Ion exchange effect

Columns of gel beads are more often used than thin layers. The process can be considered analogous to a partition system wherein molecules which are completely excluded from the gel have a distribution ratio of 0 whilst those small enough to penetrate all parts of the structure have a value of 1. Adsorption or ion-exchange effects can cause the distribution ratios of polar molecules to exceed 1. Components of a mixture are characterized by their retention volume, VR, which is determined by the distribution ratio. 

In a series of studies we recently demonstrated (29, 30, 63-67) that the resolution of peptides on reversed phase can be profoundly influenced by the addition of appropriate counterionic reagents to a mobile phase of deflned pH, ionic strength, and water content. Retention under these conditions can be discussed in terms of ion-air associations between the ionized peptide and a counterion in the mobile phase and subsequent sorption of the complex onto the stationary phase. Alternatively, adsorption of the counterion, particularly if it is lipophilic, onto the nonpolar stationary phase may occur, and peptide retention would then be mediated by dynamic liquid-liquid ion-exchange effects. Arguments in favor of the participation of one, the other, or both of these alternative pairing-ion phenomena in ion-pair chromatography have been extensively reviewed (16, 28b, 62, 68, 68a). It can be shown (62, 68) that retention behavior in ion-pair systems can be described by... 

As elucidated from the adsorption experiment on BSA, adsorption of gallium ion by simple ion-exchange effect is negligible. The characteristics of metal ions such as ionic radius and hardness/softness, and the conformation of the metallothionein probably affect the selectivity of metal adsorption. The understanding of the mutual interactions among those factors would be a key dictor in designing the protein-based ligand suitable for a specific metal ion. 

The use of paper as a chromatographic medium is usually regarded as a typical partition system, where the stationary phase is water, held by adsorption on cellulose molecules, which in turn are kept in a fixed position by the fibrous structure of the paper. It is now realised, however, that adsorption of components of the mobile phase and of solutes, and ion exchange effects, also play a part, and that the role of the paper is by no means merely that of an inert support. The technique devised by Consden et al. has not undergone any fundamental changes, but there has been considerable improvement in detail, resulting partly from the wide variety of commercial apparatus which is available. Elaborate or expensive equipment is not essential, however, and good results can be obtained with quite simple apparatus and materials. 

While the separation on cellulose is mainly due to partition, adsorption again plays some part and ion exchange is also possible. The extent of adsorption is uncertain, but is partly due to the polar nature of the hydroxyl groups of the cellulose molecule, and varies according to the polarity of the solutes. The ion exchange effects are also due to the hydroxyl groups and to the small number of carboxyl groups in the cellulose (Chapter 3). 

A long spacer is often needed with ligands of small MW, so that the ligand can settle into its binding pocket without problems. To avoid hydrophobic or ion exchanger effects, the spacer should be uncharged and hydrophile. 

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