Phase equilibrium and protein partitioning in aqueous mixtures of maltodextrin with polypropylene glycol
Introduction
Aqueous two-phase systems formed by mixtures of two polymers, or one polymer and inorganic salts, are important for separation and purification of enzymes, proteins, nucleic acids, and other substances in biological processes (Albertsson, 1986, Vernau and Kula, 1990). This technology offers the advantages of high capacity, high activity yields and is easy to scale up. For large-scale processes, methods for recycling chemicals have been developed (Greve and Kula, 1991, Hustedt, 1986). Many phase diagrams for polyethylene glycol (PEG)/salt and PEG/dextran aqueous two-phase systems have been reported (Snyder et al., 1992, Zaslavsky, 1995), while liquid–liquid equilibrium data on two-phase systems containing polypropylene glycol (PPG) are scarce, thus limiting the potential application of this specific system to biotechnology.
The most common polymer/polymer system is composed of dextran and PEG, but this system is very expensive for scaling up. This problem can be solved by the use of alternative polymers (Atkinson and Johns, 1994, Christian et al., 1998, Szlag et al., 1990). Maltodextrin (MD) is a low-cost starch derivative that can be used as replacement for dextran in aqueous two-phase systems. Moreover, PPG is a polymer that is structurally closely related to PEG. PPGs of low molecular weights are soluble in water, while high molecular weight ones are only partially soluble (Molyneux, 1983).
Many authors have described the liquid–liquid equilibrium in aqueous two-phase systems (Kang and Sandler, 1987, Wu et al., 1996, Wu et al., 1998, Wu et al., 1999) utilising a thermodynamic model. When equilibrium data are not available these models are utilised to provide the basis for extrapolating experimental data and predicting phase compositions. Furthermore, phase diagram data are necessary for the development of models that can predict phase separation.
In the present work, we report liquid–liquid equilibrium data for aqueous mixtures of MD (1000 and 2000) and PPG (400 and 3500) and the partition coefficients of the bovine serum albumin (BSA), α-lactoalbumin (α-La) and β-lactoglobulin (β-Lg) in PPG 400/MD at 25°C. The non random two-liquid (NRTL) model was used to correlate the equilibrium data for PPG/MD systems.
Section snippets
Materials
PPG samples, with molecular weights of 400 and 3500, were purchased from Aldrich. MD 1000 and 2000 were supplied as courtesy by Companhia Lorenz (Blumenau, SC, Brazil). MD is a commercial polymer and therefore can be highly polydisperse. For this reason, this polymer was analysed by gel permeation chromatography (GPC) in a Waters chromatograph (USA). The polydispersity indexes (Mw/Mn) of MDs 1000 and 2000 were found to be 1.22 and 1.74, respectively. The water content of each polymer was
Liquid–liquid equilibrium
The experimental liquid–liquid equilibrium results for the aqueous two-phase systems PPG 400/MD and PPG 3500/MD are given in Table 1, Table 2. All the results are expressed as the weight percentage. For each polymer combination, four tie lines were determined.
For most systems the MD concentration in top phase is very small, and in some cases MD is almost excluded from this phase. For the system PPG 3500/MD this trend is even more pronounced (Fig. 1). Similar results were reported by
Modelling
The NRTL equation was utilised for modelling the PPG/MD aqueous two-phase systems. Due to the large difference in molecular weights between the components in the systems, the weight fraction was used as unit of concentration. This procedure was suggested by Oishi and Prausnitz (1978) for the UNIQUAC and UNIFAC models.
Eq. (2) shows the NRTL model expressed in weight fraction:where
Conclusions
Equilibrium analysis of phase behaviour for MD/PPG/water systems were conducted at 25°C for different concentrations and molecular weights of both polymers. These systems were obtained by a combination of PPGs with molecular weights 400 and 3500 and MD with molecular weights 1000 and 2000. For these systems the MD concentration in top phase is very small, and in some cases MD is almost excluded from this phase. This effect is more pronounced in PPG 3500/MD systems. In all systems the PPG
Acknowledgements
This work was supported financially by research grants from Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP — Proc. 1997/10630-7), CNPq (Proc. 521011/95-7) and CAPES/PICDT.
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