Divalent cations, selectivity

The sodium form of weakacid resins has exceptionally high selectivity for divalent cations in neutral, basic, and slightly acidic solutions. 

Several aqueous systems should be considered in a similar manner. For example, in the selective removal of divalent cations from a saturated salt solution, the hydrated resin gives up a portion of its normal water content as it contacts the salt stream. In so doing, the particles shrink, and the inner pathways for ion migration become smaller. 

Electroanalytical chemistry is one of the areas where advantage of the unique properties of SAMs is clear, and where excellent advanced analytical strategies can be utilized, especially when coupled with more complex SAM architectures. There are a number of examples where redox reactions are used to detect biomaterials (357,358), and where guest—host chemistry has been used to exploit specific interactions (356,359). Ion-selective electrodes are an apphcation where SAMs may provide new technologies. Selectivity to divalent cations such as Cu " but not to trivalent ions such as Fe " has been demonstrated (360). 

New chelating ion-exchange resins are able to selectively remove many heavy metals in the presence of high concentrations of univalent and divalent cations such as sodium and calcium. The heavy metals are held as weaMy acidic chelating complexes. The order of selectivity is Cu > Ni > Zn > Co > Cd > Fe + > Mn > Ca. This process is suitable for end-of-pipe polishing and for metal concentration and recovery. 

Kimura and coworkers [17], Diamond [18], and Damien et al. [19] have described that the polymeric calix-[4]arenes have been used as ionophores in ion selective electrodes for Na (based on calixarene esters and amides) and for Na and Cs (based on p-alkylcalixarene acetates). The electrodes are stated to function as poten-tiometric sensors as well, having good selectivity for primary ion, virtually no response to divalent cations, and being stable over a wide pH range. 

The prespective to be gained thus far is that in order to pass through a lipid layer an ion must have an appropriate polar shell provided in large part by the carrier or channel structure which by virtue of its conformation and by also having lipophilic side chains provides for the polar shell to lipid shell transition. While the relative permeability of monovalent vs divalent and trivalent ions can be qualitatively appreciated from the z2 term in Eqn 2, as indicated in Figure 1B, it is essential to know structural and mechanistic detail in order even qualitatively to understand anion vs cation selectivity and to understand selectivity among monovalent cations. 

E. Hydrated divalent cation approaching a channel with a slightly larger diameter than in D, but the energy of interaction with the divalent cation is sufficient to deform the channel drawing the walls in to make lateral coordination with the divalent cation. Since the channel is too small for a monovalent cation to pass through with its first hydration shell and since the monovalent cation channel interaction is insufficient to make the channel small enough for lateral coordination of the monovalent cation, the channel is selective for divalent cations. (Part E reproduced with permission from Ref. 68 )... 

The effect of the nature of the divalent cation is very pronounced as illustrated in Figure 2 on sample A30. Pectins were found to be much more sensitive to copper than to calcium. A scale of affinity towards divalent cations can be easily obtained this way [18]. This result corroborates what has been measured by pH titration upon addition of increasing amount of cations [28,29], where the order of decreasing selectivity was Pb = Cu Zn > Cd = Ni > Ca. This scale does not follow the size of the radius of the cations but is in agreement with the sequence of complex stability of Irving-Williams [30]. 

For a few membrane electrodes the ion selectivity is limited, e.g., the Metrohm EA301-Me2+ electrode shows the same sensitivity for various divalent cations in fact, even non-selectivity may be accepted, e.g., where only one type of cation is present as in aqueous hydrofluoric acid where the pH can be determined with a cation-exchange resin electrode40. 

The Fur protein from E. coli was isolated in one step due to its high affinity for metal-chelate columns loaded with zinc. In DNase footprinting experiments, the Fur protein was shown to bind DNA in the promoter region of several iron-regulated genes. The consensus sequence, called the Fur box, is GATAATGATAATCATT ATC. In vitro binding is dependent on the divalent cations Co2+ Mn2+ /s Cd2+ Cu2+ at 150 iM, while Fe2+ seemed to be less active at this concentration, probably due to oxidation to Fe3+ (De Lorenzo et al., 1987). The unspecificity for divalent metals observed in vitro shows that the cells have to select the ions transported carefully and have to balance their active concentrations. In addition, it is a caveat for the experimenter to test a hypothesis on metal-ion specificity not only in vitro, but also in vivo. 

Figure 5 Direct antimicrobial and immunomodulatory activities of host defense peptides. Cationic host defense peptides exert their anti-infective activities through either direct antimicrobial activity or through modulation of the host immune response. HDP-mediated direct antimicrobial activity has been demonstrated in vivo for those HDPs that are either present at physiological concentrations that match their respective MIC values or are not inhibited by high salt or divalent cation concentrations. Most natural HDPs have also been demonstrated to be involved in the induction of innate and adaptive immune responses within the host as well as the selective suppression of proinflammatory responses. Ultimately,...

Calcium ion movements are sensitive to the concentration of other cations. In one study, it was found that Ca influx into cells might occur through K+ inward rectifier channels when extracellular K+ ion concentration fell below 1 mM. These channels became Ca -permeable only when the extracellular K+ concentration decreased to 1 mM or below. The same study found that the addition of different divalent cations revealed that Ba +, but not NT+, Cd +, Sr, or Mg +, reversibly blocked the Ca influx into cells during low external Intracellular proteins requiring calcium ion must be very selective for Ca because the concentration of free Mg + (2.5 mM) and K+ (-100 mM) are much higher. Intracellular Ca + concentrations must be kept at very low levels because calcium ions inhibit the activity of Mg +-dependent enzymes. In addition, the precipitation of sparingly soluble calcium salts may cause serious problems within a cell. 

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