Stability of two-dimensional growth of a packed body of proteins on a solid surface

Adsorption of proteins from the bulk is at times accompanied by a rearrangement which leads to the formation of closed packed bodies, that may or may not be crystalline. Mass transfer of protein molecules on a surface is modeled. Forced diffusion by van der Waals and electrostatic forces leads to segregation, which is eventually a different phase that is assumed to be thermodynamically favored. The net effective force in two-dimensions has been modeled approximately and shown to be much stronger and more long ranged than in the bulk: that is, under the same conditions, the protein molecules may not aggregate in the bulk they may aggregate on a surface. These forces have been used only indirectly but equivalently as an adsorption-desorption step at the interline. Eventually, a linear stability analysis of the growing body shows it to be unstable and would give rise to whiskers that are one molecule thick. This is what is observed experimentally. The conditions that give rise to the instability have been determined. The reverse case of rinsing of the protein molecules has also been studied experimentally and has been analyzed using the same mechanisms. Here it is seen that thicker inroads into the packed body cause the interline to take on a spongy appearance. It is conjectured that eventually islands will appear as seen in the experiments.     

Mechanistic Aspects of the Effects of Stress on the Rates of Photochemical Degradation Reactions in Polymers

Abstract

This review examines the mechanistic origins of the effects of stress on the photochemical degradation rates of polymers. Recent studies have shown that tensile and shear stresses accelerate the rate of the photochemical degradation of polymers. Conversely, compressive stress generally retards the rate of photochemical degradation. After an initial discussion of the photochemical auto‐oxidation mechanism, the three primary hypotheses that purport to explain how stress affects photochemical degradation are examined. The first hypothesis is attributed to Plotnikov, who proposed that stress changes the quantum yields of the reactions that lead to bond photolysis. The second hypothesis, attributed to a number of researchers, says that stress affects the ability of the geminate radical pairs, formed in the photochemical bond cleavage reactions, to recombine. The third hypothesis proposes that stress changes the rates of radical reactions subsequent to radical formation. A further attempt to account for the effects of stress on degradation rates is a modification of the so‐called Zhurkov equation that has been used rather successfully to predict the effects of stress on degradation rates in thermal reactions. This empirical equation relates the quantum yield of degradation to a composite activation barrier for the overall photochemical reaction. Following the discussion of these hypotheses, experimental mechanistic studies of stress effects are summarized, and what little data there is is shown to be consistent with the hypothesis that proposes that stress primarily affects the ability of photochemically generated radical pairs to recombine. By decreasing the efficiency of radical–radical recombination, the effect is to increase the relative efficiencies of the radicals' other reactions and hence the rate of degradation. In addition to stress, other factors can affect the rates of polymer photodegradation. These factors include the absorbed light intensity, the polymer morphology, the rate of oxygen diffusion in the polymer, and the chromophore concentration. Each of these parameters must be carefully controlled in mechanistic studies that probe the effects of stress on degradation rates.     

Structural-phenomenological simulation of the mechanical behavior of rubbers

It is hypothesized that, during deformation of rubbers, polymer chains slip off the layers at filler particles into voids between inclusions and high-strength polymer fibers in the uniaxially oriented state are formed in the voids. As a result, the macroscopic strength of elastomers increases by an order of magnitude and the elongation at break simultaneously increases relative to the unfilled elastomer. Aggregates of carbon black particles that occur close to one another in the initial sample depart to very large distances upon stretching the material. The fibers that tie the aggregates must extend their length by a factor of a few tens in this case. A mathematical model that takes into account these processes is proposed. It was shown that the set of constitutive equations makes it possible to simulate with good accuracy both the viscoelastic behavior of rubbers and the Mullins softening effect under finite strain conditions.     

Mechanism of inhibition by copper stearate of the photo-degradation of polyolefins

The effect of a series of transition metal (Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) stearates in retarding the photo-degradation of isotactic polypropylene has been examined and it has been found that the results obtained are quite similar to those for high density polyethylene.Infra-red spectral analysis of the polyethylene and isotactic polypropylene films showed that, in the early stages of the photo-irradiation, photo-chemical reaction of the copper stearate took place in the films to change its structure and that rapid formation of terminal unsaturation occurred only in the films containing copper stearate. The results obtained suggest that the copper stearate provides a radical scavenging function, as well as an ultraviolet light absorbing function, to protect the polymers against photo-degradation.     

磷脂水解与胆固醇型胆结石时的可能生成机理

用胆酸盐-磷脂-胆固醇-水体系模拟胆汗的化学行为。在体系中发现，磷脂可 以发生降解形成脂肪酸。而在胆汗中，磷脂在增溶固醇的过程中起到了十分重要的 作用。依此，我们提出磷脂的水解可能与胆固醇型胆结石的生成有密切关系。     

Kinetics and mechanism of the polymerization of styrene–acrylonitrile in the presence of polybutadiene

The mechanism of free radical grafting of styrene–acrylonitrile with polybutadiene was studied. By assuming a copolymerization mechanism an equation that related the amount of homopolymer formed and its molecular weight to a reactivity ratio and the charged ratio of monomer to polydiene was derived. Polymerizations that contained a variety of ratios of monomer to polybutadiene, two different catalysts, and variable amounts of a mercaptan modifier were studied. The weight fraction of homopolymer and its molecular weight were determined by high-speed GPC. The results were analyzed by the new equation and all showed a constant value of the reactivity ratio, which strongly suggests that the mechanism of the grafting reaction is copolymerization. Evidence that suggests that none of the grafting is a result of a hydrogen abstraction mechanism is also presented.     

On the reality of residual entropies of glasses and disordered crystals

We show that the hypothesis that the configurational entropy of a liquid disappears when it is kinetically frozen into a single glass state implies a spontaneous decrease of entropy. We show further that this is not an innocuous exception to the second law that requires a slight modification of that law, but rather implies directly the possibility of an uncompensated conversion of heat to work. We also note that the number of microstates visited in the course of a measurement does not determine the entropy, but rather that this number is always an inconceivably small fraction of the accessible microstates. Various experimental and computational tests of the possible vanishing of configurational entropy on kinetic arrest (e.g., from studies of glass vapor pressure and solubilities, and the coexistence curve between a disordered crystal and the liquid), with the exception of one questionable case, are consistent with the view that it does not vanish. We then show that this result is actually required by the second law. These considerations apply equally to the residual entropy of disordered crystals.     

Ring Polymers of Tubulin Induced by Binding of Natural Antimitotic Peptides

A number of natural products with potent antimitotic activity are peptides and depsipeptides that bind to tubulin, provoke depolymerization of microtubules, and induce formation of single layer rings of tubulin dimers. These peptides are all hydrophobic and small relative to tubulin (3-5 amino acid residues), yet induce rings polymers with properties that can differ significantly in size and self-association. In addition, these compounds exhibit potent cytotoxicity that varies by several hundred fold from one compound to another. Cryptophycin induces unusually homogeneous rings, composed of eight tubulin dimers, that are stable to dilution at least to nanomolar tubulin concentrations.     

Klein on the origin of the concept of chemical compound

Ursula Klein has argued that Geoffroy’s table of chemical affinities, published in 1718, marked the emergence of the concepts of chemical compound and chemical combination central to chemistry. In this paper her position is summarised and then modified to render it immune to criticism that has been levelled against it. The essentials of Geoffroy’s chemistry are clarified and adapted to Klein’s picture by way of a detailed comparison of it with Boyle’s corpuscular chemistry that proceeded Geoffroy’s by over half a century. The idea that Geoffroy’s notion of chemical combination marked a significant turning point in the emergence of modern chemistry is defended against the charge that it is Whiggish.     

Dependence of the Rate of Electrodeposition of Metals at Mono- and Multielectrodes on Voltage in the Case of an Exaltation Mechanism of Mass Transport

Dependences of the rate of copper deposition on mono- and multielectrodes out of dilute solutions of acetic acid at voltages of 20–250 V are compared. It is established that the same rate of deposition in the case of a multielectrode (25 electrodes with a diameter of 0.1 mm) takes place at a voltage that is lower by 2.7 times than that in the case of a monoelectrode with a diameter of 0.5 mm and the same area. The same results are obtained during electrodeposition of cadmium and thallium. In order to explain this effect, a two-layered model of potential distribution is suggested. In accordance with this model, the resistance of electrolyte in the vicinity of a multielectrode is equal to the sum of the resistance of the layer nearest to the electrode (where it is determined by the sum of conductances to each microelectrode) and the external layer, where the potential distribution is close to hemispherical. It is shown that calculations in accordance with this model allow one to give an estimate of the effect's scale that agrees with experiment and to predict its possible limits following an increase in the number of electrodes that make up a multielectrode.     

Determination of an ensemble of structures representing the denatured state of the bovine acyl-coenzyme a binding protein

The denatured state of a protein contains important information about the determinants of the folding process. By combining site-directed spin-labeling NMR experiments and restrained computer simulations, we have determined ensembles of conformations that represent the denatured state of the bovine acyl-coenzyme A binding protein (ACBP) at three different concentrations of guanidine hydrochloride. As the experimentally determined distance information corresponds to weighted averages over a broad ensemble of structures, we applied the experimental restraints to a system of noninteracting replicas of the protein by using a Monte Carlo sampling scheme. This procedure permits us to sample ensembles of conformations that are compatible with the experimental data and thus to obtain information regarding the distribution of structures in the denatured state. Our results show that the denatured state of ACBP is highly heterogeneous. The high sensitivity of the computational method that we present, however, enabled us to identify long-range interactions between two regions, located near the N- and C-termini, that include both native and non-native elements. The preferential formation of these contacts suggests that the sequence-dependent patterns of helical propensity and hydrophobicity are important determinants of the structure in the denatured state of ACBP.     

Application of off-specular scattering of X-rays and neutrons to the study of soft matter

Recent applications of the use of off-specular reflection of neutrons and X-rays in the study of soft matter are reviewed after a brief introduction to the status of the current theoretical treatment of such data. The review is intended to highlight the range of systems that may now be studied with off-specular reflection and recent progress in understanding the results that are observed. A few specific experimental cases are discussed as well as recent technical developments that may enable further more sophisticated treatments of off-specular scattering to be investigated.     

NiY分子筛表面酸性影响其选择性吸附脱硫性能的机理研究

采用原位红外光谱技术,以噻吩、环己烯和苯为模型探针分子,分别考察单一烃分子在NiY分子筛上的吸附与反应行为以及噻吩与烯烃、芳烃间的竞争吸附和催化反应行为。单一探针分子吸附研究发现,NiY分子筛中与Ni物种相关的Lewis(L)酸位是噻吩的选择性吸附活性位;噻吩和环己烯在NiY分子筛中Brnsted(B)酸位上发生的质子化和低聚反应明显弱于HY分子筛。双探针分子竞争吸附研究发现,环己烯二聚体在NiY中强B酸位上的强化学吸附与噻吩存在显著的竞争吸附行为。另外,苯和噻吩在NiY上的竞争吸附现象在373K时明显减弱。由此,在选择性吸附脱硫过程中,减少吸附剂表面B酸中心可降低烯烃对噻吩的竞争吸附,另外适当提高吸附体系的温度可以有效避免芳烃对噻吩的竞争吸附。     

Mass spectrometric characteristics and kinetics of iron release in visceral mass of Saccostrea cucullata

Ferritins with electrophoretic homogeneity were prepared from the visceral mass of Saccostrea cucullata in batch. The native PAGE approach showed similar electrophoretic mobility among pig pancreatic ferritin, liver ferritin of Dasyatis akajei, and visceral mass ferritin of Saccostrea cucullata. SDS-PAGE indicated that the Saccostrea cucullata visceral ferritin (SCVF) consisted of a single subunit type and had a molecular weight (MW) of approximately 20 kDa, suggesting that the protein shell in SCVF was composed of a single subunit. In addition, peptide mass fingerprinting and transmission electron microscopy were used to identify SCVF further, and to observe its molecular structure. We found that the molecular structure in SCVF was similar to that of most mammalian ferritins, which are composed of a protein shell and an iron core. The results of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry under the assistance of an acidic matrix, sinapic acid, also showed that SCVF was composed of a single subunit type and its subunit MW was calculated to be 19871.042 Da in the absence of heme. Kinetics analysis revealed that the complete process of iron release fitted the law of a first-order reaction, which is similar to that of most ferritins in mammals. Similar to bacterial ferritin, studies indicated that the shell consisted of a single subunit type and showed similar kinetics of iron release, suggesting that this subunit plays two important roles in iron release and storage, and that it shows different stability and intensity of interaction in carrying out its physiological functions in SCVF.     

Thermodynamic parameters of solvation of nonelectrolytes in aqueous solutions of amides of carboxylic acids

Interaction and reorganization contributions to solvation enthalpies of nonelectrolytes in aqueous solutions of amides of carboxylic acids with different degree of N-substitution and N-methylpyrrolidone are calculated. The data are discussed using structurally thermodynamic characteristics of water-amide systems obtained by us previously. It is found that the type of concentration dependence of the solvation enthalpy of nonelectrolytes in all solutions investigated is determined by the type of reorganization component. It is shown that the highest solvation exothermicity of nonelectrolytes in water is due to the lowest value of the reorganization contribution in spite of that nonelectrolytes interact weaker with water than with non aqueous components.     

Analysis of the bioactivity of magnetically immunoisolated peroxisomes

Peroxisomes produce reactive oxygen species which may participate in biotransformations of innate biomolecules and xenobiotics. Isolating functional peroxisomes with low levels of contaminants would be a useful tool to investigate biotransformations occurring in these organelles that are usually confounded with biotransformations occurring in other co-isolated organelles. Here, we immunoisolate peroxisomes and demonstrate that the impurity level after isolation is low and that peroxisomes retain their biological activity. In this method, an antibody targeting a 70-kDa peroxisomal membrane protein was immobilized to silanized magnetic iron oxide beads (1–4 μm in diameter) coated with Protein A. Peroxisomes from L6 rat myoblast homogenates were magnetically captured, washed, and then analyzed for subcellular composition using enzymatic assays. Based on the ratio of peroxisomal to lysosomal activity, the retained fraction is 70-fold enriched relative to the unretained fraction. Similarly, the ratio of peroxisomal activity to mitochondrial content suggests that the retained fraction is >30-fold enriched relative to the unretained fraction. H₂O₂ production from the β-oxidation of palmitoyl-CoA demonstrated that the isolated peroxisomal fraction was biologically active. Capillary electrophoresis with laser-induced fluorescence detection (CE-LIF) analysis confirmed that the immunopurified fractions were capable of transforming the anticancer drug doxorubicin and the fatty acid analog, BODIPY 500/510 C1C12. Besides its use to investigate peroxisome biotransformations in health and disease, the combination of magnetic immunoisolation with CE-LIF could be widely applicable to investigate subcellular-specific biotransformations of xenobiotics occurring at immunoisolated subcellular compartments.     

Exploration of variables in the fabrication of pyrolysed photoresist

The increased use of pyrolysed photoresist films (PPF) as electrode material intensifies the need to know what fabrication variables are important in the pyrolysis process. The main factor effects of seven variables (three time/temperature heating levels plus the position in the furnace tube) in the fabrication of PPF were investigated by a Plackett and Burman, eight-run, experimental design. In the three-step pyrolysis programme, gas flow had a large effect on the surface cleanliness and roughness. It was also observed that the position in the furnace affected the resistivity of the PPF. Fabrication parameters that give rapid electron transfer to redox species in solution, that provide low surface oxide and that lead to low surface roughness were identified. The guidelines on what fabrication conditions to employ to give a variety of different electrode characteristics are presented.     

Considerations for the quantitative transduction of hybridization of immobilized DNA

Immobilized single-stranded DNA (ssDNA) can be used as a selective ‘reagent’ to bind complementary DNA or RNA for applications such as the detection of pathogenic organisms, gene therapy agents and genetic mutations. The density of ssDNA on a surface will determine the charge density due to ionizable phosphate groups. Such a negatively charged interface will attract positive counter-ions from solution, which may result in a local ionic strength, pH and dielectric constant on the surface that is substantially different from that in bulk electrolyte solution. It is the local conditions which influence the thermodynamics of hybridization, and this can studied by the melt temperature (T_m) of double-stranded DNA (dsDNA). Experimental work and theoretical models have been used to examine whether hybridization reactions on a surface can cause dynamic changes in local charge density, and therefore, changes in selectivity and drift in calibration for quantitative analysis. Organosilane chemistry has been used to covalently immobilize hexaethylene glycol linkers and to control the subsequent density of dT₂₀ that was prepared by automated synthesis. Fiber-optic biosensors based on fused silica that was coated with DNA were used in a total internal reflection fluorescence instrument to determine T_m from the dissociation of duplexes of fluorescein-labeled dA₂₀ : dT₂₀. The experimental results suggest that the thermodynamic stability of duplexes that are immobilized on a surface is dependent on the density of immobilized DNA and on the extent of hybridization of DNA. The experimental results show that the thermodynamic stability of immobilized dsDNA is significantly different than that of dsDNA in bulk solution, and include observations of the variation of enthalpy at different ionic strengths, asymmetry in the melt curves, and the possibility of a reduced dielectric constant within a DNA layer relative to that in bulk solution.     

Enhancement of Lead-Barium Exchangeability of Barium Hydroxyapatite

In order to improve lead-barium exchangeability of barium hydroxyapatite (BaHAp), the effects of the recycle employment of BaHAp and the introduction of HCl into the exchange solution have been investigated at 293 K. The ion exchange capacity of BaHAp was evidently improved by use of the recycle experiments to reach levels approximately twice that obtained via the batch experiment. XPS analyses revealed that the ion exchange appears to proceed from the surface to the bulk of the apatite. The addition of HCl to the exchange solution also resulted in the enhancement of the ion exchangeability of BaHAp. Based on the apparent solubility of BaHAp, which was estimated from the dissolution of BaHAp for 4 h, together with the results of that experiment, it is suggested that the dissolution-precipitation mechanism appears to be possible for ion exchange. Copyright 2001 Academic Press.     

Quantification of noise sources for amperometric measurement of quantal exocytosis using microelectrodes

Electrochemical microelectrodes are commonly used to record amperometric spikes of current that result from oxidation of transmitter released from individual vesicles during exocytosis. Whereas the exquisite sensitivity of these measurements is well appreciated, a better understanding of the noise sources that limit the resolution of the technique is needed to guide the design of next-generation devices. We measured the current power spectral density (S(I)) of electrochemical microelectrodes to understand the physical basis of dominant noise sources and to determine how noise varies with the electrode material and geometry. We find that the current noise is thermal in origin in that S(I) is proportional to the real part of the admittance of the electrode. The admittance of microelectrodes is well described by a constant phase element model such that both the real and imaginary admittance increase with frequency raised to a power of 0.84-0.96. Our results demonstrate that the current standard deviation is proportional to the square root of the area of the working electrode, increases ～linearly with the bandwidth of the recording, and varies with the choice of the electrode material with Au ≈ carbon fiber > nitrogen-doped diamond-like carbon > indium-tin-oxide. Contact between a cell and a microelectrode does not appreciably increase noise. Surface-patterned microchip electrodes can have a noise performance that is superior to that of carbon-fiber microelectrodes of the same area.