Small-angle neutron scattering study of structural evolution of different phases in protein solution

Small-angle neutron scattering (SANS) has been used to study the structural evolution of different phases in protein solution leading to crystallization, denaturation and gelation. The protein solution under crystallization mostly consists of monomers and dimers, and higher-mers are not observed as they are perhaps formed in very small numbers. The onset and the rate of crystallization strongly depend on the salt concentration. Protein denaturation on addition of surfactant occurs due to the formation of micelle-like clusters along the unfolded polypeptide chains of the protein. The structure of such protein-surfactant complex is found to be independent of the size of the micelles in their pure surfactant solutions. The structure of temperature-induced protein gels shows a fractal structure. Rheology of these gels shows a strong dependence on varying pH or protein concentration, whereas the structure of such gels is found to be similar.      

The Use of Small Angle Light Scattering to Study Structure of Flocs

In this paper, a new approach using small angle light scattering to study the structure of large micron sized flocs is described. The flocs were made up from uniformly sized, approximately spherical hematite particles, induced to aggregate using 250 mM KCl to ensure that growth was governed by diffusion limited aggregation alone. The fractal dimensions of the hematite aggregates changed from 1.73 to 2.23 ± 0.05 as the aggregation proceeded, indicating that restructuring occurred during aggregation. The fractal dimensions obtained from the small angle light scattering technique are compared with those obtained from the combination of volume fraction and floc size measurement. The steady state fractal dimensions (2.23 ± 0.05) obtained by small angle light scattering are comparable to the fractal dimensions obtained from determining the slope of the relationship between floc volume fraction and floc size (2.09 and 2.14 ± 0.05). The discrepancy between the fractal dimensions obtained by these techniques during the initial stages of aggregation is due to the fact that the latter method measures the average fractal dimensions, whilst the former method measures the fractal dimensions at any particular instant.     

水分子凝胶中有机凝胶因子聚集体的分形结构研究

水分子凝胶是一种新型软凝聚体系.是凝胶因子在很低的浓度下在水中聚集、自组装，使水凝胶化形成的凝聚体系.透射电镜(TEM)表明凝胶因子在水中聚集、自组装成细纤维状结构.通过对TEM照片进行数字化处理，采用Sandbox法和密度-密度相关函数法计算的结果表明凝胶因子在聚集组装过程中具有典型的分形特征.根据C++程序计算出分形维数D＝1.814—1.977.以分形理论对凝胶因子的聚集过程以及由此形成的水分子凝聚体系的分形特征进行了讨论.利用小角x射线散射(SAXS)研究进一步表明，凝聚体系的分形结构存在于尺度α 关键词： 分形 凝胶因子 水分子凝聚体系 透射电镜(TEM) 小角x射线散射(SAXS)     

Characterization of Nanoparticles and Colloids in Aquatic Systems 1. Small Angle Neutron Scattering Investigations of Suwannee River Fulvic Acid Aggregates in Aqueous Solutions

Fulvic acids (FA) and humic acids (HA) constitute 30–50% of dissolved organic matter in natural aquatic systems. In aqueous solutions, a commonly accepted view is that FA and HA exist as soluble macroligands at low concentration and as supramolecular aggregates at higher concentration. The size, shape and structure of these aggregates are still the subject of ongoing debate in the environmental chemistry literature. In this article, we use small angle neutron scattering (SANS) to assess the effects of solute concentration, solution pH and background electrolyte (NaCl) concentration on the structures of Suwannee River FA (SRFA) aggregates in D₂O. The qualitative features of the SANS curves and data analysis are not consistent with the view point that SRFA forms micelle-like aggregates as its concentration in aqueous solution increases. We find that SRFA forms fractal aggregates in D₂0 with size greater than 242 nm. The SRFA aggregates undergo a significant degree of restructuring in compactness as solution pH, solute concentration and NaCl concentration increase.     

Structure of gels and aggregates of disk-like colloids

The static structure factor (S(q)) of dispersions and gels of disk-like mineral colloids (Laponite) was investigated using time- and ensemble-averaged light scattering. The evolution of S(q) in time after increasing the ionic strength of well-dispersed Laponite suspensions shows that Laponite aggregates and forms fractal clusters. The structure of the aggregates does not depend on the ionic strength, but the rate of growth increases very strongly with the ionic strength. At concentrations below about 3 g/l (0.12% v/v) the aggregates sediment while at higher concentrations space-filling gels are formed. The gels are homogeneous on length scales larger than the correlation length which decreases strongly with decreasing ionic strength and increasing concentration. However, the local structure is the same, independent of the concentration and the ionic strength. Received 6 August 2000 and Received in final form 16 March 2001     

小角x射线散射法研究甲基改性氧化硅凝胶的双分形结构

以正硅酸乙酯［Si(OC2H5)4,TEOS］和甲基三乙氧基硅烷［CH3Si(OC2H5)3,MTES］为前驱体，通过共水解法和两步法制备出两种不同的甲基改性氧化硅凝胶，在北京同步辐射光源(BSRF)小角x射线散射(SAXS)站测量了凝胶的散射强度，计算了凝胶的平均粒径、两相间比表面积等参数，在此基础上分析了凝胶的分形特征，发现存在两个尺度上的分形结构，分别对应于从SiO2原生颗粒到一次团聚体和从一次团聚体到簇团两种尺度.辅以透射电子显微镜(TEM)观测，证实由两种方法获得的凝胶具有非常不同的微观结构.实验证明，利用SAXS技术研究甲基改性凝胶的分形特征是获得凝胶微观结构的有力工具. 关键词： 甲基改性凝胶 氧化硅 小角x射线散射(SAXS) 分形结构     

Brownian dynamics simulation of linear chain growth

Growth of linear chains is simulated by the technique of Brownian dynamics. The influence of the initial volume fraction of particles on the rate of the process and the structure of the chains is studied. Results show that aggregation is swift at the beginning of the process and very slow at the end. This is due to the decrease of the number of flocs in solution and the decrease in the efficiency of collisions when the degree of aggregation increases. The polydispersity of the system increases during aggregation and reaches a maximum value when the reaction is 96% complete. This increase is greater at higher concentrations. Results indicate that for lower concentrations aggregate growth is principally by reactive collision between aggregates of similar molecular weight, while for higher concentrations there is a relatively high fraction of reactive collisions between aggregates with very different molecular weights. The concentration effect on the structure of the chains is studied, evaluating the fractal dimensions of the chains. When the concentration increases there is a transition from very open structures (fractal dimension 1.76) to very tangled structures, where chains are collapsed (fractal dimension 3).     

Small angle neutron scattering studies on protein denaturation induced by different methods

Small angle neutron scattering (SANS) has been used to study conformational changes in protein bovine serum albumin (BSA) as induced by varying temperature and in the presence of protein denaturating agents urea and surfactant. BSA has prolate ellipsoidal shape and is found to be stable up to 60°C above which it denaturates and subsequently leads to aggregation. The protein solution exhibits a fractal structure at temperatures above 64°C, with fractal dimension increasing with temperature. BSA protein is found to unfold in the presence of urea at concentrations greater than 4 M and acquires a random coil Gaussian chain conformation. The conformation of the unfolded protein in the presence of surfactant has been determined directly using contrast variation SANS measurements by contrast matching surfactant molecules. The protein acquires a random coil Gaussian conformation on unfolding with its radius of gyration increasing with increase in surfactant concentration      

Brownian dynamics simulation of gelation in soft sphere systems with irreversible bond formation

Brownian dynamics simulation is used to study the structure and small-deformation shear rheology of three-dimensional particle gels formed from a model of soft spherical particles incorporating a combination of flexible, irreversible bond formation and non-bonded interparticle interactions. An essential feature of the model is the restriction of angular reorganization of cross-linked aggregates during and after gelation. Numerical data relating to the fractal structure and porosity of the gels are compared with those from some related particle gel simulation models in two and three dimensions. Stress relaxation and frequency dependent rheological properties are determined as a function of the strength and nature of the interparticle interactions, and the results are discussed in relation to structural properties. A linear relationship is demonstrated between the high-frequency modulus and the number of bonds in the gel network.     

Combined SANS and SAXS study of the action of ultrasound on the structure of amorphous zirconia gels

In the present work, we have studied for the first time the combined effect of both sonication and precipitation pH on the structure of amorphous zirconia gels synthesized from zirconium(IV) propoxide. The techniques of small-angle neutron and X-ray scattering (SANS and SAXS) and low temperature nitrogen adsorption provided the integral data on the changes in the microstructure and mesostructure of these materials caused by ultrasonic (US) treatment. Amorphous ZrO₂·xH₂O synthesized under ultrasonic treatment was found to possess a very structured surface, characterized by the surface fractal dimension 2.9–3.0, compared to 2.3–2.5 for the non US-assisted synthesis, and it was also found to possess a higher specific surface area, while the sizes of the primary particles remain unchanged.     

新的光散射体系测定蛋白质的研究

以1,6-己二胺二吖啶为荧光探针,建立了一种新的蛋白质共振散射检测体系。实验考察了该吖啶荧光探针的共振散射特征光谱、散射反应稳定性,研究了缓冲介质pH、探针浓度等参数对测定蛋白质的影响。结果表明,当pH 8.9时,1,6-己二胺二吖啶染料的散射波长为470 nm;加入BSA,反应约9 min后,体系光散射强度达到最大并维持稳定,2 h内无明显变化。当1,6-己二胺二吖啶探针(浓度1.00×10-4mol·L-1)用量为3.00 mL时,蛋白质与1,6-己二胺二吖啶的光散射增强作用显著,共振散射强度随着BSA的增加而明显增强,光散射强度与蛋白质浓度成良好线性关系,线性浓度范围为1.00～5.00 μg·mL-1,检测限(3σ/K)为0.085 μg·mL-1。测定了血清样品,浓度为2.00～4.00 μg·mL-1,回收率为98.0%～101.7%,测定偏差小于1.7％。     

Components of released liquid from ultrasonic waste activated sludge disintegration

Ultrasound can be applied as a pretreatment to disintegrate sludge. In this paper, by observing the solution concentration of polysaccharide, protein, DNA, Ca and Mg before and after disintegration, the main components in the released liquid are analyzed. It has been found that the predominant component of the released liquid in this research is protein. Ultrasound can destroy the extracellular polymeric substances (EPS), which is important to the sludge flocs structure. Ca2+ and Mg2+, which play a key role in binding the EPS are released into the aqueous phase. As a result, the sludge flocs are loosened. Under the effect of the hydraulic shear force, the sludge is disintegrated. Then the hydraulic shear forces destroy the cell walls, the substances inside the cells are released into the aqueous phase.     

利用酪氨酸残基的共振瑞利散射研究溶液中蛋白质构象

基于酪氨酸残基K带激发峰在较高浓度的蛋白质溶液中表现出特征散射峰,通过Δλ＝0同步荧光扫描识别,建立用荧光分光光度法测定酪氨残基K带的共振散射峰强度及其波长位移来推测溶液中蛋白质构象变化的新方法。方法不受蛋白质色氨酸残基的干扰,图谱简单明了直观。文章叙述了方法建立的实验基础,并用构建的方法研究了盐效应、酸碱性效应对蛋白质构象变化的影响,最后通过测定不同温度下酶活性的实验验证了方法对温度影响酶蛋白质构象变化的分析的正确性。     

Synchrotron small angle X-ray scattering study on the conformation of polystyrene in compressed CO2-toluene mixture

Synchrotron small angle X-ray scattering (SAXS) was performed to investigate the effect of dissolved CO2 in toluene on the conformation of polystyrene (PS) in the solution. It has been found that the second virial coefficient A2 and the radius of gyration Rg decrease with the increasing antisolvent CO2 pressure. The scattering intensity of PS chain followed l(h)∞h-αunder different antisolvent pressures (0, 0.6, 1.5, 2.4, 3.3, and 4.2 MPa), suggesting that the PS chain has self-similar structure behavior or a fractal structure in the presence of antisolvent CO2.All this reveals a large effect of antisolvent pressure or the solubility of CO2 in the solution on PS structure. The fractal dimensions increase with the increasing antisolvent pressure, indicating that the polymer chain changes from a swollen coil into a rather dense globule in the course of adding antisolvent CO2.     

Self—assembly behaviour of amphiphilic diblock copolymer in selective solvents studied by synchrotron small—angle x—ray scattering

The aggregation behaviour of styrene-vinyl benzoic acid (PS_m-b-PVBA_n) amphiphilic diblock copolymers in selective solvents with different m and n was investigated by synchrotron small-angle x-ray scattering (SAXS). We have carried out a detailed analysis of scattering intensity, dimension, shape and microstructure of the diblock copolymers of narrow distribution in water, methanol, ethanol and isopropanol selective solvents, respectively. We have found that the aggregation behaviour of the copolymer depends on the nature of the solvent and the micelle forms flat disc objects with the ratio of radius ω=0.4. The average radius gyration R_g of the copolymer decreases as solvents change from isopropanol to ethanol and to methanol, and increases with increasing pH in aqueous solution, but decreases with the addition of CoCl_2 in ethanol solvent. The scattering intensity of diblock copolymer micelle follows I(h)∝h^{-α} in different selective solutions, suggesting that the PS_m-b-PVBA_n coils have self-similar structure behaviour or a fractal structure in the selective solvents. All of these revealed that the aggregation behaviour of the diblock copolymer changes dramatically with experimental condition in the selective solvent. The increase of mass fractal dimension (D_m) from 2.12 to 2.47 indicates that the copolymer chain changes from a swollen coil to a rather compact disc in the course of changing solvents, decreasing surface fractal dimension (D_s) from 2.98 to 2.58 indicates that the copolymer micelle change from a rather rough surface to a smooth form in the course of increasing pH in aqueous solutions, and increasing D_m and D_s from 2.29 to 2.35 and 2.70 to 2.90, respectively, indicates the shrinkage of copolymer micelle to a rather compact and rough disc form by adding CoCl_2 in ethanol solvents.     

Structure of zirconium dioxide based porous glasses

This study is devoted to investigation of the micro- and mesostructure (including fractal properties) of porous zirconia glasses synthesized by precipitation from zirconium n-propoxide solutions in the presence of different hydrolyzing-agent (H₂O) quantities at different temperatures. Analysis of small-angle neutron, ultra-small-angle neutron and X-ray scattering, scanning electron microscopy allows concluding that the synthesized glasses are complex systems with a three-level hierarchical fractal structure. It is revealed that both the temperature of synthesis and the H₂O concentration in the initial solution significantly affect the structural characteristics of the glasses.     

掺杂NaCl水溶液结晶形貌的分形研究

水溶液结晶是研究分形形貌的重要实验之一。本文通过改变溶液浓度,溶质组分和结晶温度等宏观实验参数,用分形维数的方法研究了这些参数对结晶形貌的影响。指出在低浓度下的非平衡态中溶液结晶形貌具有分形结构;不同晶体的生长作用力相互竞争,混乱度最大的情况下形成的分形结构最凸出,生长作用力是影响晶体形貌的主要因素;同时,分子热运动也是影响结晶形貌的因素之一。     

二维导体粗糙面电磁散射的分形特征研究

采用二维归一化带限Brown分形函数来模拟二维分形粗糙面,主要利用基尔霍夫近似给出了该导体粗糙面的电磁散射场.导出了平均散射场、平均散射系数和散射强度方差的计算公式.讨论了散射场分布与分维的关系,获得了散射场波峰拟合线的斜率与分维D满足线性关系这一重要结论. 关键词：     

Small angle neutron scattering studies on the interaction of cationic surfactants with bovine serum albumin

The structure of the protein-surfactant complex of bovine serum albumin (BSA) and cationic surfactants has been studied by small angle neutron scattering. At low concentrations, the CTAB monomers are observed to bind to the protein leading to an increase in its size. On the other hand at high concentrations, surfactant molecules aggregate along the unfolded polypeptide chain of the protein resulting in the formation of a fractal structure representing a necklace model of micelle-like clusters randomly distributed along the polypeptide chain. The fractal dimension as well as the size and number of micelles attached to the complex have been determined.      

Swelling/deswelling of polyacrylamide gels in aqueous NaCl solution: Light scattering and macroscopic swelling study

Swelling kinetics of water-swollen polyacrylamide (PAAm) hydrogels (WSG) was investigated in various concentrations of aqueous NaCl by macroscopic swelling measurements. For lower concentration of NaCl, WSG showed exponential swelling whereas at higher concentration of NaCl it underwent deswelling at short times and exponential swelling at long times. From these studies, collective diffusion coefficient, D, of the polymer network and polymer?Csolvent interaction parameter, ??, were calculated and found to decrease with increase in [NaCl]. Collective diffusion coefficients measured from dynamic light scattering (DLS) and that obtained from macroscopic swelling measurements are found to agree well. Measured ensemble-averaged dynamic structure factor f(q,t) for WSG and salt-swollen gels (SSG) showed an initial decay followed by a plateau at long times and it can be described by harmonically bound Brownian particle (HBBP) model. Enhanced scattering intensity at low scattering angles using static light scattering (SLS) measurements revealed the presence of inhomogeneities in PAAm gels. The reasons for increased scattering intensity of SSG over WSG gel and the linear decrease of D with increase in NaCl concentration are explained.