Ionic selectivity

In a different context, a micropipette has been applied to monitor the current through a single-ion channel in a biological membrane. The patch-clamp technique invented by Sackmann and Neher [119] led to their Nobel Prize in medicine. The variations in channel current with voltage, concentration, type of ions, and type of channels have been explored. While the functions of specific channels, in particular their ionic selectivity, have been well known, only a handful of channels have the internal geometry and charge distribution determined. The development of a theory to interpret the mass of channel data and to predict channel action is still lacking. 

To determine the ionic selectivity of the pardaxin channels, various ion substitutions were performed and the bionic reversal potential, i.e., the potential at which the current across a bilayer with many open pardaxin channels changed sign, was determined. The relative permeabilities of the ions could then be determined from the general equation ... 

Then, the ionic selectivity is discussed and related to the mechanism of crosslinking with divalent counterions. The sol-gel transition is then examined for LM and HM pectins and the mechanisms described in these two cases. The physical properties of the gels are related to the microstructure of the polymers and few data are examined. 

No specific ionic selectivity is really admitted in pectins with monovalent counterions due to the relativity low charge parameter a very interesting behaviour is observed when divalent counterions are considered. Specially, it was demonstrated that when DM<50% the activity coefficient of magnesium is much larger than that of calcium. The transport parameters (f) were found following the order [45] ... 

This ionic selectivity observed in solution is directly related with the ability to form gels [48]. From circular dichro ism it is shown that stronger interaction exists with calcium compared with sodium counterions [41,48]. 

Rasola, A., L. J. Galietta, D. C. Gruenert, and G. Romeo. 1992. Ionic selectivity of volume-sensitive currents in human epithelial cells. Biochim Biophys Acta 1139(4) 319-23. 

The process of producing an ion-sensing polymer includes the following steps (i) selection and preparation of ligand monomers, (ii) synthesis of ion complexes of the monomers or linear copolymers of the complexing monomers, (iii) preparation of cross-linked copolymers with the monomeric complexes or linear copolymer complexes, (iv) the testing of the polymers for ion selectivity, (v) optimisation of polymer ionic selectivity and (vi) the use of the polymers in the construction of ion-selective electrodes (ISEs) and optical sensors. 

Figure 13.27. Selectivity of the Sodium Channel. The ionic selectivity of the sodium channel partly depends on steric factors. Sodium and lithium ions, together with a water molecule, fit in the channel, as do hydroxylamine and hydrazine. In contrast, K+ with a water molecule is too large. [After R. D. Keynes. Ion channels in the nerve-cell membrane. Copyright 1979 by Scientific American, Inc. All rights reserved.]...

Coronado, R, Rosenberg, RL. and Miller, C. (1980). Ionic selectivity, saturation, and block in a K -selective channel from sarcoplasmic reticulum. J. Gen. Physiol. 76, 425-446. 

Favre I, Moczydlowski E, Schild L. On the structural basis for ionic selectivity among Na+, K+, and Ca2+ in the voltage-gated sodium channel. Biophys J 1996 71 3110-25. 

Studies regarding the relationship between molecular structure and ionic selectivity have resulted in the development of polymer-based ISEs using a number of naturally... 

Bean That means that the ionic selectivity is actually different for the different open states. 

Eisenman, G, 1983. The molecular basis of ionic selectivity in macroscopic systems, p. 121 156. In L. Liberti and F.G. Helfferich (ed.) Mass transfer and kinetics of ion exchange. NATO-ASI Symp. Ser. no. 71 M. Nijhoff, The Hague, Netherlands. 

Hurtado, R. M. Drost-Hansen, W. (1979). Ionic selectives of vicinal water in the pores of a silica gel. In Cell-Associated Water (Drost-Hansen, W., Clegg, J. S. eds.) pp. 115-123, Academic Press, New York. 

Membranes can be characterized by their structure and function, that is how they form and how they perform. It is essential that the cation exchange membranes used in chlor-alkali cells have very good chemical stability and good structural properties. The combination of unusual ionic conductivity, high ionic selectivity and resistance to oxidative hydrolysis, make the perfluorinated ionomer materials prime candidates for chlor-alkali membrane cell separators. 

In the absence of an electrical field and in dilute solutions, the degree of ionic selectivity depends solely on the physico-chemical properties of the membrane, but in the presence of a high intensity electrical field and the resultant large field gradients in concentrated solutions, the dynamic properties of both the membrane and solution interact with the imposed electrical field to provide the anomalous permselectivity observed. 

Rinaudo M, Milas M. Ionic selectivity of polyelectrolytes in salt-free solutions. In Rembaum A, Selegny E, eds. Polyelectrolytes and Their Applications. Dor-drecht-Holland D Reidel, 1975 31-49. 

Perhaps the most recent dramatic advance has been the determination of the three-dimensional structure of a bacterial channel from S. lividans (Doyle et al., 1998). This channel is composed of four identical subunits, each with two trans-membrane sequences and a pore region, that associate in tepee shape to form the functional ion channel containing within it the selectivity filter that discriminates from other ions. The selectivity filter contains a so-called signature sequence, highly conserved residues that characterize ion channels and the Gly-Tyr-Gly components of this sequence in the four subunits bind through their carbonyl residues and are responsible for the ionic selectivity of the channel. 

Campbell DT (1976) Ionic selectivity of the sodium channel of frog skeletal muscle. J Gen Physiol 67 295-307... 

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