Cation transport rates

Fig. 11. Plot of initial cation transport rates for various carrier/alkali picrate pairs versus equilibrium extraction constants log Ke the points are experimental data, the curve is calculated [6.1, 6.4,6.17]. For analytical reasons the Ke values were determined in conditions different from those of the transport experiments the carriers are cryptands (for [2.2.C5] and [2.1.C5] see structures in [6.1]) dibenzo-18-crown-6, DB18-6 and valinomycin, VAL picrate, P.

Christensen JJ, Christensen SP, Biehl MP, Lowe SA, Lamb JD, and Izatt RM. Effect of receiving phase anion on macrocycle-mediated cation transport rates and selectivities in water-toluene-water emulsion membranes. Sep Sci Technol 1983 18 363-373. 

Lamb, J. D., Izatt, R. M. Garrick, D. G. Bradshaw, J. S. Christensen, J. J. "The Influence of Macrocyclic Ligand Structure on Carrier-Facilitated Cation Transport Rates and Selectivities through Liquid Membranes /. Membrane Sci., 1981,9, p.83. 

Olsher U, Hankins MG, Kim YD, Bartsch RA, Anion effect on selectivity in crown ether extraction of alkah metal cations. J. Am. Chem. Soc. 1993 115 3370-3371. Christensen JJ, Christensen SP, Biehl MP, Lowe SA, Lamb LD, Izatt RM, Effect of receiving phase anion on macrocycle-mediated cation transport rates and selectivities in water-toluene-water emulsion membranes. Sep. Sci. Technol. 1983 18 363-373. Deblay P, Delepine S, Minier M, Renon H, Selection of organic phases for optimal stabihty and efficiency of flat-sheet supported hquid membranes. Sep. Sci. Technol. 1991 26 97-116. 

Cox and DiNunzio ( 32) established an optimum composition for the receiver solution, 0.2 M Mg(II), 5 x lO" 4 M Al(III). Other receiver solutions show a less favourable cation transport across membranes such as permion 1010, a grafted teflon base onto styrene with sulfonic acid exchange sites. For example 0.1 M Na+ shows enrichment factors only 50% that of the above solution. This lower cation transport rate is attributed to interaction between the mobile cations and the fixed exchange sites. The function of the multivalent ions is to provide a shield between exchange sites and mobile ions and thus lower residence times in the membrane. 

More recent results have been obtained using a modified version of the previously reported H cell (see Figure 3 below). Two potentiostats are used simultaneously, one to reduce the anthraquinone ligand near the donor organic interface and the other to oxidize the complexed, neutral species near the receiving interface. Such an experimental arrangement has been reported by Saji [20] and is clearly not without its problems. Nevertheless, we have recently shown further cation transport rate enhancements using this approach. Refinements in this approach are still required. 

By considering the stability constant and the lipophilicity of host molecules, Fyles et al. synthesized a series of carboxylic ionophores having a crown ether moiety and energetically developed the active transport of alkali metal cations 27-32). Ionophores 19-21 possess appropriate stability constants for K+ and show effective K+-selective transports (Fig. 5). Although all of the corresponding [15]crown-5 derivatives (22-24) selectively transport Na+, their transport rates are rather slow compared with... 

On the other hand, Bartsch et al. have studied cation transports using crown ether carboxylic acids, which are ascertained to be effective and selective extractants for alkali metal and alkaline earth metal cations 33-42>. In a proton-driven passive transport system (HC1) using a chloroform liquid membrane, ionophore 31 selectively transports Li+, whereas 32-36 and 37 are effective for selective transport of Na+ and K+, respectively, corresponding to the compatible sizes of the ring cavity and the cation. By increasing the lipophilicity from 33 to 36, the transport rate is gradually... 

Here C0 is the concentration of a cation in the A phase before the application of an electric held, and h is the transport rate of the cation from the A to B phases. 

The rate and extent of intestinal absorption of cimetidine has been widely discussed previously, and a reasonable value of fa for this drug has been estimated as 75% [90, 91]. It has been reported that cimetidine is a substrate for both P-gp and/ or organic cation transporters (OCNT1 and OCNT2) [82, 92]. We determined the... 

Investigations of the cellular effects of radiofrequency radiation provide evidence of damage to various types of avian and mammalian cells. These effects involve radiofrequency interactions with cell membranes, especially the plasma membrane. Effects include alterations in membrane cation transport, Na+/K+-ATPase activity, protein kinase activity, neutrophil precursor membrane receptors, firing rates and resting potentials of neurons, brain cell metabolism, DNA and RNA synthesis in glioma cells, and mitogenic effects on human lymphocytes (Cleary 1990). 

A second important application of CMD has been to study the dynamics of the hydrated proton. This study involved extensive CMD simulations to determine the proton transport rate in on our Multi-State Empirical Valence Bond (MS-EVB) model for the hydrated proton. = Shown in Fig. 4 are results for the population correlation function, (n(t)n(O)), for the Eigen cation, HsO, in liquid water. Also shown is the correlation function for D3O+ in heavy water. It should be noted that the population correlation function is expected to decay exponentially at long times, the rate of which reflects the excess proton transport rate. The straight line fits (dotted lines) to the semi-log plots of the correlation functions give this rate. For the normal water case, the CMD simulation using the MS-EVB model yields excellent agreement with the experimental proton hopping... 

According to this method, Fyles analyzed the transport rate of alkali metal cations for a series of 21 synthetic transporters (Figure 14). The whole molecules were designed to elucidate the structure-function relationship. They are composed of three parts core, wall, and head units. The core units were derived from tartaric acids so that the wall units may be fixed to provide structural control by incorporating both the polar and nonpolar functionality (Y and Z in Figure 14). The head groups (X) are attached to provide an overall amphiphilic nature. 

Figure 15. Selectivity of transport rate of alkali metal cations relative to Na by active transporters (C8TrgP)4mTet ( ), (C8TrgP)6Hex(A), (G82P)2Di (o), (A82P)4Tet ( ), (G8Trg)4Tet...

The ionophores have been incorporated into the vesicle membranes and cation flux was assessed either by monitoring the fluorescence of pyranine dye encapsulated within vesicles for estimating the proton transport rate or by analyzing the line shape of Na nucleus by NMR spectroscopy to evaluate the flux rate of Na+ 106-110 Qf ionophores were expressed by percent activity relative... 

The course of treatment also resulted in considerable positive changes in morphological and functional states of mitochondrial membranes. The passive permeability of their inner membranes for univalent cations decreased down to normal level, and transport rate of Ca ions normalized to physiological level of healthy animals. The swelling rate of mitochondria in sucrose medium increased, though remained slightly lower when compared with healthy animals. The results obtained allow us to assume that the rehabilitation of CsA-sensible pore function and membrane potential was achieved. 

The mechanism by which cations are transported across a membrane is represented in Figure 18a. A cation-carrier complex is initially formed at the interface. This lipophilic species then diffuses across the membrane as an ion pair and dissociates at the other interface to water soluble ion pair and membrane-soluble carrier. The final step is back diffusion of the free carrier to the initial interface. The factors which influence transport rates and selectivity have been the subject of much research (79PAC979, B-81MI52102). 

Figure 18 (a) Mechanism of ion pair transport mediated by a cation carrier (b) plot of initial transport rates (V) of cation picrates as a function of the logarithm of the stability constants... 

Because of the complex equilibria involved (Figure 18a), transport rates depend on the delicate balance of many factors including complexation properties and lipophilicity of the carrier, the cation and anion being transported, and the nature of the membrane species itself. Importantly, transport rates are not directly proportional to the cation-carrier complex stability but present a maximum as a function of Ks. If the complex stability is too low, insufficient cation will be complexed at the initial interface and, similarly, if the complex stability is too high, insufficient cation will be released at the opposite interface. A compromise between thermodynamics (stability) and kinetics (exchange rates) of complexation is involved. 

The phosphonium cations of the type Ph3P+(CH2)wMe (n = 0-5) and tpp+ have been used in determining the transport rate through membranes of Halobacterium halobium44... 

Table 10 Rates and Selectivities of Alkali Metal Cation Transport via Cryptate Complexes...

Carrier Cation logKs (in MeOH) Cation cone, in membrane (lumoir1) Carrier saturation (%) Initial transport rate (p molh-1) Transport selectivity, K+-.Na+... 

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