How a Polymer Brush Interacts with Inclusions and Alters Their Interaction

Abstract

Inclusions in a brush are entropically disfavored, as they constrain the surrounding brush chains and limit possible chain conformations. As a result, polymer brushes can be used in lubrication or as biological coatings against toxic molecules. Here, we study the interaction of nanoparticles with a brush using self-consistent field theory (SCFT). For a large particle compressing a brush, we reproduce the linear scaling of the repulsive potential with the particle radius found previously using SCFT. Also, we find that this linearity gives way to a nonlinear (parabolic or cubic) dependence on the particle size for particles inserted deeply into the brush, consistent with earlier particle-based simulations. The insertion of particles disrupts the brush, changing polymer–particle interactions for subsequent nanoparticles, thus introducing effective particle–particle interactions mediated by the brush. When the grafting point is mobile in the plane of the grafting surface, our results suggest that the brush promotes clustering of inclusions. In contrast, inclusions tend to be dispersed when the grafting point is fixed. Finally, we discuss the biological implications of these findings: interactions of antimicrobial peptides with bacterial lipopolysaccharides and clustering of integrin on cancerous cell membranes.

Modeling the Protective Role of Bacterial Lipopolysaccharides against Membrane-Rupturing Peptides

“…Our results suggest that the presence of the LPS brush reduces the number of hydrophobically bound peptides by ∼30%. This effect is mainly due to the steric hindrance of AMPs in the brush, which tends to exert an unfavorable osmotic pressure on the peptide. − Our results also indicate that the interfacial adsorption of AMPs is insensitive to brush lengths as long as they are appreciably longer than the size of the peptide. This, however, does not exclude the possibility of kinetic slowing-down of the binding, more so for longer brushes.…”

“…The resulting free energy change or the excess free energy, denoted as Δ F brush , has been discussed in the literature. A number of theoretical studies suggest that it can be approximated as − where V p and A p (not to be confused with the peptide-induced expansion of the membrane ) are the volume and area of the peptide in the brush, respectively, and γ­(ϕ) the surface tension at the contact (see below).…”

“…Here we employ Δ F brush in eq , partly because of its recent success. , The ϕ dependence of Δ F brush is different for the two choices. This is however not so crucial for this work, since the ϕ of the LPS brush is set by biology and will remain fixed in our consideration, if we ignore its weak dependence on N p in eq .…”

Polymer brush-induced depletion interactions and clustering of membrane proteins

“…For biological membrane where the area fraction of membrane-embedded proteins is as high as 15 -30 %, the formation of protein clusters or nano-or micro-domains [42][43][44][45][46] is of great relevance to understanding the regulation of biological signal transduction and cellto-cell communication. Although other physical mechanisms are still conceivable, lateral depletion interactions between membrane embedded proteins can arise from the fluctuations of lipids 40,47,48 or other polymer-like components comprising fluid membrane 49,50 , contributing to protein-protein attraction and clustering. In this context, the formation of integrin nanodomain which enables cell-to-cell communications via signaling [51][52][53][54] , particularly, the bulky glycocalyx-enhanced integrin clusterings and the associated signaling-induced cancer metastasis observed by Paszek et al 42 make the brush polymer- induced depletion interaction between membrane proteins and their clustering a topic of great relevance to investigate.…”