Generalities About Adsorption of Polyelectrolytes onto Solid Supports

DNA or RNA derivatives are polyelectrolytes in nature. This section thus recalls some fundamental aspects of polyelectrolyte adsorption and conformation onto charged colloidal dispersions.

A given polyelectrolyte is considered as being adsorbed when, after a mixing period with solid surface, one of its units is in direct contact with the surface. The adsorption is considered as a thermodynamic balance between free poly-electrolytes and adsorbed ones. However, desorption is generally a slow process governed by the reduction in the attractive forces involved in the adsorption process.

When the interactions between the polyelectrolytes and the charged support are weak (such as in a system bearing the same charges), the maximal amount of adsorbed polyelectrolytes is very low and increases with ionic strength, since the presence of electrolytes decreases the electrostatic repulsion forces between the polyelectrolytes and solid support. If the interactions between the polyelec-trolytes and the charged surfaces are strong (oppositely charged system polymer/surface), two possibilities may exist: either a low increase or a substantial decrease of adsorption with ionic strength is observed. In the first case, the increase of the ionic strength reduces the repulsive electrostatic repulsions between the adsorbed polyelectrolytes and the free polyelectrolytes in solution; thus the amount adsorbed increases slightly. In the second case, a competition between the adsorption of polyelectrolytes and the ions present in solution occurs which participates in macromolecule desorption. Experimental facts remain the best referee.

The buffer plays a considerable role in adsorption phenomena by modulating the solubility of the polymer in the adsorption medium [8]. The less the polymer is soluble in the considered medium, the higher the adsorption. The pH, salinity, and temperature are then the drastic parameters to control while performing polyelectrolyte adsorption.

B. Methodology

Most of the studies were performed on a poly(thymidylic acid) (dT35) oligonu-cleotide whose structure is given below (Fig. 6), though concepts derived here mainly apply to other structured ODNs. The oligonucleotide bears an amine group (2HN-(CH2)6-) at the 5' position, which permits grafting reactions. In some cases, the oligodeoxyribonucleotide was labeled with 32P or with chemically grafted fluorescein isothiocynate (FITC).

The quantification of immobilized (adsorbed or grafted) nucleic acids was principally determined from the residual nucleic acids in the supernatant (depletion immobilization methodology). The amounts of immobilized nucleic acids were determined using the following equation which allows the calculation of the amount of immobilized oligonucleotide, AS (mg/g):

where V (mL) is the final volume of the solution, Ct (mg/mL) and Cf (mg/mL) are the initial and final concentrations of ODN in the solution, respectively, and m (g) is the mass of adsorbent. The total amount of immobilized ODN is generally expressed in mg/m2, to overcome the differences in particle sizes among various latexes.

FIG. 6 Structure of polythymidylic acid (dT35) bearing aliphatic primary amine.

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