Cation-binding selectivities

The gramicidin family of linear polypeptides represents a biologically viable channel system of related peptides in which specific changes in amino acid composition can be correlated with cation binding selectivity and transport. The parent molecule of this family of polypeptides, gramicidin A, has the amino acid sequence shown in Fig. 1. This relatively simple molecule is probably the best characterized ion channel (both structurally and functionally) and has, to date, been the principal proving-ground for many of our ideas about the molecular nature of ion conduction in membranes. ... 

Lynch, A., Eckhard, K., McMahon, G. et al. (2002) Cation binding selectivity of partially substituted calix[4]arene esters. Electroanalysis, 14 (19-20), 1397-1404. 

A good deal of work has been done on polymeric crown ethers during the last decade. Hogen Esch and Smid have been major contributors from the point of view of cation binding properties, and Blasius and coworkers have been especially interested in the cation selectivity of such species. Montanari and coworkers have developed a number of polymer-anchored crowns for use as phase transfer catalysts. Manecke and Storck have recently published a review titled Polymeric Catalysts , which may be useful to the reader in gaining additional perspective. 

Calixarene-based compounds PCT-22 and PCT-23 (Figure 10.25) containing one or four appended naphthalenic fluorophores, respectively, exhibit outstanding fluorescence enhancements upon cation binding and are very selective for Na+ (see Box 10.2). 

According to the same principle, in PBFI (24a)(52) and SBF1 (24b)(53) (Figure 2.11), the oxygen atom of the methoxy substituent of the fluorophore can interact with a cation binding efficiency and selectivity are thus better than that of the crown alone. 

For identification and purification, recombinant proteins are often tagged to the N-terminus with an additional sequence of histidyl residues, mostiysix (Hisg tag). This tag binds selectively to cations as nickel or copper immobilized by covalent chelators as nitrilotri-acetic acid. The method is named Metal Chelate Chromatography (MCC, MCAC, IMAC). 

The selective cation binding properties ol crown ethers and cryptands have obvious commercial applications in the separation of metal ions and these have recently been reviewed (B-78MI52103.79MI52102, B-81MI52103). Many liquid-liquid extraction systems have been developed for alkali and alkaline earth metal separations. Since the hardness of the counterion is inversely proportional to the extraction coefficient, large, soft anions, such as picrate, are usually used. 

The question of carrier design was first addressed for the transport of inorganic cations. In fact, selective alkali cation transport was one of the initial objectives of our work on cryptates [1.26a, 6.4]. Natural acyclic and macrocyclic ligands (such as monensin, valinomycin, enniatin, nonactin, etc.) were found early on to act as selective ion carriers, ionophores and have been extensively studied, in particular in view of their antibiotic properties [1.21, 6.5]. The discovery of the cation binding properties of crown ethers and of cryptates led to active investigations of the ionophoretic properties of these synthetic compounds [2.3c, 6.1,6.2,6.4-6.13], The first step resides in the ability of these substances to lipophilize cations by complexation and to extract them into an organic or membrane phase [6.14, 6.15]. 

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