Protein charge reversal

The influence of system parameters such as protein charge, size, and concentration, ionic strength, and water content on the sizes of filled and unfilled reversed micelles has been investigated [170],... 

Wiktorowicz, J. E. and Colburn, J. C., Separation of cationic proteins via charge reversal in capillary electrophoresis, Electrophoresis, 11, 769, 1990. 

Ejfect of pH It is obvious that in order to recover the protein from reverse micelles, the pH of the stripping solution needs to change toward the pi, which will result in a reduction of the protein interaction with the oppositely charged head groups. The extent of protein recovery from reverse micelles increases with increasing pH for anionic surfactants however, for cationic surfactants the opposite is true. 

Rahaman and Hatton [152] developed a thermodynamic model for the prediction of the sizes of the protein filled and unfilled RMs as a function of system parameters such as ionic strength, protein charge, and size, Wq and protein concentration for both phase transfer and injection techniques. The important assumptions considered include (i) reverse micellar population is bidisperse, (ii) charge distribution is uniform, (iii) electrostatic interactions within a micelle and between a protein and micellar interface are represented by nonlinear Poisson-Boltzmann equation, (iv) the equilibrium micellar radii are assumed to be those that minimize the system free energy, and (v) water transferred between the two phases is too small to change chemical potential. 

The electrophoretic mobility of a protein solution may also be measured as a function of pH. By this technique it may also be observed that the colloid passes through a point of zero net charge at which its mobility is zero. The point at which charge reversal is observed electrophoretically is called the isoelectric point. 

Aqueous pH alters the protein charge property and affects the extraction efficiency. Haemoglobin (Mw 64,500, pi 6.8) is a difficult protein in terms of being able to completely extract it into reverse micelles. The representative anionic surfactant, di-2-ethylhexyl sulfosuccinate (AOT), cannot extract it, and gives rise to an interfacial precipitate. In contrast, we succeeded in the complete extraction of haemoglobin using synthetic anionic surfactants, dioleyl phosphoric acid (DOLPA), as seen in... 

Figure 4 Calculated electrostatic potentials around (a) wild-type cytochrome c peroxidase and three charge reversal mutants ((b) D34K, (c) D37K, and (d) D146K). The electric potentials are generated from a simple coulombic model and are shown In red contoured at -1.8eV. The protein backbone Is shown In green.

Cunico R L, Gruhn V, Kresin L, et al. (1991). Characterization of polyethylene glycol modified proteins using charge-reversed capillary electrophoresis. /. Chromatog. 559 467-477. 

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