0 and moderate at pH 7.2. In contrast, growth of the mutants was normal on most amino acids. Those substrates yielding abnormal growth were oxidized at normal rates by the mutants, indicating the mutation did not impair their uptake or metabolism. The data suggest that the mutants are defective in their ability to assimilate inorganic nitrogen into organic forms, and this defect is strongly influenced by pH.     

Abalone nacre insoluble matrix induces growth of flat and oriented aragonite crystals

Mollusc shell formation takes place in a preformed extracellular matrix, composed of insoluble chitin, coated with proteins and dissolved macromolecules. The water-soluble matrix is known to have a strong influence on the growth of CaCO(3), whereas the role of the insoluble matrix on mineralization is unclear. Therefore, we mineralized the EDTA (ethylenediaminetetraacetic acid) insoluble organic matrix of abalone nacre with a modified double-diffusion set-up, where the diffusing solutions were constantly renewed. Control experiments were performed with cellulose and chitosan foils. The mineralized matrices/foils were analyzed with SEM. We show that the insoluble matrix of abalone nacre induces the growth of flat and roughly polygonal CaCO(3) crystals. In some of the experiments with the insoluble matrix, the growth of three-dimensional parallel sheets of densely packed platelets inside the insoluble matrix was observed. XRD on these samples revealed that they consist of oriented aragonite.