Polar embedded groups

Example of a stationary phase with polar embedded group , different selectivity for polar analytes than the classical alkyl phases, for eluents with high water content. U.D. Neue et al., Chromatographia, 54. 169 (2001) H. Engelhardt et al., Chromatographia Supp ., 53, S 154 (2001). 

Polar-Embedded Groups—The incorporation of a polar group (e.g., carbamate, amide, urea, ether, sulfonamide) in the hydrophobic chain of the bonding reagent has led to a new class of popular phases with different... 

Example of a stationary phase with polar embedded group , different selectivity for polar analytes than the classical alkyl phases, for eluents with high water content. 

The polar groups are, on the other hand, responsible for an induced polar selectivity. Analytes able to form hydrogen bonds like phenols are retarded more strongly with polar-embedded stationary phases than with the corresponding classical RP of an identical carbon content. This is demonstrated in Figure 2.5 for the separation of polyphenolic compounds present in red wine. The retention time of the polyphenolic compound kaempferol with the shielded phase is more than three times longer than with the corresponding RP column of an identical carbon content. The polar... 

FIGURE 2.8 Hydrophobic retention and selectivity with RP columns. The stationary phases are ordered according to the increasing retention of toluene in methanol-water 50-50 v-v. Dashed line Stationary phases with a silica pore diameter below lOnm. Solid line Stationary phases with a silica pore diameter >12nm. ( ) Stationary phases with polar-embedded functional groups. (( ) Stationary phase based on a wide pore silica (30 nm)). 

Because of the unique structure of a lipoid matrix consisting of phospholipids and embedded proteins, the interaction of drag molecules with polar head groups, apolar hydrocarbons, or both, can induce several changes in the membrane. Consequently, the drag behavior is changed (diffusion, accumulation, and conformation) [136]. 

Figure 1. A schematic representation of the cross section of the lipid-globular protein mosaic model of membrane structure. The globular proteins (with dark lines denoting the polypeptide chain) are amphipathic molecules with their ionic and highly polar groups exposed at the exterior surfaces of the membranes the degree to which these molecules are embedded in the membrane is under thermodynamic control. The bulk of the phospholipids (with filled circles representing their polar head groups and thin wavy lines their fatty acid chains) is organized as a discontinuous bilayer.

Use of alkylamide phases, in which alkyl chains are attached to the silica surface via an alkylamide group, reduces interactions with free silanols, by an internal masking mechanism. Residual silanols interact by hydrogen bonding with the embedded amide groups and thus become less active toward analytes. The embedded polar amide groups lessen the hydrophobicity of these phases compared to that of C18 bonded phases prepared from the same silica. Improved peak shapes of ionizable compounds were reported and this stationary phase was successfully used under IPC conditions to analyze streptomycin and its dihydrostreptomycin derivative in food... 

The nonpolar hydrocarbon skeleton of cholesterol is embedded in the nonpolar interior of the cell membrane. Its rigid carbon skeleton stiffens the fluid lipid bilayer, giving it strength. Cholesterol s polar OH group is oriented toward the aqueous media inside and outside the cell. 

Fig. 3 Structural model of the cell membrane. The membrane is composed of a bimolecular leaflet of phospholipid with the polar head groups facing the extracellular and cytosolic compartments and the acyl groups in the middle of the bilayer. Integral membrane proteins are embedded in the lipid bilayer. Integral proteins are glycosylated on the exterior surface and may be phosphorylated on the cytoplasmic surface. Extrinsic membrane proteins, peripheral proteins, are linked to the cytosolic surface of the intrinsic proteins by electrostatic interactions. (From Ref. l)...

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