Immobilized water membrane

Immobilized artificial membranes (IAM) are another means of measuring lipophilic characteristics of drug candidates and other chemicals [90-94], IAM columns may better mimic membrane interactions than the isotropic octanol/water or other solvent/solvent partitioning system. These chromatographic indices appear to be a significant predictor of passive absorption through the rat intestine [95]. 

Equation 3.56 indicates that the biofilm essentially behaves like an immobilized water layer, with a resistance that is independent of the biofilm-water partition coefficient. Evidently, when the growth rate of the biofilm and the diffusion rate of the contaminants are of similar magnitude, this highly idealized model breaks down, and it can be expected in those cases that highly hydrophobic compounds will have more difficulty in reaching the membrane than less hydrophobic (more mobile) compounds. Also, Eq. 3.56 will likely fail to predict solute transport in biofilms with sizable populations of invertebrates because of bioturbation. 

Due to their better biomimetic properties, phospholipids have been proposed as an alternative to 1-octanol for lipophiiicity studies. The use of immobilized artificial membranes (lAM) in lipophiiicity determination was recently reviewed and we thus only briefly summarize the main conclusions [108]. lAM phases are silica-based columns with phospholipids bounded covalently. lAM are based on phosphatidylcholine (PC) linked to a silica propylamine surface. Most lipophiiicity studies with lAM were carried out using an aqueous mobile phase with pH values from 7.0 to 7.4 (log D measurements). Therefore, tested compounds were neutral, totally or partially ionized in these conditions. It was shown that the lipophiiicity parameters obtained on I AM stationary phases and the partition coefficients in 1-octanol/water system were governed by different balance of intermolecular interactions [109]. Therefore the relationships between log kiAM and log Poet varied with the class of compounds studied [110]. However, it was shown that, for neutral compounds with log Poet > 1, a correspondence existed between the two parameters when double-chain lAM phases (i.e., lAM.PC.MG and IAM.PC.DD2) were used [111]. In contrast, in the case of ionized compounds, retention on lAM columns and partitioning in 1 -octanol / water system were significantly different due to ionic interactions expressed in lAM retention but not in 1-octanol/water system and due to acidic and basic compounds behaving differently in these two systems. 

Valko, K., Nunhuck, S., Bevan, C., Abraham, M.H. and Reynolds, D.P. (2003) Fast gradient HPLC method to determine compounds binding to human serum albumin Relationships with octanol/water and immobilized artificial membrane lipophilicity. Journal of Pharmaceutical Sciences, 92, 2236-2248. 

The use of partition coefficients between water and lipophilic media is of wide use in pharmaceutical research. As discussed in the last chapters, different lipophilicity scales are used to describe the lipophilicity of a compound and relate it to its absorption behaviour in vivo. Differences between the logPow and partitioning between phospholipids and water (mainly determined using liposomes) for diverse compounds have been described leading to the development of the immobilized artificial membrane chromatography system. However, also the predictivity of the IAM system is limited and only a small number of membrane systems are available. 

Immobilized Artificial Membrane (LAM) packings for HPLC provide a different environment from that of the hydrocarbon-based ODS columns [17,30—32]. In this model, IAMs are purified phospholipids that are covalently bonded to the silicon support. At this time, only IAM columns prepared from phosphatidylcholine are commercially available (Regis Technologies, Inc., Morton Grove, IL). The hypothesis is that the bonded phospholipid layer is akin to the cellular monolayer that represents a barrier to transport. Interaction of the solute with the phospholipid results in a capacity factor ( J that is proportional to the membrane partition coefficient [PCm in Eq. (1)]. In this sense, the IAM approach does not attempt to correlate with literature values of oil water partition coefficients, but seeks to establish a unique membrane interaction parameter. 

A drug is absorbed through diffusion across a series of separate barriers where the single layer of epithelial cells is the most significant barrier to absorption. Many in vitro methods have been developed for the study of this phenomenon. These methods include small animal gut studies, cell culture (i.e., Caco-2 cell culture model), octanol-water partition coefficients, measures of hydrogen bonding and desolvation energies, immobilized artificial membranes, and retention time on reversed-phase HPLC columns. 

The solvated proton assumes two basic structures in water H30 aq and H5 02+aq vvhich have almost the same potential [41, 42). The diffusion of the proton in water is a sequence of transitions between these two states of the solvated proton, where the initiation of the transition is made by the random motion of water molecules in the second solvation shell of the proton [41, 43]. Naturally, the immobilization of the water molecules, which are in contact with the membrane or the protein s surface, will reduce their rate of orientation, leading to diminished diffusivity of the proton. Indeed, measurements of proton diffusion in immobilized water, ice, yielded a diffusion coefficient that is - 30% of the value in water at the same temperature [44]. 

Membrane separations involve the selective solubility in a thin polymeric membrane of a component in a mixture and/or the selective diffusion of that component through the membrane. In reverse osmosis (3) applications, which entail recovery of a solvent from dissolved solutes such as in desalination of brackish or polluted water, pressures sufficient to overcome both osmotic pressure and pressure drop through the membrane must be applied. In permeation (4), osmotic pressure effects are negligible and the upstream side of the membrane can be a gas or liquid mixture. Sometimes a phase transition is involved as in the process for dehydration of isopropanol shown in Fig. 1.8. In addition, polymeric liquid surfactant and immobilized-solvent membranes have been used. 

Figure 3.6.2 At first a steroid is covalently bound to a gold subphase, then the remaining gaps are filled with octadecyl-thiol. A closed membrane is thus formed with 6 x 14 A gaps caused by the steroidal bottom. The gold electrode is then submersed in an aqueous ferricyanide solution, and a strong cyclovoltammetric signal is observed. Upon addition of molecules that fit into the immobile water structure within the hydrophobic gap (= hydrophobic water), the ion flow is blocked. tran -Cyclohexanediol with two equatorial hydroxy groups is optimal glucose and rhamnose also work. cw-Cyclohexane diol with an axial OH group is much less efficient. The entrapped molecules do not equlibrate with the bulk water. Only upon acidification with HCl to pH < 3 are the entrapped molecules released. (From Fuhrhop et al., 1997.)...

Immobilized artificial membranes (lAM) are solid membrane mimetics that are covalently bound to the surface of a silica chromatographic support to generate a phospholipid monolayer. lAM chromatography can be applied to measure membrane partitioning of a given compound or to predict bile salt-membrane interactions they may also be used in studies on transcellular absorption. Artificial membranes appear to be well correlated with 1-octanol/water partition coefficients however, since the latter can be well predicted in silico, the real value of AIMs is the ability to study complex molecules or extracts, where in silico prediction is poor. I AM can also be applied to generate large data sets, that in turn can be applied to train and, therefore, improve in silico models by an extensive data input [41]. 

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