Effect of pH

The investigation of the effect of pH on the adsorption of charged macromole-cules onto charged solid support is of paramount importance. In fact, the pH

FIG. 9 (a) Adsorption isotherms of oligonucleotide onto bare latex particles. Samples were mixed and incubated for 2 h at 20°C, 10-2 M ionic strength at a given pH. (b) Adsorption isotherms of oligonucleotide onto precoated latex particles (by adsorbing small amount of Triton-X405). Samples were mixed and incubated for 2 h at 20°C, 10-M ionic strength at a given pH. (From Refs. 11, 15.)

FIG. 9 (a) Adsorption isotherms of oligonucleotide onto bare latex particles. Samples were mixed and incubated for 2 h at 20°C, 10-2 M ionic strength at a given pH. (b) Adsorption isotherms of oligonucleotide onto precoated latex particles (by adsorbing small amount of Triton-X405). Samples were mixed and incubated for 2 h at 20°C, 10-M ionic strength at a given pH. (From Refs. 11, 15.)

affects the charge density and charge distribution of the considered support without altering the nucleic acid charges.

The adsorption of oligonucleotides onto negatively charged sulfate-polystyrene latexes was found to be low and marginally pH dependent. The behavior is mainly related to the negligible effect of pH on the strong acids borne by the latex. On the opposite, the adsorption of ODNs onto cationic (amidine or amine) polystyrene latexes is dramatically affected by the pH of the medium irrespective of oligonucleotide sequence (Fig. 10). The observed behavior is attributed to a decrease of cationic surface charge density on the latex particles while increasing the pH.

The effect of pH on oligonucleotide adsorption onto cationic polymer particles can be dramatically perturbed or at least modified while coating the latex surface with nonionic surfactant. The low adsorbed amount of Triton X-405 (~0.1 mg/m2) has a negligible effect on the oligonucleotide adsorption below pH 7, i.e., in the high attractive electrostatic interactions domain. On the other hand, above pH 7, the surfactant influence is more marked since the adsorption of oligonucleotides is dramatically reduced and under some conditions the ODN adsorption is totally suppressed (Fig. 11). The surfactant layer clearly acts as an adsorption barrier leading to reduced ODN amounts on the surface. Apart from charge screening, implementing a certain degree of hydrophilicity of the particle surface may also reduce the van der Waals forces, such as stacking interaction between aromatic bases of ODN and the polystyrene support.

FIG. 10 Maximal adsorbed dT35, dC35, dA30, and dT10dC12dG5 on cationic latex particles as a function of pH at 20°C and 10-3 M ionic strength. (From Ref. 15.)

Was this article helpful?

0 0

Post a comment