Mixtures membrane permeabilities

Fig. 5-9. Total number of stages and total membrane surfaee area versus membrane seleetivity for the separation of 1 kg s of a raeemie mixture at a membrane permeability of 1.6 x 10 kg m. s, yielding both enantiomers at 95 % purity [55].

The thermodynamic aspect of osmotic pressure is to be sought in the expenditure of work required to separate solvent from solute. The separation may be carried out in other ways than by osmotic processes thus, if we have a solution of ether in benzene, we can separate the ether through a membrane permeable to it, or we may separate it by fractional distillation, or by freezing out benzene, or lastly by extracting the mixture with water. These different processes will involve the expenditure of work in different ways, but, provided the initial and final states are the same in each case, and all the processes are carried out isothermally and reversibly, the quantities of work are equal. This gives a number of relations between the different properties, such as vapour pressure and freezing-point, to which we now turn our attention. 

FIG. 14 A model for the uptake of weakly basic compounds into lipid bilayer membrane (inside acidic) in response to the pH difference. For compounds with appropriate pki values, a neutral outside pH results in a mixture of both the protonated form AH (membrane impermeable) and unprotonated form A (membrane permeable) of the compound. The unprotonated form diffuse across the membrane until the inside and outside concentrations are equal. Inside the membrane an acidic interior results in protonation of the neutral unprotonated form, thereby driving continued uptake of the compound. Depending on the quantity of the outside weak base and the buffering capacity of the inside compartment, essentially complete uptake can usually be accomplished. The ratio between inside and outside concentrations of the weakly basic compound at equilibrum should equal the residual pH gradient. 

As indicated in my report, we now know the rates of lateral diffusion of phospholipids in lipid bilayers in the fluid state, and in a few cases the rates of lateral diffusion of proteins in fluid lipids are also known. At the present time nothing is known about the rates of lateral diffusion of phospholipids in the crystalline, solid phases of the substances. As mentioned in my report, there are reasons to suspect that the rates of lateral diffusion of phospholipids in the solid solution crystalline phases of binary mixtures of phospholipids may be appreciable on the experimental time scale. Professor Ubbelohde may well be correct in pointing out the possibility of diffusion caused by defects. However, such defects, if present, apparently do not lead to significant loss of the membrane permeability barrier, except at domain boundaries. 

Equation (9.1) is the preferred method of describing membrane performance because it separates the two contributions to the membrane flux the membrane contribution, P /C and the driving force contribution, (pio — p,r). Normalizing membrane performance to a membrane permeability allows results obtained under different operating conditions to be compared with the effect of the operating condition removed. To calculate the membrane permeabilities using Equation (9.1), it is necessary to know the partial vapor pressure of the components on both sides of the membrane. The partial pressures on the permeate side of the membrane, p,e and pje, are easily obtained from the total permeate pressure and the permeate composition. However, the partial vapor pressures of components i and j in the feed liquid are less accessible. In the past, such data for common, simple mixtures would have to be found in published tables or calculated from an appropriate equation of state. Now, commercial computer process simulation programs calculate partial pressures automatically for even complex mixtures with reasonable reliability. This makes determination of the feed liquid partial pressures a trivial exercise. 

Modes of action of allelochemicals are diverse and have been described for isolated compounds, as well as for mixtures. They can affect various physiological processes, such as disruption of membrane permeability,22 ion uptake,28 inhibition of electron transport in photosynthesis and respiratory chain,1,10,33 alterations of some enzymatic activities, 1 and inhibition of cell division, 1 among others. 

Proteins and lipids interact cooperatively in the membrane. The type(s) and state(s) of lipids influence the mobility and conformation of the proteins in the membrane matrix. This, in turn, may well alter the properties of the membrane proteins. Similarly, proteins affect the phase behavior of the lipids and/or promote domain formation in membranes containing mixtures of lipids. Morphological alteration of the lipid architecture leads to changes in the membrane permeability. 

Consider two liquid mixtures of a and jS separated by a membrane permeable to the solvent of species 1 and impermeable to all other species in either mixture. The equilibrium condition for species 1 requires that the pressure of the solvent must be the same in either mixture. Therefore, the solute species in either mixture would not be in equilibrium. Also, there is no hydrostatic equilibrium established between the mixtures, and the difference of pressure is... 

The mydriatic effect of topically applied corticosteroids was investigated in living monkey eyes. Instillation of dexamethasone 0.1% (Decadron) produced pupillary dilation and ptosis as well as elevation of lOP. When the steroids were tested without their vehicles but in saline solntion, the effects on lOP, pupil size, and upper eyelid did not occnr.Thns it has been snggested that an excipient in the vehicle mixture causes the effects, possibly by altering cell membrane permeability to the steroid. 

As a further note, the membrane permeability to a component i is most likely determined experimentally using only pure component i. Whereas in the application to mixtures, the projection is made that the same value for the permeability can be used if the driving force is in terms of the partial pressure of the component i. 

Since the introduction of metal-ion affinity sorbents for the fractionation of proteins [1], the method became popular for the purification of a wide variety of biomolecules. Metal-ion affinity sorbents are also widely used for the immobilization of enzymes. At present, IMAC is a powerful method for separation of phosphorylated macromolecules, particularly proteins and peptides. The significance of techniques for separation and characterization of phosphorylated biomolecules is now increasing, because phosphorylation modulates enzyme activities and mediates cell membrane permeability, molecular transport, and secretion. Phosphorylated peptides can be separated from a peptide mixture on IDA-Sepharose with Fe " ions (Fig. 2). The majority of peptides pass freely through an IMAC column, whereas acidic peptides, including phosphorylated ones, are retained and can be released by a pH gradient. 

This section considers the Donnan equilibrium which is established by the equilibrium distribution of a simple electrolyte between an aqueous protein-electrolyte mixture and an aqueous solution of the same simple electrolyte, when the two phases are separated by a semipermeable membrane. A difference in osmotic pressure is estabhshed across the membrane permeable to all other species but proteins. This difference is measurable and provides important information about the protein-protein interaction in solution [37, 109-112, 116]. The principal goal of the theory is to explain how factors such as protein concentration, pH, protein aggregation, salt concentration and its composition, influence the osmotic pressure. At the moment this goal seems to be too ambitious these systems are often complicated mixtures of highly concentrated electrolytes and protein molecules, and the principal forces are not easy to identify [117]. 

In spite of the partial success in theoretical description, we believe that more realistic models are needed for the theory to have a predicting power. For example, measurements usually take place in the presence of a large excess of simple electrolyte. The electrolyte present is often a buffer, a rather complicated mixture (difficult to model perse) with several ionic species present in the system. Note that many effects in protein solutions are salt specific. Yet, most of the theories subsume all the effect of the electrolyte present into a single parameter, the Debye screening parameter n. In the case of the Donnan equilibrium we measure the subtle difference between the osmotic pressures across a membrane permeable to small ions and water but not to proteins. We believe that an accurate theoretical description of protein solutions can only be built based on the models which take into account hydration effects. 

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