PEG-dextran-water system

The most popular aqueous two-phase systems in use today are the PEG-dextran-water system and the PEG-potassium phosphate-water system.4 Both PEG and dextran are fully water soluble, yet the two polymers are incompatible and separate into two aqueous phases in certain concentration ranges. Table 7 is a list of various aqueous two-phase partitioning systems compiled recently by Zaslavsky.8... 

Albertsson48 found that the molecular weight of the polymers strongly affects the partitioning of a protein in PEG-dextran-water systems. If it is desirable... 

The time needed for phase separation depends on polymer type and concentration, the volume ratio, the rate of coalescence of droplets, and the presence of particles. Typical times required for PEG-dextran-water systems are 5 to 30 min. Low-speed centrifugation cuts the time down to 1 min.65 In... 

The system polyethylene glycol (PEG)-dextran-water is still the most used and best-studied aqueous polymer two-phase system. A phase diagram for a typical two-phase system is shown in Fig. 10.12 for the PEG-dextran system. Both polymers are separately miscible with water in all proportions. As the polymer concentration increases, phase separation occurs, with the... 

The PEG/water, PPG/water, and PVAL/water are among the most extensively studied water-soluble polymer solutions. These systems typically show a closed-loop phase behavior (Figure 16.4). Results for some ternary systems have been reported many of these data are for PEG/Dextran/water and PEG/water/salts and related systems, which are important for separating biomolecules such as proteins. Only few data for PEG or other hydrogen bonding polymer with mixed water solvents have been reported. 

The basis for the separation is that when two polymers, or a polymer and certain salts, are mixed together in water, they are incompatible, leading to the formation of two immiscible but predominantly aqueous phases, each rich in only one of the two components [Albertsson, op. cit. Kula, in Cooney and Humphrey (eds.), op. cit., pp. 451 71]. A phase diagram for a polyethylene glycol (PEG)-Dextran, two-phase system is shown in Fig. 22-85. Proteins are known to distribute unevenly between these phases. This uneven distribution can be used for the selective concentration and partial purification of the products. Partitioning between the two phases is controlled by the polymer molecular weight and concentration, protein net charge and... 

FIGURE 16 Effect of PEG concentration difference in the two phases on partition coefficient K in PEG 6000-dextran 70-water system containing 0.15 mol /kg NaCI in 0.01 mol /kg sodium phosphate buffer at pH 7.4 (I) myoglobin, (2) cytochrome c, (3) human serum albumin. (Reprinted from Zaslavsky8 by courtesy of Marcel Dekker, Inc.)... 

The polymers generally employed in forming aqueous two-phase systems are quite hydrophilic as compared with organic solvents there is, however, a marked difference in hydrophobicity of the phase polymers. Thus, by extraction into the more hydrophobic phase it may be possible to design bioconversion in aqueous two-phase systems of substances of low water solubility. To this aim, we examined the transformation of hydrocortisone to prednisolone by Arthrobacter simplex cells in 25% PEG 8000 - 6% Dextran T40 system (18). Our studies showed that while the cells preferred the bottom phase, the Kpart of the steroid was in favour of the PEG rich upper phase (Kpart 3.0 andW.O for hydrocortisone and prednisolone respectively). The reaction rate was found to be comparable with those of systems in which the organic solvent had been included, which could be due to the efficient mass transfer between the two phases. Due to the high top to bottom phase volume ratio (8.5 1) a recovery of 98-99% could be obtained in the top phase. 

Upon addition of certain polymers such as polyethylene glycol (PEG) and dextran or salt to water, a phase boundary forms even though the system consists of only one solvent, water. When a mixture of biomolecules such as a fermentation broth or a solution of lysed cells is added to such a system, each type of biomolecule partitions uniquely between the two phases, achieving separation (Kula, 1979,1990 ... 

A number of water-soluble polymers will cause phase separation when present together at concentrations of a few percent. The most widely used polymers are polyethylene glycol (PEG) and dextran. Proteins, other macromolecules, and cell components such as mitochondria distribute in the phases or collect at the interface. Proteins are destabilized at organic solvent/water interfaces, but when each solvent is water, the interfacial tension is negligible. Some salts such as potassium phosphate will also induce phase separation when a polymer is present, but the salt concentration must be high. Two-phase aqueous systems provide a mild method for purification of proteins, and scale-up to large volumes presents no engineering problems. The polymers... 

For the extraction of proteins, aqueous two-phase systems (ATPS) are preferred over organic solvents, which usually denature the proteins and render them biologically inactive. They consist of polyethylene glycol (PEG), and a salt (e.g., potassium phosphate) or dextran in water. At concentrations above a critical value, the mixture separates into two phases—one rich in PEG and the other in dextran or salt. In industrial systems, salts are more commonly used because they are relatively inexpensive as compared to dextran. The MW, charge and surface properties of the protein decide how the protein partitions in the system. The nature of the phase components, the MW of the polymer, and the concentration and type of salt used also affect the distribution. ... 

Kakisaka, K. et al.. Partition coefficients of amino acids, peptides, and enzymes in Dextran/PEG/water aqueous two phase systems, J. Chem. Eng. Jpn., 31, 991, 1998. 

The well documented phenomenon of separation of an aqueous solution of two different water-soluble polymers into individual phases, during recent years, has shown widespread potential in biotechnology (1). A number of polymers have been employed for the preparation of these bi-phasic systems. (2). The n st commonly used systems have been those of poly(ethylene glycol) (PEG) and dextran. The molecular weight of the polymers used plays an important role in determining the characteristics of the phase system. Phase systems formed by mixing one polymer and a high concentration of certain salts in aqueous solutions, have also been reported. (2). 

Production of bulk chemicals. The production of solvents is normally characterized by a general inhibition phenomenon which has been mainly attributed to the changes in membrane permeability, or to the toxic effects on the metabolic pathway. Aqueous two-phase systems have been shown to be effective as media for the extractive fermentation of a number of solvents which include ethanol, acetone-butanol and acetic acid (3). Improved productivity has been achieved in most of the cases as compared to the conventional fermentations, which is significantly due to the elimination of product inhibition. However, there is an indication that changes in the microenvironment of the microbial cells due to the presence of non-metabolizable polymers could also contribute, in the initial phases, to the increased production. The addition of PEG and dextran to a growth medium, for instance, was shown to give increased initial ethanol yields, as a result of decrease in the chemical potential of water (8). 

Consider an aqueous solution of dextran (dex) and polyethylene glycol (PEG). One important rule about polymer solutions is that two polymers arc rarely compatible with one another, and phase separation into dex-iich and PEG-rich phases take place. (This is the reason why it is difficult to develop solid polymer blends.) Using the gradient theory of Section 5 and properties of polymers, Viij (1968) showed that the interfacial traision in such phase separated systems is vray low (s 1 mN/m), irrespective of the common solvent content In the water-dex-... 

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