Dissociation and aggregation

Often aggregation follows dissociation according to the Lumry and Eyring model  

Depending on the rate of the irreversible step, this model will lead to different situations. When the equilibrium between the native and the unfolded states is always established but the irreversible step is dominant, the following expressions hold [216]  

For reasonable values of 7iupp, 2 In/a can be treated as a constant the In C. ioi versus n trend is therefore a straight line with slope - / (P 1)7.  

Harwalkar and C.Y. Ma (Eds.), Thermal Analysis of Foods, Elsevier Appl. Sci. Pub ., Barking, England, 1990. 

Schiraldi and M. Riva, in L. Bonati, U. Cosentino, M.Lasani, G. Moro, D. Pitea, A. Schiraldi (Eds.), Trends in Ecological Physical Chemistry, Elsevier Publ., Amsterdam, 1993, 167. 

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Recently, a study was made of the electrophoretic as well as other physical and chemical properties of three types of soybean protein fractions (19). It was found that heating generally reduced the solubility of a suspension of these fractions and also increased their viscosity. These changes were attributed to subunit dissociation and aggregation. 

Since food systems usually contain a heterogeneous mixture of proteins and polysaccharides differing in chemical nature, conformation, chain rigidity, size, and shape of molecules, degrees of hydrolysis, denaturation, dissociation and aggregation, information on model systems containing two biopolymers is not sufficient to interpret phase behaviour and functionality of real dispersed food systems. 

Theoretical Effect of Swelling, Dissociation, and Aggregation on the Electrostatic Interaction Factor w at SS C... 

In the case of long chain surfactants used as collectors, a signiflcant correlation between their adsorption in the form of aggregates and flotation of minerals with them was established in the 1960 s by Somasundaran et al. (1964). Somasundaran et al. (1976, 1985) developed the relevant dissociation and aggregation equilibria for flotation reagents in the solution as well as at the mineral-solution interface. Importantly, the ion-molecule complexes phenomenon was proposed later by Somasundaran (1976), Hanna and Somasundaran (1976) to account for the flotation maximum exhibited at certain pH values by hydrolyzable surfactants. 

Figure 2 gives some characteristics of the size separation techniques that can be used to study the distribution of trace elements associated with various constituents of natural waters. It is obvious that the dimensions given in the figure are tentative as various factors influence the association/dissociation and aggregation/dispersion processes. However, preservation of real equilibria and labile species of elements, especially at concentrations of less than 10 g 1 prior to analysis is a much more serious problem encountered with methods that are not based on a direct physical separation. From this point of view, membrane filtration as well as some variants of field-flow fractionation (FFF) have advantages, although some uncertainties connected with equilibria shifts always exist. 

This three volume series contains equilibrium data on liquid-liquid distribution reactions. The equilibria of concern Include distribution, dissociation and aggregation of the extractant, reactions of the extractant with diluents and with other solvents, extraction of the water, equilibria of the extraction metal Ions, and extraction of metal ions with the extractant as the ligand. Volumes 1 and 2 cover organophosphorous extractions and alkylammonium salt extractants, respectively. Volume 3 deals with distribution reactions of carboxylic and sulfonic acid extractants, and the distribution of Inorganic acids, salts, and complexes between aqueous solutions and both Inert solvents and solvents which have oxygen donor atoms. Each reaction entry Includes the equilibrium constant, the temperature for which the data exists, the conditions, and a reference(s) to the source literature. 

A partially purified glucoamylase from A. niger has been characterized by biochemical, physicochemical, and optical methods. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate indicated that the enzyme consists of two principal components (mol. wts. 6.30 x 10 and 5.75 X 10 ). Small proportions of dissociated and aggregated species are also present, but the size of the monomer (3.0 x 10 ) was deduced from sedimentation studies, etc. Chemical modification of the enzyme indicated that tyrosyl residues are located at the active site. The tertiary structure of the molecule contains 15—25% of a-helix, as well as /9-structure and disordered segments. 

It seems that, at least in some animals, hemocyanin undergoes great seasonal modifications. Roche and Roche (1935) in fact showed that the average size of the hemocyanin molecule in Helix is different during the hibernation and estivation periods. Such naturally occurring dissociation and aggregation modifications are perhaps similar to those observed in vitro and may be related to the different calcium content of the hemolymph in the two periods. Should it be demonstrated that there are also seasonal modifications of the pH, a strict correlation between in vitro and in vivo phenomena would be provided. 

Reaction conditions can be modified to accelerate the rate of lithiation when necessary. Addition of tertiary amines, especially TMEDA, facilitates lithiation53 by coordination at the lithium and promoting dissociation of aggregated structures. Kinetic and spectroscopic evidence indicates that in the presence of TMEDA lithiation of methoxybenzene involves the solvated dimeric species (BuLi)2(TMEDA)2.54 The reaction shows an isotope effect for the o-hydrogcn, establishing that proton abstraction is rate determining.55 It is likely that there is a precomplexation between the methoxybenzene and organometallic dimer. 

Patapoff, T. W., Mrsny, R. J., and Lee, W. A., The application of size exclusion chromatography and computer simulation to study the thermodynamic and kinetic parameters for short-lived dissociable protein aggregates, Anal. Bio-chem., 212, 71, 1993. 

When attempting to use NMR to measure a dissociation constant, the basic experiment will be to vary the ligand concentration in the presence of a fixed concentration of protein. (The converse experiment, varying the protein concentration, may sometimes be carried out, but is generally less satisfactory because of problems with protein solubility and aggregation.) What one sees in this experiment will depend critically on the rate of... 

If the solute i is monodisperse—that is, if no dissociation or aggregation of the (macro)molecules occurs and each one has exactly the same molecular weight at every position in the cell—then M, is the same for all macromolecular species in the solution. If, furthermore, these solute molecules do not interact with each other—that is, if they behave ideally—the term d In yt/dcj = 0. Under these circumstances, In Ci varies hnearly with as shown in hne A of Figure 21.3. If the molecular weight of species i is unknown, it can be determined from the slope of line A, because Equation (21.33) becomes... 

The association of amphiphiles is strongly solvent-, temperature-, pH- and concentration-dependent. The studies of various aggregates formed by a cholesterol derivative 91 [31 ] illustrate this point (Figure 4.11). The dissociation and reassembling of tobacco mosaic virus into its constituent parts discussed in Section 5.2.1 provides an elegant example of a more complicated assembly... 

Conversely, the presence of macromolecular aggregates are not detected adequately by VPO. A suitable change of solvent or a temperature increase can lead to dissociation and a corresponding effect on Mn. Tables III and IV clearly show that this associative phenomenon is not relevant in the case of two typical samples. 

Another useful physical property of the crystal is its density, which can be used to determine several useful microscopic properties, including the protein molecular weight, the proteinlwater ratio in the crystal, and the number of protein molecules in each asymmetric unit (defined later). Molecular weights from crystal density are more accurate than those from electrophoresis or most other methods (except mass spectrometry) and are not affected by dissociation or aggregation of protein molecules. The proteinlwater ratio is used to clarify electron-density maps prior to interpretation (Chapter 7). If the unit cell is symmetric (Chapter 4), it can be subdivided into two or more equivalent parts called asymmetric units (the simplest unit cell contains, or in fact is, one asymmetric unit). For interpreting electron-density maps, it is helpful to know the number of protein molecules per asymmetric unit. 

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