Carrier complex formation

Detailed information about the mechanism of carrier complex formation can be obtained by relaxation techniques (17) and NMR studies (80, 100—102). The rate constants of the formation reaction for monactin/Na+ (sound absorption) and valinomycin/Na+ (sound absorption, T-jump) in methanol are about 2 108 and 7 10 M-1 sec-1 respectively, and the corresponding rate constants of the dissociation reactions are 4 105 and 5 105 sec-1 (17). In contrast, the dissociation rate constant for some cryptates is much smaller (42, 103, 122). 

Detailed information about the mechanism of the carrier complex formation can be obtained from kinetic measurements. Various relaxation techniques have been applied to both equihbrium and rate studies with biological alkali ion carrier complexes. These methods were described in detail in several review articles cf. ref. 31. 

The mechanism of the carrier complex formation must involve stepwise substitution of solvent molecules of the inner coordination sphere... 

The indicator should be applicable in the concentration range of 10 to 10 moles of alkali ions per liter, since the dissociation constant of carrier complex formation is to be expected in this range. 

The consideration made above allows us to predict good chromatographic properties of the bonded phases composed of the adsorbed macromolecules. On the one hand, steric repulsion of the macromolecular solute by the loops and tails of the modifying polymer ensures the suppressed nonspecific adsorptivity of a carrier. On the other hand, the extended structure of the bonded phase may improve the adaptivity of the grafted functions and facilitate thereby the complex formation between the adsorbent and solute. The examples listed below illustrate the applicability of the composite sorbents to the different modes of liquid chromatography of biopolymers. 

Emi, N., Kidoaki, S., Yoshikawa, K., and Saito, H. Gene transfer mediated by polyarginine requires a formation of big carrier-complex of DNA aggregate. Bio-chem. Biophys. Res. Commun. 1997, 231, 421-424. 

Since Co(II)-complexes are well known oxygen carriers the formation... 

There appear to be two major ways by which ionophores aid ions to cross membrane barriers. Ionophores such as valinomycin and nonactin enclose the cation such that the outside of the complex is quite hydro-phobic (and thus lipid-soluble). The transport behaviour thus involves binding of the cation at the membrane surface by the antibiotic, followed by diffusion of the complexed cation across the membrane to the opposite surface where it is released. Such carrier type ionophores can be very efficient, with one molecule facilitating the passage of thousands of ions per second. A prerequisite for efficient transport by this type of ionophore is that both the kinetics of complex formation and dissociation be fast. 

For the supported catalyst it is expected that the ligand does not leach since it is chemically bonded to the carrier. In contrast, the rhodium metal bound to the ligand is subject to leaching due to the reversible nature of the complex formation. The amount will depend on the equilibrium between rhodium dissolved in the organic phase and that bound to the ligand. When an equilibrium concentration of 10 ppb Rh is attained, the yearly loss of Rh for a 100 kton production plant will be about 1 kg Rh per year. Compared to the reactor contents of rhodium (see Table 3.9, 70 kg Rh) this would result in a loss of 1.5% of the inventory per year, which would be acceptable. 

A series of model siderophore molecular recognition studies coupled with host-guest carrier facilitated model membrane transport studies was reported (198-202). Three approaches were taken which incorporate (i) second coordination shell host-guest complexation, (ii) ternary complex formation, and (iii) a combination of ternary complex - second coordination shell host-guest complex formation. Examples of these approaches are described below. 

BLM transport systems for ferrioxamine B were also devised based on first coordination shell recognition via ternary complex formation utilizing vacant coordination sites on the Fe(III) center (Fig. 29) (199). The tetra-coordinated substrate complex selectively transported was partially dechelated diaqua-ferrioxamine B and coordinately unsaturated di-hydroxamato iron(III) complexes, which utilized a hydrophobic membrane bound bidentate chelator as a carrier for selective transport. Active transport for these systems was accomplished using a pH gradient (199). 

It is not overly difficult to include the effects of interconversion of hydrogen among its charge states if these are equilibrated with the local carrier concentrations and if we continue to neglect complex formation and assume that the spatial scale of the diffusion-migration phenomena is large... 

For the biological limitation of trace metal internalisation, complex formation will invariably decrease the concentration of free metal ion and thus decrease the biouptake fluxes and carrier-bound metal (FIAM, BLM). In the case of a diffusion-limited internalisation, complex labilities and mobilities become much more pertinent when determining uptake fluxes. As shown earlier, few experiments have been designed to identify diffusion limitation of metal uptake fluxes, despite the fact that such a limitation is possible (Figure 10). Competition experiments that can distinguish a kinetic from a thermodynamic control are rare. In these areas, an important research focus is... 

For the alkali metal cations, the stability (14) and permeability (43) sequences for dicyclohexyl-18-crown-6 have been found to be the same (K+ > Rb+ > Cs+ > Na+ > Li+). Thus, a direct relationship exists between the ability of a macrocyclic compound to complex a particular cation (as measured by the log K value for complex formation) and its influence on the biological transport of that cation. Furthermore, it would appear that the biological ion-transport mechanism may in part be due to the complexation properties of the macrocyclic carrier molecules. This subject with respect to cyclic antibiotics has been treated extensively by Si wow and co-workers (2). 

Figure 1. Schematic representations of significant biological functions displayed by host-guest complexation in homogeneous solutions or at membrane surfaces, (a) Separation (e.g., antibody-antigen complex formation), (b) Transformation (e.g., enzymatic reaction), (c) Translocation (e.g., carrier- or channel-mediated transport), (d) Transduction (e.g., receptor-mediated transmembrane signaling).

Oxine (5) fonns complexes of analytical applicability with various metal ions. A RP-HPLC-FLD method (Xex = 370 nm, Xg = 516 nm) was proposed for simultaneous determination of Al(III) and Mg(II), using a Cjg column. Various details of the method are noteworthy Optimization of the method showed that for both ions it is best to have also precolumn and in-column complex formation, caused by the presence of 5 in the injection loop and in the carrier solution FLD detection is preferable to simple UVD because it avoids the background of 5 and interference of various ions forming nonfluorescent chromogenic complexes, e.g. Ca(II) and Zn(II) the intensity of the fluorescence can be increased by micelle formation on addition of SDS and neutralized Af,Af-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid (6). The LOD (SNR = 3) were 0.74 (xM (18 ppb) Mg(n) and 0.60 (xM (16 ppb) Al(III) the latter was attributed in part to residual impurities in the purified water -... 

To obtain theoretical expressions for E corresponding to (1), additional restrictions must be incorporated, besides those given in the preceding list. Assuming cations Iz+ and Jz + to be of the same charge z+ and the formation of any or all l n (n = 1, 2,..., N, respectively, M) cation-carrier complexes possible, the expressions given in Table 11 are obtained.33 These are based on the assumption that the concentration [cs(x)] of the uncomplexed carrier S in the liquid membrane remains constant and that... 

Independent of the assumptions A to C the cation selectivity of the membranes in the equilibrium domain is therefore controlled by the ratio of the complex formation constants (6) and should therefore be identical for different types of neutral carrier membranes.18 Figure 2 indicates that there is indeed a close parallelism between the selectivities of solvent polymeric membranes (SPM) and bilayer lipid membranes (BLM) modified with valinomycin 1, nonactin 2, trinactin 5, and tetranac-tin 6 (see also Ref. 18). This is in good agreement with findings from Eisenman s45 and Lev s15 research groups. 

Uncomplexed valinomycin has a more extended conformation than it does in the potassium complex.385,386 The conformational change results in the breaking of a pair of hydrogen bonds and formation of new hydrogen bonds as the molecule folds around the potassium ion. Valinomycin facilitates potassium transport in a passive manner. However, there are cyclic changes between two conformations as the carrier complexes with ions, diffuses across the membrane, and releases ions on the other side. Tire rate of transport is rapid, with each valinomycin molecule being able to carry 104 potassium ions per second across a membrane. Tlius, a very small amount of this ionophore is sufficient to alter the permeability and the conductance of a membrane. 

The performance of calixarenes as cation carriers through H20-organic solvent H20 liquid membranes has also been studied.137 In basic metal hydroxide solutions, the monodeprotonated phenolate anions complex and transport the cations, while [18]crown-6 does not, under the same conditions. Low water solubility, neutral complex formation and potential coupling of cation transport to reverse proton flux have been cited as desirable transport features inherent in these molecules.137... 

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