Model partitioning systems, selection

Huang, S-W., Frankel, E.N., German, J.B. and Aeschbach, R. Partition of selected antioxidants in com oil-water model systems. /. Agric. Food Chem. 45, 1991-1994 (1997). 

Figures 7.31a-c clearly show that after some critical soy content in dodecane, Pe values decrease with increasing soy, for both sink and sinkless conditions. [This is not due to a neglect of membrane retention, as partly may be the case in Fig. 7.23 permeabilities here have been calculated with Eq. (7.21).] Section 7.6 discusses the Kubinyi bilinear model (Fig. 7.19d) in terms of a three-compartment system water, oil of moderate lipophilicity, and oil of high lipophilicity. Since lipo-some(phospholipid)-water partition coefficients (Chapter 5) are generally higher than alkane-water partition coefficients (Chapter 4) for drug-like molecules, soy lecithin may be assumed to be more lipophilic than dodecane. It appears that the increase in soy concentration in dodecane can be treated by the Kubinyi analysis. In the original analysis [23], two different lipid phases are selected at a fixed ratio (e.g., Fig. 7.20), and different molecules are picked over a range of lipophilicities.

This permeability barrier shows selectivity in that small hydrophobic molecules can partition into and diffuse across the lipid bilayer of the cell membrane, whereas small hydrophilic molecules can only diffuse between cells (i.e., through the intercellular junctions). In addition, the presence of uptake and efflux transporters complicates our ability to predict intestinal permeability based on physicochemical properties alone because transporters may increase or decrease absorptive flux. The complexity of the permeability process makes it difficult to elucidate permeability pathways in complex biological model systems such as animals and tissues. For this reason, cultured cells in general, and Caco-2 cells in particular, have been used extensively to investigate the role of specific permeability pathways in drug absorption. 

This microscopic interaction model can be used to explain more specific interactions between drug molecules and lipids. Such specific interactions could be a selective coupling between a drag molecule and a particular chain conformation of the lipid (kink excitation). This could have a dramatic effect on the fluctuation system. The drug molecule would then control the formation of interfaces between lipid domains and bulk phase in the neighborhood of the transition. First results on an extended model of this type [50] have confirmed this view and demonstrated that the partition coefficient can develop non-classical behavior by displaying a maximum near the transition. And such a maximum has in fact been observed experimentally... 

Phase compositions of VLLE in the systems glucose + acetone + water + carbon dioxide and carbohydrates + 2-propanol + water + carbon dioxide have been determined experimentally. Like for VLE of related systems from literature, the carbohydrate solubility in a phase rises when the phase becomes more similar to the water-rich lower liquid phase. At the same time separation of different carbohydrates becomes more difficult because selectivity decreases. Theoretically based models can help to find an optimum of capacity and selectivity and to minimize the number of necessary experiments. A simple model based on the Soave-Redlich-Kwong EOS which can reproduce glucose partitioning between the two liquid phases in VLLE in the glucose + acetone + water + carbon dioxide system is presented. 2-Propanol is shown to be a better modifier for these systems than acetone, but denaturation of carbohydrates in the carbohydrate + 2-propanol + water + carbon dioxide system limits industrial applications. 

Quantifying the influence of physicochemical form on metal uptake from solids is an important prerequisite to developing realistic models of metal dynamics in animals. In this paper -we statistically assess the effect of selected chemical characteristics of estuarine sediments on the concentration of Zn in bivalves which ingest those sediments while feeding. Our objective is to determine what proportion of the variation in Zn concentrations of deposit-feeding bivalves from natural systems may be explained by the physicochemical partitioning of Zn in the sedimentary food of the animals. 

The choice of a solvent model for the hydrophobic interaction. Obviously the selectivity of the biological system cannot be modeled by a solvent. However, solvents can be chosen to model the partial desolvation observed in protein-protein interactions (see above) because there is a qualitative analogy between water and non-polar solvents and the interaction of small molecules with biological systems. This can be expressed as a quantitative relation between partition coefficients and the binding to biological systems (receptors, proteins, membranes, etc.) It... 

A significant amount of work has demonstrated the feasibility and the interest of reversed micelles for the separation of proteins and for the enhancement or inhibition of specific reactions. The number of micellar systems presently available and studied in the presence of proteins is still limited. An effort should be made to increase the number of surfactants used as well as the set of proteins assayed and to characterize the molecular mechanism of solubilization and the microstructure of the laden organic phases in various systems, since they determine the efficiency and selectivity of the separation and are essential to understand the phenomena of bio-activity loss or preservation. As the features of extraction depend on many parameters, particular attention should be paid to controlling all of them in each phase. Simplified thermodynamic models begin to be developed for the representation of partition of simple ions and proteins between aqueous and micellar phases. Relevant experiments and more complete data sets on distribution of salts, cosurfactants, should promote further developments in modelling in relation with current investigations on electrolytes, polymers and proteins. This work could be connected with distribution studies achieved in related areas as microemulsions for oil recovery or supercritical extraction (74). In addition, the contribution of physico-chemical experiments should be taken into account to evaluate the size and structure of the micelles. 

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