Macromolecules charged

Charged macromolecules, such as proteins or polymers, are often separated elec-trophoretically. The rate of migration through an electric field increases with net charge and field strength. Molecular size of analytes and viscosity of separation media both have inverse relationships with rate of migration. These variables must all be taken into account in order to optimize the conditions for an efficient electrophoretic separation. 

Detection of charged macromolecules by means of field-effect devices (FEDs) possibilities and limitations... 

Most of the experiments for detecting charged macromolecules with FEDs, reported in literature, have been realized using a transistor structure [11-36], Recent successful experiments on the detection of charged biomolecules as well as polyelectrolytes with other types of FEDs, namely semiconductor thin him resistors [39 11], capacitive MIS [42] and EIS structures [43-50], have demonstrated the potential of these structures - more simple in layout, easy, and cost effective in fabrication - for studying the molecular interactions at the solid-liquid interface. A summary of results for the DNA detection with different types of FEDs is given in Table 7.1. 

Detection of Charged Macromolecules by Means of Field-Effect Devices (FEDs)... 

FIGURE 7.3 Simplified equivalent circuit of an original (unmodified) EIS structure (a) and EIS biosensor functionalized with charged macromolecules (b). Cj, Cx and CML are capacitances of the gate insulator, the space-charge region in the semiconductor, and the molecular layer, respectively / u is the resistance of... 

Generally, there are a number of ways in which the adsorption and binding of charged macromolecules (in particular, DNA immobilization and hybridization) can affect the electrochemical properties of the analyte-FED interface. In the case of field-effect devices, two basic effects are usually considered ... 

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 preparation of the FEDs, the experimental set-up and measuring conditions for the detection of DNA immobilization and hybridization as well as for the monitoring of the layer-by-layer adsorption of the polyelectrolyte multilayers are described in detail elsewhere [46-50], The attachment of these charged macromolecules to the FED surfaces has been systematically characterized by means of capacitance-voltage,... 

The results obtained with PE multilayers as well as DNA on top of the capacitive EIS sensor could verify their feasibility as transducer for a label-free detection of adsorption, binding, and interactions of charged macromolecules. Nevertheless, our experiments do not enable us to clearly distinguish between the contributions in the signal generation from each of the mechanisms discussed in sections 7.3 and 7.4. Probably, both basic mechanisms, namely, the intrinsic charge of molecules and the ion-concentration redistribution in the intermolecular spaces or in the multilayer, affect the sensor signal by superposition. 

At the same time, however, it must be concluded that the practical development of FEDs for a label-free detection of DNA and other charged macromolecules by their intrinsic molecular charge seems to be more complicated than originally expected. Although the discussed results are highly exciting, they are rather diverse and even sometimes inconsistent. Factors influencing the DNA immobilization and hybridization detection by FEDs are ... 

A. Poghossian, M.H. Abouzar, F. Amberger, D. Mayer, Y. Han, S. Ingebrandt, A. Offenhauser, and M.J. Schoning, Field-effect sensors with charged macromolecules characterisation by capacitance—voltage, constant capacitance, impedance spectroscopy and atomic-force microscopy methods. Biosens. Bioelectron. 22, 2100-2107 (2007). 

MJ. Schoning, M.H. Abouzar, S. Ingebrandt, J. Platen, A. Offenhauser, and A. Poghossian, Towards label-free detection of charged macromolecules using field-effect-based structures scaling down from capacitive EIS sensor over ISFET to nano-scale devices, in Mater. Res. Soc. Symp. Proc. 915, 0915-R05-04 (2006). 

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... 

Even complicated, charged macromolecules like proteins can be succesfully displaced. As an example we give in Figure 6 the displacement isotherm for human plasma albumin from silica by morpholine (21). Of course, in this case where charge effects and a variety of segment/surface interactions play a role, our simple Equation 5 does not apply. Nevertheless, for practical work it is important to realize that most macromolecules, often thought to be irreversibly adsorbed, can be removed completely from the adsorbent surface by the concerted action of a large number of small molecules. 

The principle of hard and soft Lewis acids and bases, proposed by Pearson (1963), is useful to describe these reactions. A Lewis acid is any chemical species that employs an empty electronic orbital available for reaction, while a Lewis base is any chemical species that employs a doubly occupied electronic orbital in a reaction. Lewis acids and bases can be neutral molecules, simple or complex ions, or neutral or charged macromolecules. The proton and all metal cations of interest in subsurface aqueous solutions are Lewis acids. Lewis bases include H, O, oxyanions, and organic N, S, and P electron donors. A list of selected hard and soft Lewis acids and bases found in soil solutions is presented in Table 6.1. 

Various theoretical approaches have been utihzed to describe the performance of charged macromolecules at interfaces, hi particular, scahng theories... 

Polymer composed of negatively charged macromolecules and an equivalent amount of... 

In Section 15.6, the retention of proteins in ion exchange chromatography is discussed. The ions in the surrounding electrolyte form an electrical double layer around a charged macromolecule, e.g., a protein. The interaction between a protein and an oppositely charged surface can, therefore, be described as taking place between two overlapping double-layer systems. 

Polyelectrolytes can be used to investigate intermolecular interactions because parameters such as ionic strength and concentration can be changed to monitor reactions between oppositely charged macromolecules. Upon formation of a polyelectrolyte complex, there is also the formation of a low molecular weight... 

SW CNT functionalized with DNA showed a 10 times more effective penetration and expression of genes in vitro, in comparison with molecular DNA. Other charged macromolecules such as polypeptides and liposomes, can provide more effective transport, but they can cause destabilization of the cellular membrane exhibiting a cytotoxic effect. Whereas, using nanotubes for gene delivery has not caused any cytotoxic effects. 

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