Zwitterion phases

Rgure 7 Packings with other liinclional groups (A) pentafluorophenyl, (B) r anopropyl, (C) nitrophenyl, (D) aminopropyl, (E) zwitterionic phase, and (F) diol packing. 

In the case of electrically neutral stationary phases the zwitterionic phases, on the surface of which a betaine is bonded, have the advantage over the diol phases that with the same mobile phase the retention of the stationary phase is much higher. This is because due to the ionic groups the more polar mobile phase near the surface can form a thicker layer and, therefore, the retention of the polar analytes is increased. The selectivity of zwitterionic stationary phases is similar to that of diol phases. 

A zwitterion phase has some unique properties. Sample anions are attracted to the N sites and cations are attracted to the negative sites. If the positive and negative sites are separated by a sufficient distance, these sites have little effect on one another. But in the examples shown, anions are attracted to the positive sites and are simultaneously repulsed by the nearby negative sites. Cations undergo a similar attractive-repulsive effect. Thus, the chromatographic properties of zwitterionic phases differ in several ways from the anion- and cation-exchange columns used in conventional chromatography ... 

Anions and cations are taken up simultaneously by zwitterion phases. The strength by which they are retained is affected by the properties of both the anion and the cation. 

With zwitterion phases, the charged group nearest to the hydrophobic chain determines whether the sample anions or cations will be retained more strongly. Thus, in a sulfo-betaine (SB) phase, sample anions will be retained more strongly because of the close proximity of the N to the alkyl chain. 

The ionic concentration of the mobile phase has much less effect on the retention times of sample ions on a zwitterion column than it does on a conventional IC column. At the eluent concentrations generally used with a zwitterion phase, an increase in eluent concentration results in only a slight decrease in the retention times. Sometimes longer retention times are observed as the eluent concentration increased. 

Macka and Haddad [8] separated the sodium salts of several inorganic anions on a column coated with a CHAPS zwitterion phase with only water as the eluent. 

Johnsen et al. separated nucleoside triphosphates on a polymeric zwitterionic phase (ZlC-pHlLlC) that provided a better separation than the respective... 

Phospholipids. For the removal of ionic contaminants from raw zwitterionic phospholipids, most lipids were purified twice by mixed-bed ionic exchange (Amberlite AB-2) of methanolic solutions. (About Ig of lipid in lOmL of MeOH). With both runs the first ImL of the eluate was discarded. The main fraction of the solution was evaporated at 40°C under dry N2 and recryst three times from n-pentane. The resulting white powder was dried for about 4h at 50° under reduced pressure and stored at 3°. Some samples were purified by mixed-bed ion exchange of aqueous suspensions of the crystal/liquid crystal phase. [Kaatze et al. J Phys Chem 89 2565 7955.]... 

The pFL-dependent partitioning of the ionizable, cationic dye thymol blue has also been investigated [6]. In its neutral, zwitterionic, and monoanionic forms, the dye preferentially partitions into the IL phase (from acidic solution), the partition coefficient to the IL increasing with increasing IL hydrophobicity. Under basic conditions, the dye is in the dianionic form and partitions into water (Figure 3.3-9). 

It has been found that the tris(tert-butyloxycarbonyl) protected hydantoin of 4-piperidone 2, selectively hydrolyses in alkali to yield the N-tert-butyloxycarbonylated piperidine amino acid 3. The hydrolysis, which is performed in a biphasic mixture of THF and 2.0M KOH at room temperature, cleanly partitions the deprotonated 4-amino-N -(tert-butyloxycarbonyl)piperidine-4-carboxylic acid into the aqueous phase of the reaction with minimal contamination of the hydrolysis product, di-tert-butyl iminodicarboxylate, which partitions into the THF layer. Upon neutralization of the aqueous phase with aqueous hydrochloric acid, the zwitterion of the amino acid is isolated. The Bolin procedure to introduce the 9-fluorenylmethyloxycarbonyl protecting group efficiently produces 4.8 This synthesis is a significant improvement over the previously described method9 where the final protection step was complicated by contamination of the hydrolysis side-product, di-tert-butyl iminodicarboxylate, which is very difficult to separate from 4, even by chromatographic means. 

The development of monoalkyl phosphate as a low skin irritating anionic surfactant is accented in a review with 30 references on monoalkyl phosphate salts, including surface-active properties, cutaneous effects, and applications to paste and liquid-type skin cleansers, and also phosphorylation reactions from the viewpoint of industrial production [26]. Amine salts of acrylate ester polymers, which are physiologically acceptable and useful as surfactants, are prepared by transesterification of alkyl acrylate polymers with 4-morpholinethanol or the alkanolamines and fatty alcohols or alkoxylated alkylphenols, and neutralizing with carboxylic or phosphoric acid. The polymer salt was used as an emulsifying agent for oils and waxes [70]. Preparation of pharmaceutical liposomes with surfactants derived from phosphoric acid is described in [279]. Lipid bilayer vesicles comprise an anionic or zwitterionic surfactant which when dispersed in H20 at a temperature above the phase transition temperature is in a micellar phase and a second lipid which is a single-chain fatty acid, fatty acid ester, or fatty alcohol which is in an emulsion phase, and cholesterol or a derivative. 

Girod, L., Martel, S., Carrupt, P. A. The lipophilicity of zwitterionic compounds by RP-HPLC the marked influence of 1-octanol in mobile phase. Unpublished results. 

FIG. 9 A scheme representing lateral phase separation for anionic lipids from zwitterionic lipids in a mixed lipid bilayer induced by the peptide antibiotic, polymyxin-B. (Reprinted by permission from Ref. 41, copyright 1998, Elsevier Science.)... 

The activation free energies computed both in the gas phase and in aqueous solution (Table 2.6) suggest that the generation of an alkylating QM (QM-NI) becomes a much easier process, passing from the protonated quaternary ammonium salt NI to its zwitterionic form NI. ... 

The water-soluble calix[n]arenes 6.3 (n = 4, 6 and 8) containing trimethylammonium groups act as efficient inverse phase-transfer catalysts in the nucleophilic substitution reaction of alkyl and arylalkyl halides with nucleophiles in water (Eq. 6.19).40 In the presence of various surfactants (cationic, zwitterionic and anionic), the reactions of different halides and ketones show that the amount of ketone alkylation is much higher and that the reactions are faster in the presence than in the absence of surfactant aggregates.41 The hydrolysis of the halide is minimized in the presence of cationic or zwitterionic surfactants. 

In cyclohexane the same two ketones (12) and (13) are obtained from the photolysis of (11) but in aqueous dioxane two phenols are isolated as well as the bicyclic ketone (12). Swenton(10) suggested that the gas-phase reaction involves diradical species, whereas in polar solvents zwitterionic intermediates are favored ... 

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