Conformational switching of chiral colloidal rafts regulates raft–raft attractions and repulsions

Abstract

Membrane-mediated particle interactions depend both on the properties of the particles themselves and the membrane environment in which they are suspended.Experiments have shown that chiral rod-like inclusions dissolved in a colloidal membrane of opposite handedness assemble into colloidal rafts, which are finite-sized reconfigurable droplets consisting of a large but precisely defined number of rods. Here, we systematically tune the chirality of the background membrane, and find that in the achiral limit colloidal rafts acquire fundamentally new structural properties and interactions. In particular, rafts can switch between two chiral states of opposite handedness, which dramatically alters the nature of the membrane-mediated raft-raft interactions. Rafts with the same chirality have long-ranged repulsions, while those with opposite chirality acquire attractions with a welldefined minimum. Both attractive and repulsive interactions are explained by a continuum model that accounts for the coupling between the membrane thickness and the local tilt of the constituent rods. These switchable interactions enable assembly of colloidal rafts into intricate higher-order architectures with unusual symmetries, including stable tetrameric clusters and "ionic crystallites" of counter-twisting domains organized on a binary square lattice. Furthermore, the properties of individual rafts, such as their sizes, are controlled by their complexation with other rafts. The emergence of these complex behaviors can be rationalized purely in terms of generic couplings between compositional and orientational order of fluids of rod-like elements. Thus, the uncovered principles might have relevance for conventional lipid bilayers, in which the assembly of higher-order structures is also mediated by complex membrane-mediated interactions. Significance Statement:We describe hierarchical assemblages of colloidal rods that mimic some of the complexity and reconfigurability of biological structures. We show that chiral rod-like inclusions dissolved in an achiral colloidal membrane assemble into rafts,

All twist and no bend makes raft edges splay: Spontaneous curvature of domain edges in colloidal membranes

“…Furthermore, for sufficiently small chiral bias, negative twist stiffness should give rise to metastable rafts with twist that is opposite of the native one. Intriguingly, such counter-twisted rafts have been observed in membranes with a low net chirality (22). Furthermore, initial observations indicate that raft twist in fully achiral membranes is comparable or greater than in the chiral membrane ( fig.…”

“…4F ). To clarify the contribution of chiral effects, we repeated measurements for background membranes with different chirality strengths by mixing different ratios of left-handed (M13KO7) and right-handed (M13KO7-Y21M) ( 22 , 30 ). Using this method, the background membranes were tuned to be either strongly left-handed (ϕ BG = 1.0) compared with the above weakly chiral membranes (ϕ BG = 0.4), or effectively achiral (ϕ BG = 0) ( Fig.…”

“…For short-rod rafts, the chirality of the background membrane was adjusted by mixing the left-handed M13KO7 with the right-handed M13KO7-Y21M ( 30 ). A mixture of 37% M13KO7 and 63% M13KO7-Y21M shows no evidence of spontaneous twist ( 22 ). We define where .…”

“…Furthermore, our findings demonstrate a chirality-independent mechanism that drives spontaneous interfacial twist in membranes composed of rods with sufficiently different lengths. This mechanism could stabilize assembly of finite-sized colloidal rafts even in weakly chiral or achiral systems ( 22 ). More generally, these results show that the interplay between orientation and composition in phase-separated membranes gives rise to shape selection mechanisms for nonconvex 2D “droplets” that are reminiscent of those studied in 3D vesicles ( 23 ).…”

Geodesic fibrations for packing diabolic domains

“…Where might these textures be detected? Thin rafts of rigid rods (22), when sufficiently chiral, would present themselves as in Fig. 1, Lower Right.…”