Negatively charge compounds

Fig. 17.1. Representation of the forces that influence a negatively charged compound in an electric field.

Since the negatively charged compound, naphthalenesulfonate, is not expected to interact with the solid support by specific interactions, the study by McCalley clearly indicates that, at least in this case, the tailing of the peaks has the electrostatic origin discussed here. Traditionally, the tailing of basic compounds has been attributed to interactions between the base and the solid support, e.g., silanol groups, acidic sites, etc. However, further quantitative studies in this area are needed to separate the electrostatic effect on peak asymmetry from other types of interactions. 

Beyond these generalizations, it is important to consider possible reactions specific to the contaminant and soil combination of interest. For example, if the soil contains iron oxides, the pH-dependent binding of weak acids is possible. Below the ZPC, electrostatic interaction of the positively charged oxide is possible with negatively charged compounds such as 2,4-D, 2,4,5,-T, and MCPA if present in their deprotonated form (Schwertmann et al., 1986). 

Polycations such as protamine or streptomycin may also be used for differential protein precipitation. These materials bind to negatively charged compounds and hence neutralize a large proportion of the charge they possess. Polycations can be used most advantageously if the desired protein is not precipitated by them. As shown in Table 10-3, PEP carboxykinase from R. rubrum remains soluble while three-quarters of the undesired proteins precipitate with protamine sulfate. On the other hand, the fact that polycations irreversibly remove many anionic proteins somewhat limits their use. When polycations are used they must be... 

Figure 2 Schematic illustration of the (transport) properties of the blood-brain barrier. Shown is the influence of astrocyte endfeet at the brain capillary endothelial cell. This cell has narrow tight junctions, low pinocytotic activity, many mitochondria, and luminal anionic sites that hinder the transport of negatively charged compounds. Passive hydrophilic transport occurs via paracellular diffusion (tight junctions), whereas passive lipophilic transport is a transcytotic process. Adsorptive-, receptor-, and carrier-mediated transport has been indicated. The metabolic properties of the BBB are illustrated by the various enzymes at the BBB [from (157), with permission].

CE/MS was used by Loos et al. for the analysis of polar hydrophilic aromatic sulfonates in wastewater treatment plants. One disadvantage of the method developed was that two and threefold negatively charged sulfonates could not be detected as these very polar negatively charged compounds migrate in the opposite direction from the electroosmotic flow (EOF). By comparison, LC/MS offered higher separation efficiency and sensitivity for LC/MS than the method developed. 

Lacey s group [98, 99, 100] modified the N-terminal end of the peptide with a negatively charged compound. In this case, y-ion type fragmentation is seen, a, b and c types of ions are suppressed due to the negative charge carried by the sulfonate on the N-terminus. 

Use of Equation 11.27 to estimate values of Ki gave the following results sodium acetate 92 litre moP S sodium benzoate 707 litre moP and sodium p-toluene sulphonate 950 litre moP The results of the study suggest that a negatively charged compound capable of hydrophobic interactions is required to prevent effective interaction of the CTAB micelles with the substrate. With amphipathic compounds such as the p-toluene sulphonates there is the possibility of the type of interactions that were discussed by Tomlinson et al [23]. Independent measurement of binding or interaction constants thus would be required before this mechanism of inhibition was elucidated. Charge reduction at the surface is likely to inhibit the access of OH ions to the substrate incorporated into mixed micelles . 

Table 1. Permeability [27] and Diffusion coefficients [28] for different substrates and metabolites. The permeability of DCFH2 and DCF have been set zero since both these molecules dissociate in wata-solution in the working conditions (pH around 7.0) giving negatively charged compounds.

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