Exchange Kinetics

The exchange process in the adsorption of macromolecules onto colloidal particles has been investigated by Pefferkorn et al. [14] and was related to the establishment of adsorption equilibrium, which was found to be dependent on incubation conditions (temperature, pH, salinity, solvent nature, etc.). Similarly, the exchange processes and kinetics of oligonucleotides adsorption using dT35 and radiolabeled dT35-32P were performed on bare and coated cationic polystyrene latexes. The radiolabeled ODN was first adsorbed and the supernatant (or fraction of the supernatant) containing free ODN molecules was replaced by nonra-diolabeled ODN. After incubation time, the radiolabeled ODNs were detected either in the supernatant or directly on the particles.

The exchange kinetics investigated at basic pH in which the attractive electrostatic interactions are low were found to increase slightly as a function of

FIG. 12 Effect of ionic strength on the maximal adsorbed dT35 onto (a) bare and (b) precoated latex particles (by adsorbing a small amount of Triton-X405) as a function of pH. (From Refs. 11, 15.)

FIG. 13 Amount of dT35 exchanged as a function of time. The adsorbed amount was 0.5 mg m- and 0.3 mg m- onto bare and precoated latex respectively (at 20°C, pH 9.2, and 10-2 M ionic strength). The exchange was performed using surfactant-free buffer. (From Ref. 11.)

time before reaching a plateau (Fig. 13). Exchange between adsorbed and free oligonucleotides doubtless exists but remains small. The initial exchange rate determined from the slope of the exchanged ODN amount vs. time was found to be 1.3 pg m-2 h-1.

H. Thermodynamic Aspects

The adsorption of single-stranded DNA or RNA fragments (i.e., oligonucleo-tides) onto colloidal particles bearing negative or positive surface charges turned out to depend mainly on electrostatic forces. The attractive electrostatic interactions between the cationic support and the ODNs favor their immobilization, whereas for anionic latexes for which hydrophobic interactions slightly compete with electrostatic repulsions, very few chains adsorb on the particle surface.

Similar to polymer adsorption onto solid support, the ODN adsorbed amount can be related to the adsorption energy (AG) of each monomer constructing the ODN chain and the ODN bulk concentration (C) as expressed by the following relationship [15]:

where n is the number of bases and k is a constant depending on the nature of the system and the experimental conditions. The adsorption energy of each molecule can be considered as the sum of all contributions (®electrostatic and Ohydrophobic energies):

FIG. 14 Maximal adsorbed dT35 (Ns in mg.m-2) on latex particles as a function of surface charge density (pH 5.0, 25°C, and ionic strength 10-2). (From Ref. 16.)

FIG. 14 Maximal adsorbed dT35 (Ns in mg.m-2) on latex particles as a function of surface charge density (pH 5.0, 25°C, and ionic strength 10-2). (From Ref. 16.)

n AG Ohydrophobic + O electrostatic (4)

The hydrophobic adsorption energy is neglected so that only the electrostatic term is considered. This electrostatic adsorption energy can be expressed as the product of oligonucleotide charge (ooiigo assumed constant) and the surface charge density (afatex) of the colloidal support as expressed below:

Oelectrostatic ^oligo.^ latex (5)

To point out the relationship between ODN adsorption and pH, the plot of log(Ns) vs. the surface charge density (i.e., Z potential at a given pH) is reported in Fig. 14. Extrapolation of linear variation behavior at zero charge density (i.e., zero Z potential) gives the maximal ODN adsorbed amount reached via exclusive hydrophobic interactions (~0.3 mg/m2) [10].

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