Stationary phases donor-acceptor type

New brush-type phases (donor-acceptor interactions) are appearing all the time. " Examples are stationary phases comprising quinine derivatives and trichloro-dicyanophenyl-L-a-amino acids as chiral selectors. Quinine carbamates, which are suitable for the separation of acidic molecules through an ionic interaction with the basic quinine group, are also commonly used but in general they are classified with the anion-exchange type of chiral selectors (see further) because of their interaction mechanism, even though r-donor, r-acceptor properties occur. (Some separations on Pirkle-type CSPs are shown in Table 2.)... 

For example, cyclodextrins form chiral cavities which adsorb the corresponding enantiomers with different affinity while cellulose triacetate crystallizes in the form of helical substructures in which the enantiomers may be incorporated with different rates. For amino acid derived stationary phases there are two types of enantiomer differentiating interactions a brush-like hydrogen bond and dipole interaction plus a /[-complex donor or acceptor interaction with the aromatic residues in the amino acid. 

Mobile phases are usually binary or ternary mixtures of solvents. Selectivity is affected mostly by mobile phase composition rather than strength, and peak shape and retention are both influenced by the addition of organic modifiers.101 Some compounds naturally have 77-donor or 77-acceptor groups and can be resolved directly. In many cases, however, introduction of 77-donating groups by derivatization steps is necessary. Figure 2.20 shows the proposed three-point interaction of 3-aminobenzo[a]pyrene, a polycyclic aromatic hydrocarbon (PAH), with a Pirkle-type stationary phase.111 Two possible interactions are illustrated, showing the best orientations for maximum interaction. 

Here, only general categories of chiral stationary phases will be mentioned. One of the more popular types of GC and HPLC columns use donor-acceptor interactions such as those illustrated in Figure 2.16 for enantiomer separation. 

The retention behavior of solutes will depend on the type of interactions with the micelles and with the surfactant-modified stationary phase. Nonpolar solutes should only be affectedby hydrophobic interactions (Fig. 5.1a). For these solutes, different proportions of nonpolar, dipole-dipole and proton donor-acceptor interactions between solutes and... 

The retention characteristics of over 40 small polar molecules (phenols, phenyl alcohols, imines, phenylcarboxylic acids, phenyl esters, phenyl. ethers, phenyl ketones, nitro- and cyanophenols) were studied on C4 and C g stationary phases [117]. Experimental results explain the advantages of methanol over other organic mobile phase components (e.g., acetonitrile) in the separation of aliphatic alcohols, phenols, and carboxylic acids. Methanol, through its reciprocal hydrogen bond donor/acceptor character forms stable complexes with these solutes in the stationary phase, giving enhanced selectivity for these solute types. The k values for these 40 solutes are tabulated for 20/80, 25/75, and 50/50 methanol/water mobile phases. 

The interaction between a solvated peptide or protein and a chemically modified RPC and HIC stationary phase in a fully or partially aqueous solvent environment can be discussed in terms of the interplay of weak physical forces. The main types of physical interactions that are involved in order of relevance and dominance for the establishment of the selective recognition and binding between a peptide or protein and RPC and HIC ligates are (I) hydrophobic interactions and related phenomena mediated by polarized electron donor or electron acceptor processes, (2) Lifshitz-London forces and van der Waals and associated weak dipolar interactions, (3) tt 7t and n ->dipole interactions, (4) hydrogen bond interactions, (5) electrostatic interactions, (6) metal ion coordination interactions, and (7) secondary macromolecular interactions involving force field effects. 

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