Modes of adhesion of spherocylindrical nanoparticles to tensionless lipid bilayers

Abstract

The adhesion modes and endocytosis pathway of spherocylindrical nanoparticles (NPs) are investigated numerically using molecular dynamics simulations of a coarse-grained implicit-solvent model. The investigations is performed systematically with respect to the adhesion energy density ξ, NP's diameter D, and NP's aspect ratio α. At weak ξ, the NP adheres to the membrane through a parallel mode, i.e. such that its principal axis is parallel to the membrane. However, for relatively large ξ, the NP adheres through a perpendicular mode, i.e. the NP is invaginated such as its principal axis is nearly perpendicular to the membrane. The value of ξ at the transition from the parallel to the perpendicular mode decreases with increasing the D or α, in agreement with theoretical arguments based on the Helfrich Hamiltonian. As ξ is further increased, the NP undergoes endocytosis, with the value of ξ at the endocytosis threshold that is independent of the aspect ratio but decreases with increasing D. The kinetics of endocytosis depends strongly on ξ and D. While for low values of D, the NP first rotates to a parallel orientation then to a perpendicular orientation. At high values of ξ or D, the NP endocytoses while in the parallel orientation.

Membrane-mediated dimerization of spherocylindrical nanoparticles

“…A spherocylindrical NP, of diameter D and length l = rD, where r is its aspect ratio, is constructed as a fairly rigid triangulated mesh following the details provided in Section SII (ESI †). We recently used this NPs model to investigate the adhesion modes of a single SCNP on planar membranes 41 and the adhesion modes of Janus spherical NPs on lipid vesicles. 47 The advantage of this model lies on the fact that the NPs are hollow, which leads to a significant reduction in the number of degrees of freedom associated with the NPs as opposed to models in which NPs are constructed from a three-dimensional arrangement of beads in some lattice structure.…”

“…This choice of initial condition is in line with earlier studies on single SCNPs, which demonstrate that regardless of their initial orientation with respect to the membrane plane, upon their adhesion, the SCNPs quickly rotate to the parallel mode. 37,41 The time evolution of the angle y(t), starting from y 0 , and the distance between the two SCNPs are shown in Fig. 2(A) and (B), respectively.…”

“…This concurs with the fact that the distance between their centers of mass is slightly higher than D. The fact the SCNPs dimerize for all values of d 0 t 70 nm implies that the energy barrier between the monomeric and dimeric states at this value of x must be at a relatively high value of d that exceeds 70 nm. This can be explained by the fact that at x = 1.45k B T nm À2 , monomeric SCNPs are in the parallel mode, 41 and induce a relatively large deformation of the membrane, which must lead to relatively long-range membrane curvaturemediated attraction between them.…”

“…A more detailed understanding of the interaction of nanorods with lipid membranes has been mainly extracted from computer simulations, which thus far have only been carried in the context of a single particle adhering to a planar membrane. [36][37][38][39][40][41] These studies demonstrate that the details of the geometry of the nanorods play a major role on their adhesion mode and the process of their internalization process. Namely, for low values of x, an adhering nanorod lies mainly parallel to the membrane.…”

“…However, as x is increased, the nanorod adhesion mode undergoes a first order transition to the normal mode, in which it is in a tubular pit that is mainly perpendicular to the membrane. 38,41 The value of x at the transition from the parallel to the normal mode decreases with increasing D or r. 41 The nanorod undergoes spontaneous endocytosis at adhesion strengths beyond a threshold value x*, which decreases with D but is independent of r. 41 Membrane-mediated interaction between nanorods was investigated analytically in the asymptotic limit of NPs with infinitely long aspect ratio. 42,43 These calculations therefore consider the case where the long axes of the NPs are parallel and can only account for the side-wise adhesion mode of the NPs.…”