Effect of Polymer Grafting on the Bilayer Gel to Liquid-Crystalline Transition

Abstract

Grafted polymers on the surface of lipid membranes have potential applications in liposome-based drug delivery and supported membrane systems. The effect of polymer grafting on the phase behavior of bilayers made up of single-tail lipids is investigated using dissipative particle dynamics. The bilayer is maintained in a tensionless state using a barostat. Simulations are carried out by varying the grafting fraction, G(f), defined as the ratio of the number of polymer molecules to the number of lipid molecules, and the length of the lipid tails. At low G(f), the bilayer shows a sharp transition from the gel (L(beta)) to the liquid-crystalline (L(alpha)) phase. This main melting transition temperature is lowered as G(f) is increased, and above a critical value of G(f), the interdigitated L(betaI) phase is observed prior to the main transition. The temperature range over which the intermediate phases are observed is a function of the lipid tail length and G(f). At higher grafting fractions, the presence of the L(betaI) phase is attributed to the increase in the area per head group due to the lateral pressure exerted by the polymer brush. The areal expansion and decrease in the melting temperatures as a function of G(f) were found to follow the scalings predicted by the self-consistent mean field theories for grafted polymer membranes. Our study shows that the grafted polymer density can be used to effectively control the temperature range and occurrence of a given bilayer phase.

Investigations on the melting and bending modulus of polymer grafted bilayers using dissipative particle dynamics

“…For the melting transition studies, polymer grafting is carried out in a manner similar to that used in our earlier work. 8 A sphere of given radius, r A , is drawn from bead A of the polymer. The head beads H of the lipid that lie in a distance r Ͻ r A are attached with a harmonic potential ͓Eq.…”

“…For all simulations, the bilayer is maintained in a tensionless state using a modified Andersen barostat. 8 Maintaining a zero tension across the bilayer is important while studying the melting transition and becomes crucial while determining the low q scalings while computing the bending modulus of the membrane.…”

“…Thus the presence of the polymer directly influences the bilayer thickness and area per head group. [6][7][8] These changes influence the mechanical properties, such as the area stretch modulus ͑k A ͒ and bending modulus ͑͒ of the membrane. Theoretical analysis 9 suggests that the area stretch modulus decreases with addition of polymer.…”

“…25 In the work by Pal and Seidel, 24 the scaling for the brush length with grafting density was found to agree well with the theoretical predictions. In a recent study we investigated the influence of polymer grafting on the melting transition of lipid bilayers 8 and compared the changes in the melting transition and areal expansion with scaling theories for grafted membrane systems. Simulations were carried out using a modified Andersen barostat to obtain tensionless membranes made up of single tailed lipids, with polymers in the brush regime.…”

“…The simulations were found to capture the areal expansion and lowering of the gel to liquid crystalline transition due to grafting, and the corresponding scalings were found to follow the self-consistent mean field results. In this paper we expand our earlier analysis 8 and study the influence of grafting on the melting of smaller chain lipids and determine the bending modulus as a function of the grafting fraction using dissipative particle dynamics. We also analyze the changes to the bending modulus and partition the contributions arising from bilayer thinning and the presence of grafted polymer.…”