Partitioning membrane-water

Avdeef, A., Box, K. J., Comer, J. E., Hibbert, C., Tam, K. Y. pH-metric logP 10. Determination of liposomal membrane-water partition coefficients of ionizable drugs. Pharm. Res. 1998,... 

The solubility-diffusion theory assumes that solute partitioning from water into and diffusion through the membrane lipid region resembles that which would occur within a homogeneous bulk solvent. Thus, the permeability coefficient, P, can be expressed as... 

Although the relationship in Fig. 5.6 is somewhat coarse, it is still useful in predictions. Since octanol-water log P prediction programs are omnipresent and adequately reliable, it can now be said that they can predict membrane-water partitioning, by using the equation in Fig. 5.6. Better yet, if one measures the value of loS Ka. one can estimate the membrane partition coefficient with the confidence of the variance expressed in Fig. 5.6. 

In the introductory discussion in Chapter 2, it was indicated that the effective permeability Pe linearly depends on the apparent membrane-water partition... 

Membrane uptake of nonionized solute is favored over that of ionized solute by the membrane/water partition coefficient (Kp). If Kp = 1 for a nonionized solute, membrane permeability should mirror the solute ionization curve (i.e., membrane permeability should be half the maximum value when mucosal pH equals solute pKa). When the Kp is high, membrane uptake of nonionized solute shifts the ionization equilibrium in the mucosal microclimate to replace nonionized solute removed by the membrane. As a result, solute membrane permeability (absorption rate) versus pH curves are shifted toward the right for weak acids and toward the left for weak bases (Fig. 7). 

Gobas, F.A.P.C., Lahittete, J.M., Garofalo, G., Shiu, W.Y., Mackay, D. (1988) A novel method for measuring membrane-water partition coefficients of hydrophobic organic chemicals Comparison with 1-octanol-water partitioning. J. Pharm. Sci. 77, 265-272. 

Understanding and estimating membrane/water partition coefficients Approaches to derive quantitative structure property relationships, Chem. Res. Toxicol. 11 847 (1998). 

Silver occurs naturally in several oxidation states, the most common being elemental silver (Ag°) and the monovalent ion (Ag+). Soluble silver salts are, in general, more toxic than insoluble salts. In natural waters, the soluble monovalent species is the form of environmental concern. Sorption is the dominant process that controls silver partitioning in water and its movements in soils and sediments. As discussed later, silver enters the animal body through inhalation, ingestion, mucous membranes, and broken skin. The interspecies differences in the ability of animals to accumulate, retain, and eliminate silver are large. Almost all of the total silver intake is usually... 

Derivation of Membrane-Water Partition Coefficients of a Charged or Neutral Compound or Species. 223... 

Since the membrane water partition coefficient is a relevant parameter for the pharmacokinetic behaviour, i.e. drug uptake, many more studies on the speciation of drugs at membrane interphases can be found in the... 

Principal differences between bulk media water and membrane water partition coefficients are listed in Table 2. These differences are essentially based on the several orders of magnitude difference in surface-to-volume ratio. In the liposomal system, charges built up due to sorption of charged species can be electrically neutralised by counter-ions from the electrolyte (diffuse double... 

Often, the concentration of compound or species i in the membrane is given in molality (mol kg ), which yields the membrane water partition coefficient, w (in units of dm kg 1) ... 

Table 3. Examples of membrane-water partitioning of HIOCs and the difference in partitioning between neutral and charged species...

The major route for bioaccumulation of hydrophobic organic compounds in aquatic animals is passive diffusion over cell membranes. In fish, the gill epithelia are the predominant port of entry, with less than 40% of uptake across the skin [181]. Since permeability of the membrane is a direct function of the membrane-water partition coefficient and the diffusion coefficient across the membrane interior [182], the bioconcentration factor (logBCF) can be directly correlated with log K0Vl. or log Km%v for compounds with intermediate hydro-phobicity [183,184],... 

K Dimensionless membrane-water partition coefficient of species or compound i ... 

Estimation is easier and less time-consuming because use is made of empirical relationships between the BCF and physicochemical properties of the compound, such as water solubility (S) [42-48], Km, (solid organic carbon/water partition coefficient) [48], Kmw (membrane water partition coefficient), iipw (liposome water partition coefficient) [49], critical micelle concentration (CMC) [45], steric factors, molecular weight [47,48], and others. The most common regression method is the estimation of BCF from the octanol-water partition coefficient (Kovl) [18,42,44-48,50,51],... 

By using a PES with a different thickness, one can conveniently change the AV ratio. This approach permits some control over the time required to reach equilibrium concentrations. Bartkow et al. (2004) has reported an excellent example of the impact of ratio or thickness on the time to equilibrium. These investigators showed that a 200 pm thick PE sheet took twice as long to reach equilibrium in air as a 100 pm thick PE sheet. In theory, changing membrane thickness will not affect polymer diffusivity and equilibrium membrane-water partition coefficients (I mwS) or solubility coefficients ( p). However, in practice different values of (membrane-air partition coefficient) and membrane... 

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