Protein diffusion factors affecting

Rate of protein transfer to or from a reverse micellar phase and factors affecting the rate are important for the practical applications of RME for the extraction and purification of proteins/enzymes and for scale-up. The mechanism of protein exchange between two immiscible phases (Fig. 2) can be divided into three steps [36] the diffusion of protein from bulk aqueous solution to the interface, the formation of a protein-containing micelle at the interface, and the diffusion of a protein-containing micelle in to the organic phase. The reverse steps are applicable for back transfer with the coalescence of protein-filled RM with the interface to release the protein. The overall mass transfer rate during an extraction processes will depend on which of these steps is rate limiting. 

Table 3.9 shows diffusion coefficients of some p-hydroxybenzoate (paraben) preservatives and typical proteins in aqueous solution. Although the trend for a decrease of D with increase of molecular size (as predicted by equation 3.91) is clearly seen from the data of this table, it is also clear that other factors, such as branching of the p-hydroxybenzoate molecules (as with the isoalkyl derivatives) and the shape of the protein molecules, also affect the diffusion coefficients. The diffusion... 

Cyclic voltammetry of spinach plastocyanin portrays an interesting view of how these factors affect its ability to transfer electrons. It was observed [99] that the eonditions required to promote electrochemistry at a PGE electrode were broadly similar to those pertaining to the photosynthetic electron-transport system. For a solution of the protein (oxidized or reduced) at low ionic strength, well-defined diffusion-controlled voltammetric waves were observed upon addition of Mg " or by acidification to pH 4. The peak-current response as a function of these variables is shown in Fig. 12. At pH 7 (3 °C), E was found to be 375 mV, in good agreement with the potentiometric value reported by Katoh and co-workers [156]. The electrode reaction was found to be essentially reversible at pH 4. Whilst this appears at first to be in conflict with the evidence for plastocyanin being inactive at this pH, closer consideration shows that this is a consistent result. The corresponding electrode reaction may be written as in Eqs. (11)-(12)... 

Lipid-based systems can achieve sustained release by providing a barrier to diffusion of water, necessary for dissolution of solid protein dispersed in a hydrophobic matrix, and by a subsequent barrier to diffusion or convection of protein solution entrapped within the matrix. The rate-limiting step for aqueous solute release is often dissolution or disintegration of the lipid mass or particle. Factors affecting release of proteins from various matrices have been reviewed in detail by Pitt (1990a) and Park et a/.(1993). 

Diffusion describes the random motion that transports matter from one part of a system at high concentration to another at low concentration. Mathematically, this process relates the mass-transfer rate of a substance through unit area to the concentration gradient normal to the section by a proportionality constant, D (cmVsec), also referred to as the diffusion coefficient (Crank, 1975). Factors that affect protein diffusion in polymers include properties that alter polymer chain segmental mobility (degree of crystallinity, chain stiffness, degree of cross-linking), deformations that alter the free volume, and factors that can immobilize or denature the protein (Rabek,1980). 

Poorly absorbed compounds have been identified as those with a PSA>140Af Considering more compounds, considerable more scatter was found around the sigmoidal curve observed for a smaller set of compounds [74]. This is partly due to the fact that many compounds do not show simple passive diffusion only, but are affected by active carriers, efflux mechanisms involving P-glycoprotein (P-gp) and other transporter proteins, and gut wall metabohsm. These factors also con-... 

If staining is used to visualize the separated proteins, the proteins are usually first fixed by precipitating them in the gel with a chemical agent like acetic acid and methanol. This prevents diffusion of proteins out of the gel when submersed in the stain solution. The amount of dye taken up by the sample is affected by many factors, such as the type of protein and the degree of denaturation of the proteins by the fixing agents. 

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