Thermodynamics of water–biopolymer interactions: Irreversible sorption by two uniform sorbent phases

In order to understand the mutual interactions between water and a biopolymer, thermodynamic analysis of sorption isotherms of water vapor by the biopolymer is necessary. These isotherms are irreversible and show sorption hysteresis. The reasons for such behavior are not established. As a continuation of previous work, general relationships for thermodynamic quantities of sorption are derived for the general case when the sorbent consists of two uniform phases. As in the case of a single sorbent phase, the Clausius–Clapeyron equation can be used to obtain differential entropies of sorption. Two special cases for the two-phase situation—equilibrium hysteresis and partial equilibrium hysteresis—are plausible models for the irreversibility seen in water–biopolymer interactions. When differential entropy of sorption is plotted as a function of amount of water sorbed per mole of biopolymer, irregularities are generally seen. It is suggested that these irregularities reflect changes in conformation and/or dynamics of the biopolymer molecule.     

Thermodynamic functions of biopolymer hydration. I. Their determination by vapor pressure studies,discussed in an analysis of the primary hydration process

The primary hydration process of native biopolymers is analyzed in a brief review of the literature, pertaining to various aspects of biopolymer–water systems. Based on this analysis, a hydration model is proposed that implies that the solution conformation of native biopolymers is stable at and above a critical degree of hydration (h_p′ = 0.06–0.1 g H₂O/g polymer). This water content corresponds to the fraction of strongly bound water, and amounts to ～20% of the primary hydration sphere. In order to test this model, detailed sorption–desorption scanning experiments were performed on a globular protein (α-chymotrypsin). The results obtained are consistent with the proposed hydration model. They show that under certain experimental conditions, sorption isotherms can be obtained that do not exhibit hysteresis. These data represent equilibrium conditions and are thus accessible to thermodynamic treatment. Valid thermodynamic functions, pertinent to the interaction of water with biopolymers in their solution state, can be obtained from these sorption experiments.     

Thermodynamics of water–biopolymer interactions: Irreversible sorption by a single,uniform sorbent phase

Sorption isotherms of water vapor by solid biopolymers are necessary for the determination of thermodynamic quantities for water–biopolymer interactions. Such isotherms are generally irreversible, so equilibrium thermodynamics may not be applicable. General relationships are derived for thermodynamic quantities of sorption when the sorbent is a single uniform phase. In general, use of the Clausius–Clapeyron equation allows correct determination of differential entropies of sorption. However, calorimetric data are also necessary for the correct determination of other thermodynamic quantities. The single uniform phase model appears more useful than a domain model in explaining the hysteresis seen in water–biopolymer sorption isotherms.     

Hydration of stratum corneum

A model for the hydration behavior of human stratum corneum has been developed from measurements on in vitro samples isolated in a manner which minimized tissue treatment and trauma. Water sorption/desorption rate measurements as a function of water activity, temperature, and tissue integrity are reported. These data, together with thermodynamic data (infrared and nmr results reported earlier) are consistent with a model in which rapidly sorbed/desorbed water (ca. 0.5 mg water/mg stratum corneum) is associated with (“bound” by) the tissue, while slowly sorbed/desorbed “free” water (up to 12 mg water/mg stratum corneum) is kinetically restricted and probably intracellular in location. Both equilibrium water binding and desorption kinetic data suggest structural changes of this cellular water barrier upon hydration. Evidence for hysteresis in water sorption isotherms (reported by others) could not be reproduced.     

The hydration of proteins in nearly anhydrous organic solvent suspensions

Water sorption isotherms at 27°C have been measured for lysozyme and chymotrypsin in suspensions of toluene, di(n-butyl) ether, n-propanol, and a solution of 1M n-propanol in benzene. Sorption isotherms for the different suspensions are compared by converting solvent water content to the thermodynamic activity of water in each solvent. The sorption behavior is also compared to that for the two proteins hydrated from the vapor phase. At low water activities, all sorption isotherms are similar when compared on the basis of water activity. However, at higher activities, water sorption by the proteins in the organic suspensions is suppressed relative to the sorption of water vapor. The greatest suppression is observed for n -propanol, which suggests that the suppression may be due to a competition for water-binding sites on the protein by the organic solvent. Sorption isotherms at low water activities have also been predicted using a thermodynamic model in which it is assumed that water binds selectively to the ionizable residues on the surface of the protein. A comparison of predicted and measured sorption isotherms shows that the model can provide reasonable estimates of water sorption in nonpolar or moderately polar organic solvent suspensions at low levels of hydration. © 1993 John Wiley & Sons, Inc.     

Structural Relaxation During Drying and Rehydration of Food Materials��the Water Effect and the Origin of Hysteresis

The state of water in foodstuffs is a guiding principle in food design, and the equilibrium concept of water activity (Aw) is ubiquitous. It is regarded as a primary variable or “hurdle” in preservation technology, and a key variable influencing chemical reaction during storage. However, the amount of water in any system differs as function of water activity depending whether it is determined by water sorption or desorption. Even though this hysteresis behaviour has already been described in the literature, no physical interpretation of its origin has yet been proposed with respect to detailed molecular organisation. This work shows, for two different food powders, gluten and a milk-based product that the hysteresis disappears when either go through their glass transition. A more complete DSC analysis for gluten during different sorption/desorption cycles demonstrates that the hysteresis is dependent on the ageing of the material, which evolves in the glassy state and is induced by structural relaxation.     

A non-intrusive method for the measurement of water vapour sorption by bacterial spores

Single particle levitation (SPL) is used to measure the sorption and desorption of water vapour from microparticles comprising of Bacillus spores. Water gain is determined from increases in the weight-balancing levitation voltages. Spore water isotherms are compared to values previously reported using bulk gravimetric methods. Two salient differences are found between the SPL and bulk data: (1) greater osmotic swelling is observed in the SPL data; and (2) the SPL data exhibit open loop hysteresis while bulk data exhibit closed loop hysteresis.     

表面活性剂对链霉素在内蒙古牧区土壤中吸附解吸的影响

通过静态吸附实验,研究了3种不同类型的表面活性剂(阳离子:十六烷基三甲基溴化铵,CTAB;阴离子:十二烷基苯磺酸钠,SDBS;非离子,曲拉通100,TX-100)对链霉素(Streptomycin,STR)在内蒙古牧区土壤中吸附解吸的影响。结果表明,3种表面活性剂的存在均会增加STR在土壤中吸附的线性程度。CTAB的存在抑制了STR在土壤表面的吸附同时抑制了STR的解吸,然而SDBS的存在降低了STR在土壤中的吸附量并增加了吸附过程的可逆性。与CTAB和SDBS不同,TX-100对STR在土壤中的吸附的影响取决于其添加浓度。低浓度的TX-100对STR的吸附促进作用大于高浓度。随着TX-100浓度的增加,TX-100对STR的增溶作用加剧。促使吸附在土壤表面的STR迁移到水相中,从而抑制了STR的吸附,进而促进了STR的解吸。     

Interpretation of water desorption isotherms of lysozyme

The interrelationship between water desorption isotherms and the humidity dependences of the Young's modulus and volume strains of different lysozyme crystals and films has been analysed. The strain of protein samples is shown to be dependent on their elastic properties and to be induced by Laplace's pressure which can be calculated from the Thomson (Kelvin) relation. The energy of the mechanical deformation of protein samples expended for repacking and deformation of protein molecules considerably contributes to the integral change of free energy upon dehydration. Solid protein samples should be considered as deformable porous materials whose water sorption isotherms are determined not only by the number and properties of individual hydration centres but also by the macroscopic properties of protein solids.     

Deuterium NMR of water in immobilized protein systems.

Deuterium NMR spectra are reported for lysozyme crystals, powders, and frozen solutions. At high water contents the spectrum is a superposition of a narrow central component and a quadrupole doublet. The quadrupole splitting and the relaxation rates of both components, monitored as a function of water content and temperature, are discussed in terms of models for the water-protein interaction. The anisotropy of the water molecule motion is clearly demonstrated by the deuterium quadrupole splitting observed in the protein single crystal, but such splittings were not found in protein powders and frozen protein solutions. We therefore suggest that the most useful view of such data is to consider the water-protein interactions at the surface to be mixed rapidly and that a distribution of interactions be invoked rather than an oversimplified view often taken of a two or n-site mixing where n is small.     

Lysozyme kinetics in low water activity media. A possible hydration memory

The influence of water activity on initial lysozyme kinetics is studied. Enzyme catalytic capacity exponentially increases with thermodynamical water activity of the reactant medium; this relation seems independent of the chemical structure of the water activity depressors but dependent on the structuration of water molecules they induce. This influence of water organization on initial enzyme activity is immediate and may be preserved even after a large dilution, thus lysozyme presents a "hydration memory" phenomenon. This effect is in accordance with sorption/desorption isotherms, an hysteresis loop being observed.     

Effect of water content on the structural reorganization and elastic properties of biopolymer films: a comparative study

In this work, the effect of water uptake on the structural reorganization and elastic properties of three types of biopolymer films was studied. The water-biopolymer interaction for hydroxypropyl cellulose (HPC), gelatin, and cassava starch films prepared from aqueous solutions was studied and compared using Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), X-ray diffraction, dynamic vapor sorption (DVS), and dynamic mechanical thermal analysis with humidity generator and controller (DMTA) techniques. The FTIR spectral variations due to the water sorption were generalized into two-dimensional (2D) correlation graphs for each biopolymer, and the effect of water on the molecular conformation was compared. The water sorption isotherms were fitted with Guggenheim-Anderson-De Boer (GAB) and D'Arcy and Watt models. The water content in the mono- and multilayers predicted by both models for each biopolymer was discussed and compared. The correlation of the fitted data obtained from the sorption isotherms to the DMTA data allowed us to conclude that the elastic properties of the HPC films depended on the total water content in contrast to the elastic properties of the gelatin and cassava starch films, which decrease only with the appearance of multilayer water.     

Thermodynamic models for water-protein sorption hysteresis

Sorption isotherms of water by proteins show hysteresis that appears to be due to interactions at the molecular level. Four thermodynamically consistent models for this irreversible process are presented. Hysteresis could be the result of slow, incomplete conformational changes occurring upon addition and removal of water. Conformational hysteresis would occur if a number of different conformations, each corresponding to a local free energy minimum, could be present at each pressure of water vapor. Hysteresis might result from an incomplete process of intermolecular phase annealing. Finally, hysteresis might be due to incomplete phase change if two different protein phases are present. Experimental tests for these models are presented. Further study should lead to more insight into the effects of the presence of water on protein conformation and dynamics.     

Plant lipases: biocatalyst aqueous environment in relation to optimal catalytic activity in lipase-catalyzed synthesis reactions.

Adsorption and desorption isotherms of two commercial enzyme preparations of papain and bromelain were determined with a Dynamic Vapor System. The Guggenheim-Anderson-deBoer (GAB) modeling of the obtained sorption isotherms allowed the definition of different levels of hydration of those samples. Afterward, these enzyme preparations were used as biocatalysts in water and solvent-free esterification and alcoholysis reactions. The evolution of the obtained fatty acid ester level as a function of the initial hydration level of the biocatalyst, i.e., thermodynamic water activity (a(w)) and water content, was studied. The results show an important correlation between the initial hydration level of the biocatalyst and its catalytic activity during the lipase-catalyzed synthesis reactions. Thus, the Carica papaya lipase (crude papain preparation) catalytic activity is highly dependent on the biocatalyst hydration state. The optimized synthesis reaction yield is obtained when the a(w) value of the enzyme preparation is stabilized at 0.22, which corresponds to 2% water content. This optimal level of hydration occurs on the linear part of the biocatalyst's sorption isotherm, where the water molecules can form a mono- or multiple layer with the protein network. The synthesis reaction yield decreases when the a(w) of the preparation is higher than 0.22, because the excess water molecules modify the system equilibrium leading to the reverse and competitive reaction, i.e., hydrolysis. These results show also that an optimal storage condition for the highly hydrophilic crude papain preparation is a relative humidity strictly lower than 70% to avoid an irreversible structural transition leading to a useless biocatalyst. Concerning the bromelain preparation, no effect of the hydration level on the catalytic activity during esterification reactions was observed. This biocatalyst has too weak a catalytic activity which makes it difficult to observe any differences. Furthermore, the bromelain preparation is far more hydrophobic as it adsorbs only 18 g of water per 100 g of dry material at a(w) around 0.90. No deliquescence of this enzymatic preparation is observed at this a(w) value.     

Equilibrium and Kinetic Studies on the Removal of Basic Violet 10 from Aqueous Solutions Using Activated Carbons Prepared from Industrial Wastes

Low-cost adsorbents prepared from industrial wastes such as sugarcane bagasse (BC), rice husk (RC), and textile waste cloth (TC) are identified as suitable sorbents for removing basic violet 10 (BV). Fourier transform infrared (FTIR) and scanning electron microscopic (SEM) studies were carried out to characterize the prepared sorbents. The effects of dosage, time, and pH on dye removal were examined. It was observed that BV sorption takes place in acidic, neutral, and alkaline aqueous solutions. The work discussed the best-fit sorption isotherms among Freundlich and Langmuir, in addition to the reaction- and diffusion-based kinetic models. Based on the data obtained, it was found that the BV sorption took place in acidic, neutral, and alkaline aqueous solutions and sorption kinetics found to be controlled by pseudo-second-order and pore diffusion models. Also, thermodynamic parameters such as ΔG ^o, ΔH ^o, ΔS ^o, and E _a were calculated in order to understand the nature of the sorption process. The maximum dye removal capacity (DRC) was found to be 5608, 1244, and 27,495 mg/kg for BC, RC, and TC, respectively. Collectively, it can be concluded that the activated carbon sorbents, prepared from the named wastes, can used to adequately remove the basic violet dye from its aqueous waste solution.     

第一类鱼抗冻蛋白对冰晶生长界面层结构的影响

根据冰晶在水溶液中生长的基本热力学性质,应用多层界面模型,分别得到了冰晶在纯水及抗冻蛋白溶液中生长界面层的吉布斯自由能.由冰晶生长界面层的吉布斯自由能,分析了冰晶在三种不同第一类鱼抗冻蛋白分子溶液中,热平衡状态下生长界面层的微观平衡结构,发现冰晶在抗冻蛋白溶液中生长与其在纯水中生长相比,界面层结构有明显变化,结合抗冻蛋...     

Naproxen Adsorption-Desorption in a Sandy Aquifer Matrix: Characterisation of Hysteretic Behavior at Two Different Temperature Values

Adsorption/desorption processes (sorption isotherms) of Naproxen in a sandy aquifer matrix sediment were investigated using batch tests to compare Naproxen sorption behavior at 15°C and 25°C. Both temperatures are representative of the aquifer media and environmental conditions. Adsorption was well described by linear isotherms with low sorption affinity to aquifer material (K_d of 0.4 μg kg^?1) at both temperatures (15°C and 25°C). Desorption isotherm coefficients at 15°C and 25°C were 5.0 and 4.9, respectively. Naproxen hysteresis indices were between 9.98 and 10.8, indicating that a Naproxen fraction may be irreversibly fixed in the aquifer media, being higher at 25°C (10.88) compared to 15°C, showing a decreasing trend with increasing compound concentration at 15°C. The low sorption of Naproxen leads to potential leaching to groundwater if present in irrigation water, and its prevalence in an aquifer media when directly injected in wells for groundwater recharge.     

Adsorption of Five Model Organic Compounds on a Peat at Different Stages of Drying

Acridine orange (AO), dinitrobenzoic acid (DNB), bromocresol green (BCG), bromophenol blue (BPB), and methylene blue (MB) were chosen as model aromatic compounds of different polarity, charge, and solubility in water to examine the effects of solute properties on hydrophobic adsorption. These compounds show strict structural similarities to some herbicides and other potential xenobiotic pollutants and exhibit distinct absorption maxima in the visible region, which allows for their easy determination. A well-decomposed peat (medisaprist) at four different stages of drying was used to determine compound adsorption/desorption influences based on the degree of hydrophobicity and charge density of an organic surface. Adsorption and desorption isotherms were investigated using the batch equilibration method and determining the concentration of free chemicals by UV-Vis spectrophotometry. AO had a high tendency of adsorption and was strongly sorbed on peat samples that had been air-dried for 12 months. The lower Freundlich coefficient values found for MB when compared with AO at all the drying stages of the peat indicated that electrostatic attraction has a secondary contribution to sorption. On the contrary, the higher energy that must be spent to break solute-solvent interactions in the case of charged or polar molecules is one of the main factors in determining the position of the equilibrium. For a given solute, K_f values varied with the degree of hydrophobicity and the charge density of the surface, but again solute-solvent interactions appear to be much more important in the overall energy balance of hydrophobic pollutants than the electrostatic sorbate-sorbent interactions. A change in the solution pH does not improve the adsorption of the relatively polar DNB molecule, but sorption increases strongly for BCG and BPB when these molecules are in non-dissociated forms. The larger increase in BPB sorption observed on H⁺ saturated peat suggests that the degree of interaction increases with the suppression of the negative charge, but charge repulsion has a small effect in preventing adsorption of molecules bearing hydrophobic groups such as BCG. Desorption results differed depending on the chemical structure of the compound examined. For example, with AO there was no desorption from the more hydrophobic peat surfaces. A negative hysteresis was observed for DNB; the magnitude of hysteresis, evaluated using the ratio of Freundlich coefficients for adsorption and desorption, increased with the drying stage of the sorbent and was larger on oven-dried samples.     

NMR-based structural biology of proteins in supercooled water

NMR-based structural biology of proteins can be pursued efficiently in supercooled water at temperatures well below the freezing point of water. This enables one to study protein structure, dynamics, hydration and cold denaturation in an unperturbed aqueous solution at very low temperatures. Furthermore, such studies enable one to accurately measure thermodynamic parameters associated with protein cold denaturation. Presently available approaches to acquire NMR data for supercooled aqueous protein solutions are surveyed, new insights obtained from such studies are summarized, and future perspectives are discussed.     

Analysis of Water Sorption Isotherms of Superabsorbent Polymers by Solution Thermodynamics

Water sorption isotherms of superabsorbent polymers were measured, and their affinity for water was evaluated by solution thermodynamics. The results provide basic data for the functional packaging of food to control the water content of food during its transportation or storage. Water activity above 0.9 was measured by adding a specific amount of water to the samples, while that below 0.9 was measured with apparatus for evaluating water sorption isotherms. Thus, water sorption isotherms for superabsorbent polymers were obtained up to a water activity of approximately 0.98. The amount of water sorbed by the superabsorbent polymers was influenced by the type of functional groups in the polymers, and not by the degree of cross-linking in the polymers. The integral Gibbs free energy, which is the most suitable parameter for evaluating the affinity of a material for water, was evaluated from the water sorption isotherms by using solution thermodynamics.