Dextran systems

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 use of absorption optics with the ultracentrifuge has allowed us to monitor the rapid transport of PVP in the standard PVP/dextran system as a function of g. It was demonstrated that while the rate of the PVP transport increases with increasing g acting on the system, the rate is rather insensitive to the magnitude of the gravitational force. We found 51) that the linear time rate of the transport varies as g°19. Note, however, that although rapid PVP transport has been found at various values of g, we cannot be sure whether structured flows exist. 

A comparison of the data obtained by using McDougal and Turner s treatment of the prediction of structured flow formation in the PVP-dextran system with the ternary diffusion data in Fig. 16 indicates that their theory is not compatible with our observations 36). 

Effects of Dextrans on Electrostatic Repulsion. In the absence of externally applied forces, the net aggregation energy (Ea) in a rouleau is equal to Eb minus electrostatic repulsive energy (Ee). The aggregation behavior of each cell-dextran system reflects the Ea. Since electrostatic... 

Most recently, an in vivo investigation of a NIR Con A RET system was reported and the host response characterized.114 In this work, Cy-7-labeled Con A and Alexa Flour 647-labeled dextran system were entrapped with a hollow microdialysis fiber immobilized on the tip of an optical fiber. The device was characterized in vitro, the result of which indicated a response range of 36 150 mg/dL. More important, the device was implanted subcutaneously and evaluated in vivo for 16 days. Results were promising, as the implant readout retained a high degree of correlation with blood glucose fluctuations (as measured by blood-draw methods). An increase in response time was observed at the end of the experimental period, with fibrous encapsulation cited as the cause. 

Figure 7.7. Effect of molecular weight on glass transition temperature of dextrans at various water activities (Reprinted from Carbohyd. Polym., 62, Icoz, Moraru, and Kokini, Polymer-polymer interactions in dextran systems using thermal analysis, pp. 120-129, Copyright (2005), with permission from Elsevier.)...

Icoz, D.Z., Moraru, C.I., and Kokini, J.L. (2005). Polymer-polymer interaetions in dextran systems using thermal analysis. Carbohyd. Polym. 62(2), 120-129. 

Some general rules apply to proteins in PEG/dextran systems and are summarized as follows ... 

The method, however, fails to retain viscous, low interfacial tension polymer phase systems such as polyethylene glycol (PEG)-dextran systems due to its intensive mixing effect which tends to produce emulsification, resulting in carryover of the stationary phase. This problem is largely eliminated by the crossaxis CPC described below. 

The polymer-phase system composed of PEG and potassium phosphate has a relatively large difference in density between the two phases, so that it can be retained well in both XL and XLL column positions which provide efficient mixing of the two phases. On the other hand, the viscous PEG-dextran system, with an extremely low interfadal tension and a small density difference between the two phases, has a high tendency of emulsification under vigorous mixing. Therefore, the use of either the XLLL or L column position, which produces less violent mixing and an enhanced lateral force field, is required to achieve satisfactory phase retention of the PEG-dextran system. 

Classical LLPC using aqueous-aqueous polymer systems based on Albertsson s [9] PEG-dextran system has provided a versatile tool for the separation of proteins and nucleic acids, thus increasing the arsenal of biopolymer purification methods currently dominated by gel filtration, ion-exchange chromatography, and affinity chromatography RPC. The technique operates... 

Liquid-liquid partition chromatography techniques based on aqueous-aqueous systems have successfully been employed in the fractionation of crude human serum, purification of steroid hormone-binding proteins from human serum, isolation of basic proteins from crude bacterial extracts, purification of immunoglobulins and monoclonal antibodies, DNA fractionations by size, topology and base sequence, as well as the isolation of soluble and ribosomal RNAs in preparative amounts from bulky mixtures [10]. Highspeed CCC using PEG-dextran system has also been employed in the separation of proteins [6]. 

Muller, W. New phase supports for liquid-liquid partition chromatography of biopolymers in aqueous poly(ethyleneglycol)-dextran systems. Synthesis and application for the fractionation of DNA restriction fragments, Eur. J. Bio-chem., 1986, 155, 213. 

The semicontinuous production of -amylase and cellulase has been studied in PEG-Dextran systems using Bacillus subtilis and Tricoderma reesel, respectively (21, 22). Some improvements in the yields have been observed in both the cases. In case of cellulase production, the economics of the process could be improved by using a cheap, lignocellulosic waste as the substrate, and replacing the fractionated dextran with a crude one. 

In an unmodified PEG/dextran system, cells and cellular components partition to the dextran-rich bottom phase (2), leaving the top phase available for product. a-amylase has been partitioned to the top phase and Bacillus subtllis cells to the bottom phase (22). Production of biologically active materials may be enhanced by removal of product from cells or cellular components for two reasons 1 ) Degradation of product by extra- or intracellular enzymes still present in the broth is prevented. 2) Removal of product reduces negative feedback inhibition of growth or production of cells. The method is unique in speed and ease for handling bulk quantities which is critical for sensitive systems. 

Various forms of DNA behave differently. In PEG/dextran systems, native DNA exhibits higher values of fC than denatured DNA. Both materials exhibit higher K values as the pH is increased (i.e. (H2P04)- is shifted to (P04)3-) (J6). 

Quite interestingly and importantly, the synergism between Ce(IV) and Pr(III) described above (sect. 2.4) is also operating in these homogeneous solutions, and the catalytic activity of homogeneous Ce(lV)/dextran system was synergistically promoted by Pr(III). The activity for DNA hydrolysis shows a maximum at the [Ce(IV)]/[Pr(III)] ratio of 2. Other lanthanide ions also synergetically promoted the DNA hydrolysis by the Ce(TV)/dextran system. 

L (central position). The off-center position is used for both organic/aqu-eous and aqueous PEG—potassium phosphate systems, while the central position is used for viscous, low interfacial tension PEG-dextran systems. 

The different types of cross-axis coil planet centrifuges (cross-axis CPCs) were tested for the retention of the stationary phase of aqueous-aqueous pol3fmer-phase systems such as poly(ethylene glycol) (PEG) l(X)0-potassium phosphate and PEG SOOCf iextran T500. The XL and XLL CPC are suitable for the PEG 1000-potassium phosphate, which has a relatively large difference in density between the two phases. On the other hand, the XLLL and L CPC are required to acheive satisfactory phase retention phase retention of the PEG-dextran system. These cross-axis CPCs were utilized for the separation of several proteins. 

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