The effective diffusion coefficient and the distribution constant for small molecules in calcium-alginate gel beads

The effective diffusion coefficient, D(e), and the distribution constant, K(i), for selected mono- and disaccharides and organic acids were determined in homogeneous calcium-alginate gel with and without entrapped bacteria. Results were obtained from transient concentration changes in well-stirred solutions of limited volume, in which the gel beads were suspended. The effective diffusioncoefficients and the distribution constants were estimated by fitting mathematical model predictions to the experimental data using a nonlinear model fitting program (MODFIT). Both single solute diffusion and multiple solute diffusion were performed. A small positive effect was obtained onthe values of D(e) for the system of multiple solute diffusion; however, the values of K(i) were not significantly influenced. For the nine solutes tested, D(e) for 2% Ca-alginate gel beads was found to be approximately 85% of the diffusivity measured in water. The effects on D(e) and K(i), for lactose and lactic acid were determined for variations of alginate concentration, pH, temperature, and biomass content in the beads. D(e) decreased linearly for both lactose and lactic acid with increasing cell concentration in the Ca-alginate gel. K(i), was constant for both lactose and lactic acid with increasing cell concentration. D(e) was significantly lower at pH 4.5 than at pH 5.5 and 6.5 for both lactose and lactic acid. Furthermore, D(e) seemed to decrease with increased alginate concentration in the range of 1% to 4%. The diffusion rate increased with increasing temperature, and the activation energy for the diffusion process for both lactose and lactic acid was constant in the temperature range tested. (c) 1995 John Wiley & Sons Inc.     

Assessment of protein solution versus crystal structure determination using spin- diffusion-suppressed NOE and heteronuclear relaxation data

A spin-diffusion-suppressed NOE buildup series has been measured for E. coli thioredoxin.The extensive 13C and 15N relaxation data previously reported for this protein allow fordirect interpretation of dynamical contributions to the 1H-1H cross-relaxation rates for a largeproportion of the NOE cross peaks. Estimates of the average accuracy for these derived NOEdistances are bounded by 4% and 10%, based on a comparison to the corresponding X-raydistances. An independent fluctuation model is proposed for prediction of the dynamicalcorrections to 1H-1H cross-relaxation rates, based solely on experimental structural andheteronuclear relaxation data. This analysis is aided by the demonstration that heteronuclearorder parameters greater than 0.6 depend only on the variance of the H-X bond orientation,independent of the motional model in either one- or two-dimensional diffusion (i.e., 1– S2 = 3/4 sin2 2 ). The combination of spin-diffusion-suppressed NOEdata and analysis of dynamical corrections to 1H-1H cross-relaxation rates based onheteronuclear relaxation data has allowed for a detailed interpretation of various discrepanciesbetween the reported solution and crystal structures.     

蛋白质晶体的优化生长

蛋白质晶体的优化生长是获得高质量蛋白质晶体, 进而得到高精度晶体结构的有效途径.针对不同的晶体生长方法,已尝试了不同的优化手段,这对改善某些蛋白质晶体的质量显示了明显的成效.然而,鉴于蛋白质晶体生长的多样性与复杂性,这些方面均未发展成为实用的技术.文章综述了这类研究进展,分析了各手段的利弊,并指出了应着重解决的问题.     

Turing bifurcations with a temporally varying diffusion coefficient

This paper is concerned with the possibility of Turing bifurcations in a reaction-diffusion system in which the diffusion coefficient of one species varies periodically in time. This problem was introduced and investigated numerically by Timm and Okubo (J. Math. Biol. 30, 307, 1992) in the context of predator-prey interactions in plankton populations. Here, I consider the simple case in which the temporal variation in diffusivity has a square-tooth form, alternating between two constant values, with a period that is long compared with the time scale of the kinetics. The analysis is valid for any set of reaction kinetics. I derive explicit expressions for the Floquet multipliers that determine the stability of the steady state, and thereby obtain the conditions for diffusion driven instability to occur. These conditions imply that, depending on the kinetics, the homogeneous equilibrium may be either more or less stable than when the diffusion coefficient is a constant equal to the mean of the variable diffusivity. I go on to consider the form of the solution when diffusion driven instability does occur, and I use perturbation theory to determine the effect of a small temporal variation in the diffusion coefficient on the spatial wavelength of the pattern that results from diffusion driven instability.     

Removal of toluene in waste gases using a biological trickling filter

The removal of toluene from waste gas was studied in a trickling biofilter. A high level of water recirculation (4.7 m h^–1) was maintained in order to keep the liquid phase concentration constant and to achieve a high degree of wetting. For loads in the range from 6 to 150 g m^–3 h^–1 the maximum volumetric removal rate (elimination capacity) was 35±10 g m^–3 h^–1, corresponding to a zero order removal rate of 0.11±0.03 g m^–2 h^–1 per unit of nominal surface area. The surface removal was zero order above the liquid phase concentrations of approximately 1.0 g m^–3, corresponding to inlet gas concentrations above 0.7–0.8 g m^–3. Below this concentration the surface removal was roughly of first order. The magnitude of the first order surface removal rate constant, k_1A , was estimated to be 0.08–0.27 m h^–1 (k_1A a=24–86 h^–1). Near-equilibrium conditions existed in the gas effluent, so mass transfer from gas to liquid was obviously relatively fast compared to the biological degradation. An analytical model based on a constant liquid phase concentration through the trickling filter column predicts the effluent gas concentration and the liquid phase concentration for a first and a zero order surface removal. The experimental results were in reasonable agreement with a very simple model valid for conditions with an overall removal governed by the biological degradation and independent of the gas/liquid mass transfer. The overall liquid mass transfer coefficient, K_La, was found to be a factor 6 higher in the system with biofilm compared to the system without. The difference may be explained by: 1. Difference in the wetting of the packing material, 2. Mass transfer occurring directly from the gas phase to the biofilm, and 3. Enlarged contact area between the gas phase and the biofilm due to a rough biofilm surface.     

Dynamic modelling of a helical peptide in solution using NMR data: Multiple conformations and multi-spin effects

Summary Nuclear Overhauser effect (NOE) measurements on molecules in solution provide information about only the ensemble-averaged properties of these molecules. An algorithm is presented that uses a list of NOEs to produce an ensemble of molecules that on average agrees with these NOEs, taking into account the effect of surrounding spins on the buildup of each NOE (spin diffusion). A simplified molecular dynamics simulation on several copies of the molecule in parallel is restrained by forces that are derived directly from differences between calculated and measured NOEs. The algorithm is tested on experimental NOE data of a helical peptide derived from bovine pancreatic trypsin inhibitor.     

脂肪酸气相色谱图鉴别方形黄鼠蚤各亚种的初步研究

方形黄鼠蚤４个亚种在脂肪酸组分和百分含量上具有明显不同,其中雌虫在１８个组分上有区别雄虫在１４个组分上有区别。此外,雌虫在Ｃ１６：１和Ｃ２０：０的含量上,雄虫在Ｃ１６：１的含量上,４个亚种之间均具有显著性差异。在Ｃ１２－Ｃ２２区间内有９个含量较高的主要组分,是４个亚种所共有的。方形黄鼠蚤在我国境内分布有４个亚种即松江亚种、蒙古     

气相色谱法测定RuBp羧化酶活性的研究

ＲｕＢｐ羧化酶作用后反应体系中能为加入的ＨＣｌ释放出的剩余,作为ＲｕＢｐ羧化酶活性指示。所释放的ＣＯ＿２以气相色谱法检测,用黄瓜叶片中ＲｕＢｐ羧化酶的反应时间曲线和酶量曲线验征,并与现行的分光光度,￣（１４）Ｃ标记测试方法比较分析,认为本方法具有应用简便、快速、准确、重复性好等优点。所以,可被认为是一种有效的测定ＲｕＢｐ羧化酶活性的方法。     

Accurate and efficient method for quantification of urinary histamine by gas chromatography negative ion chemical ionization mass spectrometry

Because of the recognized inaccuracy and unreliability of currently available methods for the quantification of histamine in biological fluids, a method for quantification of urinary histamine by stable isotope dilution assay with negative ion chemical ionization mass spectrometry has been developed. Following the addition of [²H₄]histamine to 1 ml of urine, histamine is extracted into butanol, back-extracted into HCl, derivatized to the pentafluorobenzyl derivative (CH₂C₆F₅)₃-histamine, extracted into methylene chloride, and then quantified with negative ion chemical ionization mass spectrometry by selected ion monitoring of the ratio of ions $\text{m}\text{z}$$\text{430}\text{434}$. Twenty samples can be assayed in 2 days. Precision of the assay is ±2.7% and the accuracy is 97.6%. Lower limits of sensitivity are approximately 100–500 fg injected on-column. This assay provides a very sensitive, accurate, and efficient method for the quantification of histamine in human urine.     

The glycoprotein 71 of ecotropic Friend murine leukemia virus. Structure of the oligosaccharides linked to asparagine-12

The glycoprotein from Friend murine leukemia virus was digested with protease from Staphylococcus aureus V8. A glycopeptide comprising the N-terminal glycosylation site (Asn-12) was isolated from the mixture of fragments and analyzed by amino acid sequencing and methylation-capillary gas chromatography-mass spectrometry before and after treatment with sialidase from Vibrio cholerae. Asn-12 was thus found to be substituted by a family of partially sialylated, fucosylated, and intersected glycoprotein N-glycans of the hybrid type.