Charged macromolecules, electrostatic

In summary, a practical realization of FEDs for the pure electrostatic detection of charged macromolecules by their intrinsic molecular charge, especially in high ionic-strength solutions such as physiological conditions, seems to be problematic. All the above discussed disturbing factors, together with a possible undesired adsorption or... 

The electrochemical response of analytes at the CNT-modified electrodes is influenced by the surfactants which are used as dispersants. CNT-modified electrodes using cationic surfactant CTAB as a dispersant showed an improved catalytic effect for negatively charged small molecular analytes, such as potassium ferricyanide and ascorbic acid, whereas anionic surfactants such as SDS showed a better catalytic activity for a positively charged analyte such as dopamine. This effect, which is ascribed mainly to the electrostatic interactions, is also observed for the electrochemical response of a negatively charged macromolecule such as DNA on the CNT (surfactant)-modified electrodes (see Fig. 15.12). An oxidation peak current near +1.0 V was observed only at the CNT/CTAB-modified electrode in the DNA solution (curve (ii) in Fig. 15.12a). The differential pulse voltammetry of DNA at the CNT/CTAB-modified electrode also showed a sharp peak current, which is due to the oxidation of the adenine residue in DNA (curve (ii) in Fig. 15.12b). The different effects of surfactants for CNTs to promote the electron transfer of DNA are in agreement with the electrostatic interactions... 

Concerning the electrostatic interaction of charged macromolecules with lipids and segregation in mixed films. I would like to point out that (to be published in Electrical Phenomena at Membrane Level, p. 273 Elsevier, Amsterdam, 1977) ... 

Biesheuvel, P.M., Cohen Stuart, M.A. (2004). Electrostatic free energy of weakly charged macromolecules in solution and intermolecular complexes consisting of oppositely charged polymers. Langmuir, 20, 2785-2791. 

Electric fields can be used, for instance, to reduce fouling phenomena in systems involving electrically charged macromolecules (e.g., proteins). In microsystems used for capillary zone electrophoresis an external electric field applied across the capillary tube induces electrostatic repulsion between the macromolecules and the inner surface. The reduced adsorption of macromolecules enhances separation resolution and efficiency. 

Ionic Strengths If the protein-polymer complex is formed as a result of electrostatic interactions, increased ionic strength should serve to reduce the attraction between the oppositely charged macromolecules, and decrease the precipitation efficiency. This is observed at pH 4.2 in Figures 3 and 4 for lysozyme and ovalbumin, respectively, and in Figure 5 for lysozyme at pH 5.8 and 7.5. 

Apart from polymer adsorption for uncharged macromolecules, charged macromolecules (polyelectrolytes), such as proteins can also adsorb at surfaces [20, 21]. Adsorption of a charged macromolecule is different from adsorption of an uncharged polymer in that there is a high dependency on the salt concentration. At a low salt concentration, repulsive electrostatic forces between charged polymer chains will inhibit formation of loops and tails (Fig. 4). This has been predicted and confirmed, for instance for adsorption of humic acids on iron-oxide particles [22]. 

Layer-by-layer deposition from aqueous solutions of two types of oppositely charged polymers, usually positively charged polydiallyldimethylammonium chloride (PDADMAC, polycation) and negatively charged sodium polystyrene sulfonate (PSS, polyanion) produces a multilayer film via electrostatic interaction between oppositely charged macromolecules. The thickness of such films can be controlled precisely by a number of deposited layers from approximately 1.2 nm for one layer to 33.9 nm for 21 layers [5]. Finally, oppositely charged (relatively to PDADMAC) QDs can be deposited on the surface of the polyelectrolyte spacer as a submonolayer film. 

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