Cation specificity

Polymers like those in the poly aniline family interchange protons and anions with the solution, allowing a local modulation of pH. Composites that interchange cations allow the modulation of any cation concentration. Efforts are being devoted to the synthesis of polymer or polymeric derivatives having great cationic specificity. 

Morris, E.R., Rees, D.A. and Robinson, G. (1980) Cation-specific aggregation of carrageenan helices Domain model of polymer gel structure. Journal of Molecular Biology, 138, 349-362. 

ElectroCell System AB [99], EL-TECH[269], ICI [271,272], and de Nora [129,273] are now developing electrohydrolysis of sodium sulfate for commercial applications. In the electrohydrolysis process sodium sulfate is fed as anolyte to an electrochemical cell divided by a cation specific membrane. Protons are generated in the anolyte, hydroxyl ions at the cathode. Sodium ions cross the membrane to produce a catholyte solution of sodium hydroxide. The net reaction is ... 

Figure 1. Kinetics of K efflux from Chlorella under Og stress. External K measured with cation-specific electrode (Beckman Instruments, Fullerton, model 39137) with 10" cells/ml suspended in a 10 mM Tris-Cl, ImM CaCU, pH 9 solution (10 ml, total volume). Control efflux (0—0) is linear after 20 min (denoted as 0 time here). The addition of Og ( — ) for 30 min and continuous Og (O—O), 26 fimoles/liter air flow (25 cc/min) are shown. The electrode output is amplified by an electrometer coupled to an antilog converter (12,13).

Sprunger, L. et al., Characterization of room temperature ionic liquids by the Abraham model cation-specific and anion-specific equation coefficients, /. Chem. Info. Model, 47,1123,2007. 

In contrast to the hydrolysis and synthesis of ATP connected with proton translocation in mitochondria, chloroplasts and bacterial membranes, the energy linked movement of calcium ions gives rise to the appearance of an acid-stable phosphorylated intermediate in the membranes. A cation specific phosphorylation also occurs in the membranes of the sodium potassium transport system183. However, due to the inability to correlate phosphorylation and ion movement in the latter membranes, membrane phosphorylation has been questioned as being a step in the reaction sequence of ion translocation184,18s. Solely the sarcoplasmic calcium transport system allows to correlate directly and quantitatively ion translocation with the phosphoryl transfer reactions. 

Donors present in appended groups may also cause gradations in stability and selectivity, for example in systems based on (115), 6a 474 there is an enhanced Na+ stability shown, and this is also seen when pyridinyl, furanyl and thiophenyl units are introduced.475" 77 Careful manipulation of the donors, in conjunction with the properties discussed below, can lead to reasonably discrete metal cation specificities. 

The cation plays a prominent structure-directing role in zeolite crystallization. The unique structural characteristics of zeolite frameworks containing polyhedral cages (62, 63) have led to the postulate that the cation stabilizes the formation of structural subunits which are the precursors or nucleating species in crystallization. The many zeolite compositions and complex cation base systems studied allow a test of the structuredirecting role of the cation and the cation templating concept. Table I summarizes the cation base systems from which zeolites have been synthesized. The systems used before 1969 are indicated to illustrate the number and complexities of new cation systems investigated since that time. Table II presents a summary of zeolite framework structure types, the cation systems in which they have been formed, and a proposal for a cation specificity for the formation of each framework type. A similar... 

Zeolite Structure Building Units Double Cation Specificity for Framework... 

Building Synthesis Zeolite Framework Types Containing Cation Specificity... 

Table IV. Cation Specific Building Units in Zeolite Structures...

Since the discovery in 1964 that the antibiotic valinomydn exhibited alkali cation specificity in rat liver mitochondria, a new area of research has developed, based not only on biological systems but also on model systems such as crown ethers.484 The ability of neutral compounds to form lipid-soluble alkali and alkaline earth complexes was observed in 1951. The structure of the corresponding ligand, the anion of the antibiotic nigericin (78), was characterized as its silver salt in 1968.488 486 Silver was used as a heavy atom crystaUographically, since the Ag+ cation had a radius between that of Na+ and K+, which were the two alkali cations with which nigericin was most active. 

While gramicidin and other channel formers can show high transport rates, they do not show the high selectivity that characterizes natural channels. There is much interest at present in a class of proteins called porins, which form natural pores in the outer membranes of Gram-negative bacteria. Several different porin proteins have been isolated from Escherichia coli. These form water-filled channels of various sizes in membranes. Thus the proteins OmpC and OmpF seem to be cation-specific channels while other proteins give larger diameter channels that seem to be specific for anions.34,35... 

Two distinct receptor groups have been identified for acetylcholine, the nicotinic and the muscarinic groups (Table 11.1). Furthermore, there are at least four subtypes of nicotinic and five subtypes of muscarinic receptors. Nicotinic receptors are ubiquitous and exist at the neuromuscular junctions of skeletal muscles and on ganglion cells in the autonomic nervous system. Nicotinic receptors located on cation-specific ion channels, when opened, evoke fast, transient depolarizations of the recipient cell. Muscarinic receptors are found in smooth muscle receiving parasympathetic innervation and elsewhere, and can be blocked by atropine. Muscarinic receptors are coupled indirectly to slow and fast ion channels via G proteins. 

Sodium and potassium cations are often encountered in the same biological environment and the transmembrane movements of both are required as part of an enzymatic pathway as in Na+, K+-ATPase. Under these circumstances it is essential that cation-specific channels are formed. What features of the channels contribute to the selectivity Earlier the preferred geometries of Na+and K+, sixfold octahedral and eightfold cubic respectively, were proposed as the main discriminatory factors. A computational analysis by Dudev and Lim [35] has considered the effect of coordinated water, number of available coordination sites in the channel walls, and the dipoles of the coordinating groups. The researchers investigated cation complexes with valinomycin and the protein KcsA, both K+-selective, and compared these with a non-selective NaK channel. 

This is most clearly seen in the work of Beer whose group linked cation specific crown ethers to electrochemically responsive ferrocene reporter groups shown in Fig. 6.3 [1],... 

While the hydration force was associated with the structuring of water in the vicinity of a lyophilic surface [12], there is no consensus about its microscopic origin. This incertitude is probably due to the apparent contradictory experimental results for phospholipid bilayers, the hydration force is apparently independent of the electrolyte concentration and has a decay length of about 2 A [11], while for mica surfaces the hydration is strongly dependent not only on the electrolyte concentration, but also on the nature of the cation (cation-specific effects) [17] and has a decay length of about 10 A. 

Cation specificity has been tested performing syntheses in the same conditions as those giving cancrinite (see the first of three oxide compositions reported above), substituting the small or the large cation with other cations. 

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