Hydrophobic anions, membrane-binding

Especially sensitive and selective potassium and some other ion-selective electrodes employ special complexing agents in their membranes, termed ionophores (discussed in detail on page 445). These substances, which often have cyclic structures, bind alkali metal ions and some other cations in complexes with widely varying stability constants. The membrane of an ion-selective electrode contains the salt of the determined cation with a hydrophobic anion (usually tetraphenylborate) and excess ionophore, so that the cation is mostly bound in the complex in the membrane. It can readily be demonstrated that the membrane potential obeys Eq. (6.3.3). In the presence of interferents, the selectivity coefficient is given approximately by the ratio of the stability constants of the complexes of the two ions with the ionophore. For the determination of potassium ions in the presence of interfering sodium ions, where the ionophore is the cyclic depsipeptide, valinomycin, the selectivity coefficient is Na+ 10"4, so that this electrode can be used to determine potassium ions in the presence of a 104-fold excess of sodium ions. 

The principle of a liquid-based ion-selective electrode was described in Figure 15-6. Figure 15-13 shows a Ca ion-selective electrode, featuring a hydrophobic poly(vinyl chloride) membrane saturated with a neutral Ca -binding ligand (L) and a salt of a hydrophobic anion (Na R ) dissolved in a hydrophobic liquid (Figure 15-14). The response is... 

The squaraine probe 9g was tested for its sensitivity to trace the formation of protein-lipid complexes [57]. The binding of dye 9g to model membranes composed of zwitter-ionic lipid phosphatidylcholine (PC) and its mixtures with anionic lipid cardiolipin (CL) in different molar ratios was found to be controlled mainly by hydrophobic interactions. Lysozyme (Lz) and ribonuclease A (RNase) influenced the association of 9g with lipid vesicles. The magnitude of this effect was much higher... 

The results may provide a guidepost for designing hybrid assemblies which are capable of performing effective binding of anionic and hydrophobic guests that are not incorporated into the anionic bilayer membrane itself macrocyclic... 

In considering the differential or selective binding of one anion or another, the intrinsic properties of anions mean that we are not on a level playing field it is considerably easier to bind some anions than others. In general, in the absence of specific chemical recognition between anion and host, anion binding selectivity, particularly in solvent extraction experiments or in the detection of anions by membrane-based ion selective electrodes, follows the order of anion hydrophobicity. This order is termed the Hofmeister series, or lyotropic series and was first outlined in 1888 from experiments based on the... 

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