Micellization of bovine beta-casein studied by isothermal titration microcalorimetry and cryogenic transmission electron microscopy

The association behavior, critical micellization concentration (CMC), and enthalpy of demicellization (DeltaHdemic) of bovine beta-casein were studied, for the first time by isothermal titration calorimetry, in a pH 7.0 phosphate buffer with 0.1 ionic strength and in pure water. In the buffer solutions, the CMC decreased asymptotically from 0.15 to 0.006 mM as the temperature was raised from 16 to 45 degrees C. DeltaHdemic decreased with increasing temperature between 16 and 28 degrees C but increased from 28 to 45 degrees C. Thermodynamic analysis below 30 degrees C is consistent with the Kegeles shell model, which suggests a stepwise association process. At higher temperatures, this model exhibits limitations, and the micellization becomes much more cooperative. The CMC values in water, measured between 17 and 28 degrees C, decreased with increasing temperature and, expectedly, were higher than those found in the buffer solutions. beta-Casein micelles were visualized and characterized, for the first time in their hydrated state, using advanced digital-imaging cryogenic transmission electron microscopy. The images revealed small, oblate micelles, about approximately 13 nm in diameter. The micelles shape and dimensions remained nearly constant in the temperature range of 24-35 degrees C.     

Maltodextrin-anionic surfactant interactions: isothermal titration calorimetry and surface tension study

Interactions between maltodextrin (DE = 10) and an anionic surfactant (sodium dodecyl sulfate, SDS) were studied in a buffer solution (pH 7.0, 10 mM NaCl, 20 mM Trizma, 30.0 degrees C) using isothermal titration calorimetry (ITC), surface tension, differential scanning calorimetry (DSC), and turbidity techniques. ITC measurements indicated that the binding of SDS to maltodextrin was exothermic and that, on average, one SDS monomer bound per 24 glucose units of maltodextrin at saturation. Surface tension measurements indicated that there was a critical surfactant concentration ( approximately 0.05 mM SDS) below which surfactant and maltodextrin did not interact and that the amount of surfactant bound to the maltodextrin above this concentration increased with increasing maltodextrin concentration. Turbidity measurements indicated that the solutions remained transparent at all maltodextrin (0-1 wt %) and SDS (0-20 mM) concentrations studied, which suggested that phase separation did not occur. DSC measurements indicated that no phase transitions occurred between 10 and 110 degrees C for maltodextrin solutions (0.5 wt %) in the presence or absence of surfactant. A phase diagram was developed to describe the interactions between SDS and maltodextrin.     

Thermodynamic parameters of beta-lactoglobulin-pectin complexes assessed by isothermal titration calorimetry

Isothermal titration calorimetry (ITC) was used to determine the binding constant, stoichiometry, enthalpy, and entropy of beta-lactoglobulin/low- and high-methoxyl pectin (beta-lg-LM- and HM-pectin) complexes at 22 degrees C and at pH 4. The binding isotherms revealed the formation of soluble intrapolymer complexes (C1) further followed by their aggregation in interpolymer complexes (C2). The interaction between beta-lg and LM- or HM-pectin in C1 and C2 occurred spontaneously with a Gibbs free energy around -10 kcal/mol. The C1 were enthalpically driven, whereas enthalpic and entropic factors were involved in the C2 formation. Because ITC did not allow the dissociation of different enthalpic contributions, the values measured as pectin and beta-lg interacted could partially be attributed to conformational changes. The C1 had a binding stoichiometry of 8.3 and 6.1 beta-lg molecules complexed per LM- or HM-pectin molecule, respectively. The C2 had about 16.5 and 15.1 beta-lg molecules complexed per LM- and HM-pectin, respectively.     

Effect of different dextrose equivalent of maltodextrin on the interactions with anionic surfactant in an isothermal titration calorimetry study

Isothermal titration calorimetry (ITC) was used to study interactions between an anionic surfactant (sodium dodecyl sulfate, SDS) and maltodextrins with different dextrose equivalents (DE) in a buffer solution (pH 7.0, 10 mM NaCl, 20 mM Trizma, 30.0 degrees C). The interaction between SDS and maltodextrin was exothermic, which was attributed to incorporation of the hydrocarbon tail of the surfactant into a helical coil formed by the maltodextrin molecules. ITC measurements indicated that the number of SDS molecules bound per gram of maltodextrin increased with decreasing maltodextrin DE, i.e., increasing molecular weight. It was proposed that SDS only binds to maltodextrin molecules that have a DE greater than 10 glucose units.     

Binding between bixin and whey protein at pH 7.4 studied by spectroscopy and isothermal titration calorimetry

Bixin is the major coloring component of annatto used in manufacturing colored cheeses, but its presence in liquid whey causes undesirable quality of the recovered whey protein ingredients. The objective of this work was to study molecular binding between bixin and three major whey proteins (β-lactoglobulin, α-lactalbumin, and bovine serum albumin) at pH 7.4 using UV-vis absorption spectroscopy, fluorescence spectroscopy, isothermal titration calorimetry, and circular dichroism. These complementary techniques illustrated that the binding is a spontaneous complexation process mainly driven by hydrophobic interactions. The complexation is favored at a lower temperature and a higher ionic strength. At a lower temperature, the binding is entropy-driven, while it changes to an enthalpy-driven process at higher temperatures. The binding also increases the percentage of unordered secondary structures of proteins. Findings from this work can be used to develop whey protein recovery processes for minimizing residual annatto content in whey protein ingredients.     

Isothermal titration calorimetry study of pectin-ionic surfactant interactions

Isothermal titration calorimetry (ITC) was used to measure enthalpy changes resulting from injection of anionic (sodium dodecyl sulfate, SDS) or cationic (dodecyl trimethylammonium bromide, DTAB) surfactants into aqueous 1 wt % pectin solutions (30, 60, or 90% methoxylated). In the absence of pectin, the critical micelle concentrations (cmc) determined by ITC were 14.7 mM for DTAB and 7.7 mM for SDS. Binding of DTAB to pectin was endothermic and was attributed to electrostatic attraction between the cationic surfactant and anionic biopolymer. Binding of SDS to pectin was exothermic and was attributed to hydrophobic interactions. Pectin reduced the cmc of SDS, probably because of long-range electrostatic repulsion between the molecules. Above a particular concentration, which depended on pectin and surfactant type, both ionic surfactants promoted pectin aggregation (monitored by turbidity increase). This study demonstrates the potential of ITC for providing valuable information about interactions between polysaccharides and amphiphiles.     

Interactions between flavan-3-ols and poly(L-proline) studied by isothermal titration calorimetry: effect of the tannin structure

Interactions of proline-rich proteins (PRPs) with flavan-3-ols was studied using poly(L-proline) as a model protein by means of isothermal titration calorimetry (ITC). Several parameters were varied: (i) the galloylation and B-ring trihydroxylation of the flavan-3-ols (catechin, epicatechin, epicatechin gallate, and epigallocatechin gallate) and (ii) the degree of polymerization (monomers were compared to a mixture of oligomers with average degree of polymerization of 3.85). Large differences were observed between the flavan-3-ol monomers: no enthalpy change was measured when catechin and epicatechin were titrated by poly(L-proline), whereas thermodynamic parameters were determined in the case of galloylated monomers and mixture of oligomers. Stoichiometry ranged from 1 oligomer bound for each 12 proline units to 1 galloylated monomer for each 8 or 10 proline units. Association constants were in the range of 10(4)-10(5) M(-1), indicating a relatively high affinity of galloylated flavanols toward poly(L-proline), and the coexistence of both enthalpy- and entropy-driven phenomena was suggested. Finally, the binding of grape seed tannins to proteins was shown to be a cooperative process.     

Influence of agricultural practices on soil microbial activity measured by microcalorimetry

To investigate the influence of different agricultural practices and vegetations on soil microbial activity and diversity, soil samples from different habitats were studied with their microbial activities measured by the microcalorimetric technique. Seven soil samples were collected in Wuhan, China from different locations with primary and regeneration forest, nursery and crop land, and uncultivated land. The number of microorganisms in soils was measured by viable count, and some physicochemical parameters were determined. Power–time curves were recorded for soil samples supplemented with glucose and ammonium sulphate, and the total heat changes of the microbial growth reaction, Q_total (Jg⁻¹) and the microbial growth rate constant, k (min⁻¹) were calculated from the curves. All power–time curves presented typical changes of microbial activity. The curves of soil samples could be divided into two groups differing in agricultural practices and vegetations. The same grouping could also be reached according to the values of peak time (t_max). The most soil samples showed a higher correlation between the values of k and the counted bacterial numbers. The Q_total correlated well with vegetation abundance and probably with microbial diversity. In conclusion, microbial activity of the soil samples determined by microcalorimetry reflected differences in soil vegetation and agricultural management.     

Automatic determination of calcium and magnesium by coulometric titration

An automatic system based on the use of a PC-compatible computer was developed for the implementation of coulometric titrations by release of EDTA from its Hg complex on electrochemical reduction of the Hg²⁺ ion. A variety of indicators electrodes were assayed to obtain potentiometric titration curves. The method thus developed allows the straightforward, convenient determination of Ca and Mg and was applied to the analysis of various real samples with good results.     

硝酸汞滴定法测定复混肥料中氯离子的含量

采用GB/T 24890—2010中推荐的沸水提取样品前处理方法外,对比了超声提取的样品前处理方法,确定了硝酸汞滴定法测定复混肥料中氯离子含量的最佳试验条件,结果与硝酸银滴定法(摩尔法)的结果基本吻合,相比之下硝酸汞滴定法操作简单,终点稳定且容易判断,回收率为94. 5%～103. 3%,准确度高。此外,还首次采用艾斯卡-半熔法-硝酸汞滴定法测定了复混肥料中总氯,该方法和GB 18877—2009 《有机-无机复混肥料》中爱斯卡混合试剂(Na2CO3和MgO按1∶2研细混匀)-硝酸银滴定法测定氯含量的方法一致,特别适用于沸水提取和超声提取的提取液浑浊或带有深颜色的不易观察滴定终点的复混肥料和有机-无机复混肥料中氯的测定,具有可批量熔样、干扰少、结果准确的特点。     

通过微量量热法研究Cd2+在针铁矿上的吸附机制

A study on energy changes and mechanisms of Cd sorbed on goethite was performed using the technique of microcalorimetry.The data of the amount of Cd sorpion(Aq) and concentration of Cd in equilibrium solution(Ce),and the data of Aq and the heat effect(AH) caused by Cd^2 sorption on goethite all fitted Langmuir isotherm.The amount of heat released from Cd sorption on goethite increased with the amount of Ce or Aq.The reaction process of Cd sorption on goethite may be divided into five stages and three plateaus,depending on the variation of enthalpy change(ΔaHm) of Cd sorption with Aq,which implied three mechanisms of interaction between Cd and goethite.The experimental results showed that the microcalorimetry may be useful for determination of microcalorie variation in soil.     

自动电位滴定法测定土壤有机质含量

采用丘林滴定法与自动电位滴定法测定土壤中有机质含量,并对自动电位滴定法做了精密度和加标回收率试验。同时进行了用自动电位滴定法与手工滴定法标定硫酸亚铁标准溶液试验。结果表明,两种方法标定硫酸亚铁标准溶液结果一致,并无显著差别;丘林滴定法和自动电位滴定法均能满足土壤中有机质含量的分析要求,测定结果无显著性差异。自动电位滴定法显示了很好的准确度和重现性,加标回收率在98.43%～101.8%之间,相对标准偏差(RSD)为0.693%～0.936%,可见自动电位滴定法是一种测定土壤有机质含量的好方法。     

微量热法在土壤微生物研究中的应用进展

一切生命过程都伴随着热效应。微量热法可对这些热效应进行精确的跟踪,并表征其过程。微量热法已广泛应用于医药、生化等研究领域,但在土壤微生物研究中却鲜有应用。本文简要介绍微量热法的原理和实验方法,综述其在土壤微生物研究中的应用现状并展望其应用前景,以期为土壤微生物研究提供新的思路和技术。     

Fractional determination of functional groups of humic acids by conductometric titration

The conductometric replacement titration proposed in a previous report disclosed that the functional groups of humic acids are fractionated into three clusters of different dissociation constants, designated as VW, W and S clusters. In this paper, the method was examined to ascertain the relationship between the concentration of a sample and the amounts of fractionated clusters. The total amounts of fractionated clusters as well as the amount of the S cluster were directly proportional to the concentration of the sample, whereas the amount of the VW cluster decreased a little as concentration increased. The same method, applied to various types of humic acids in Japan, showed that the sums of the functional groups and the ratios of the S cluster to the sums likewise decreased in the order of A type > B and P types > Rp type. The total acidities determined by conductometry were nearly equal to the sums of carboxyl determined by Blom's method and phenolic hydroxyl determined by Folin's method.     

Analysis for total sulfite in foods by using rapid distillation followed by redox titration

A rapid and accurate analysis for total sulfite as sulfur dioxide has been developed for foods and food products. The method, which combines a selective distillation cleanup procedure with the selective redox titration of sulfite ion by iodine, has been applied to a variety of foods and food products over a period of time with no significant interference encountered in any matrixes other than garlic and leeks. For the foods analyzed, the method typically shows a detection limit of 10 ppm, a relative standard deviation of 7.5% (compared with 10.4% for similar matrixes by the Monier-Williams method), and recoveries of 97.9 +/- 6.4%. Comparison of results for this method with those obtained using the Monier-Williams method showed a mean value for the distillation/titration method of 241 ppm compared with 242 ppm for the Monier-Williams method. A correlation of 0.991 and odds of a difference between methods of 10.7% (Student's paired t-test (1-alpha) X 100) were obtained for those matrixes where no interferences were encountered with either method.     

Rapid and simple determination of oxyphenbutazone in dosage forms by biamperometric titration

Oxyphenbutazone was determined in the presence of glacial acetic acid, hydrochloric acid, and potassium bromide by bromometric titration with biamperometric end point indication. The course of the titration was followed with double platinum electrode with 100 mV of polarizing voltage.     

温度滴定法测定复合肥中钾离子浓度的研究

复合肥中钾元素的检测可有效控制复合肥产品的质量。本研究选取合适的前处理方法,并应用温度滴定法对复合肥中的钾离子进行了定量研究。该法测定复合肥中的钾离子以四苯硼钠与钾离子生成沉淀反应为基础,由于反应是放热反应,当滴定到等当点时,反应室的温度变化率最大,指示终点的到达。该方法具有操作简便、快速、灵敏度高等优点,非常适合对复合肥中钾离子含量的监控。研究结果表明,使用温度滴定法和离子色谱法及四苯硼酸钾重量法测得的复合肥中钾离子含量一致,该方法准确度高,重现性好,回收率可达97%以上。     

Soil microbial activity measured by microcalorimetry in response to long-term fertilization regimes and available phosphorous on heat evolution

Microcalorimetric technique was used to investigate the effects of balanced versus nutrient-deficiency fertilization on soil microbial activity in a long-term (19-year) fertilizer experiment. The number of microorganisms in soils was measured by viable cell count, and the power-time curves were recorded for soil samples supplemented with glucose and ammonium sulphate, also with or without sodium dihydrogen phosphate for P-deficiency fertilization. Both the bacterial and the fungal populations were significantly higher (P < 0.05) under balanced fertilization than under nutrient-deficiency fertilization. The microbial activity presented by heat dissipation per cell unit indicated that microorganisms under balanced fertilizer treatments had more efficient metabolism, while decreased microbial activity under nutrient-deficiency treatments was firstly due to soil available P, followed by N and K (P < 0.05). In addition, microbial growth in soils under P-deficiency fertilization was stimulated by adding available P, while the lower growth rate, less peak heat, and longer peak time all indicated the low activity of soil microorganisms. We emphasize the importance of balanced fertilization, as well as the role of available P, in maintaining and promoting soil microbial activity.     

The silicon availability in paddy soils as predicated by isothermal adsorption curve

Journal of Soils and Sediments - Silicon (Si) is important for rice growth and development. Hence, it is urgent and necessary to evaluate the silicon availability of paddy soils. At present, the...     

梯度恒温水热处理饲料的糊化时间温度特性研究

糊化作为淀粉热加工过程中的一种功能特性,在以淀粉源原料为主的饲料工业中应用广泛,而时间和温度是饲料调质等热加工中2个相互依存的重要工艺参数。为探究饲料糊化过程中对时间和温度的敏感性、掌握饲料糊化变性规律,该文以育肥猪配合饲料粉料为研究对象,基于饲料糊化的黏度特性,利用快速黏度分析仪(rapid visco analyzer,RVA)在25～95℃范围内测定了5、10、15℃/min3个升温速率对饲料糊化行为的影响,并在此基础上,采用自定义的RVA梯度恒温加热程序对饲料进行水热处理,分析饲料糊化的时间和温度依赖性,利用黏度差值Δμ及其导数分析得到饲料糊化的温度阈值。结果显示:饲料的糊化行为受升温速率影响(P0.05),当升温速率由5增加到15℃/min,峰值黏度由295增加到364 mPa·s,起始糊化温度由71.90增加到72.85℃;72、78和86℃3个温度阈值将饲料糊化过程中黏度的增长趋势划分为4个阶段;温度梯度范围为64～95℃、恒温保持时间分别为1、3、5和10min的梯度恒温加热程序,证实了饲料糊化的温度依赖性和时间依赖性,且在饲料糊化的不同阶段所表现出的温度和时间依赖性显著程度不同;过长的恒温或加热处理时间会降低饲料糊化过程中的黏度值,使饲料的糊化表现出剪切稀化现象;在育肥猪配合饲料调质工艺参数的设定中,调质温度选择高于起始糊化温度72℃为宜,且延时熟化保持能带来更好的调质效果。研究结果为饲料糊化过程的研究提供了一种新思路,也为配合饲料调质等热加工过程的工艺优化提供参考。