Vesicular budding induced by a long and flexible polymer

– We investigate the shape transformation of a vesicle incorporated with a long and flexible polymer. We evaluate the grand canonical partition function of the polymer to consider the effects of chain conformational changes on the membrane. It is found that the polymer loops with the ends adsorbed or anchored on the membrane play a major role on vesicular budding transition. The vesicle, one of self-assembling aggregates of lipids in fluid state, is very flexible and thus exhibits a variety of shape transformations [1]. A prominent example is the budding of smaller vesicle [2], which plays an essential role in the vesicle-mediated transport along endocytic and exocytic pathways. Several attempts have been made to understand the shape transformations in terms of elastic-bending energy concept [1]. In biological membranes, the lipid bilayers are associated with a various kind of macromolecular structures, such as integral proteins, intracellular-side cytoskeletons, and extracellular-side glycocalix [3]. The macromolecular association varies the elastic properties of the membranes over a large range [4] and thus can affect the vesicular shape. Futhermore, the conformational fluctuations of the involved macromolecules may play a key role in the vesicular shape transformations. In particular, the flexible polymers interacting with the membranes have been adopted as tractable model systems to investigate entropic effects on the membrane curvatures [5-8]. The evidences for the dramatic vesicular shape changes induced by flexible polymers have been observed in recent experiments. Giant vesicles subject to amphiphilic polymer solutions have been observed to break into “strings of pearls” of large curvature [9]. Simon et al. [10] have also observed that the amphiphilic polymers carrying both long hydrophilic sections and short hydrophobic anchors on membranes undergo expanded-collapsed transition, leading to pronounced budding of thin-walled vesicles. The way how the polymer and membrane interact and drastically affect the conformations one another poses an interesting problem. In this letter, we discuss the problem, focusing on the polymer-induced vesicular budding transition which has been little studied theoretically as yet. The polymer is associated with the membrane via either anchorage or adsorption. Here we will first consider the case of adsorption developing the grand canonical ensemble approach [11] which is flexible enough to be adapted to the case of