Adsorption of macromolecules onto responsive surfaces. Monte Carlo simulations

Protein and polymer adsorption onto lipid bilayers and monolayers is fundamentally interesting and has great importance in medicine, biology and technological applications such as biosensors, implants, food and pharmaceutical formulations. Here we use a simple membrane model that includes lateral diffusion and/or vertical excursions out of the bilayer (protrusions) to study the influence of such parameters on the adsorption degree of macromolecules and the conformational behavior of polyelectrolytes with different characteristics.

Polyion adsorption at responsive catanionic surfaces

The adsorption of a single and negatively charged polyion onto a surface carrying both negative and positively charges, representing a charged membrane surface, has been investigated by using a simple model employing Monte Carlo simulations. The surface charges were either frozen in a liquid-like structure or laterally mobile. With a large excess of positive surface charges, the classical picture of a strongly adsorbed polyion with an extended and flat configuration emerges. However, adsorption also appears at a net neutral surface or at a weakly negatively charged surface, and at these conditions the adsorption is stronger with a flexible polyion as compared to a semiflexible one, two features not appearing in simpler models containing homogeneously charged surfaces. The presence of charged surface patches (frozen surface charges) and the ability of polarization of the surface charges (mobile surface charges) are the main reasons for the enhanced adsorption. The stronger adsorption with the flexible chain is caused by its greater ability to spatially correlate with the surface charges. Polyions with both linear and ring architectures were also considered. At weakly attractive surfaces, the ring polyion adsorbs more strongly since it loses less entropy on adsorption than a linear chain. The adsorption of the ring is also enhanced at the fluid surfaces, since its more compact conformation increases the polarization of the surface. However, the linear polyion shows a significant adsorption at a neutral fluid surface, while the ring chains are totally desorbed, suggesting a delicate balance between the entropy of the surface groups and that of the chains. Although ring polyions show a stronger adsorption and a more compact conformation both in- and out-of-plane, at weakly attractive surfaces, no significant influence of the architecture was found on the polyion induced surface polarization (fluid surfaces) or opposite charge patch detection (frozen surfaces), at the monomer level. The adsorption profiles are, however, very different. For linear polyions at weakly attractive surfaces, it was observed a strong predominance of one-tail conformations, which was independent of the state of the surface.

People: Rita Dias, and Alberto Pais (Univ. Coimbra, Portugal)

References:

• Polyion Adsorption onto Catanionic Surfaces
  Dias, R.S.; Pais, A.A.C.C.; Linse, P.; Miguel, M.G.; Lindman, B.
  J. Phys. Chem. B 109 (2005) 11781-11788.
• Effect of the architecture on polyelectrolyte adsorption and condensation at responsive surfaces
  Dias, R.S.; Pais, A.A.C.C.
  J. Phys. Chem. B 116 (2012) 9246-5409.