Finite size and inner structure controlled by electrostatic screening in globular complexes of proteins and polyelectrolytes

Abstract

International audienceWe present an extended structural study of globular complexes made by mixing a positively charge protein (lysozyme) and a negatively charged polyelectrolyte (PSS). We study the influence of all the parameters that may act on the structure of the complexes (charge densities and concentration of the species, partial hydrophobicity of the polyion chain, ionic strength). The structures on a 15 scale range lying from 10Å to 1000Å are measured by SANS. Whatever the conditions, the same structure is found, based on the formation of dense globules of ~ 100Å with a neutral core and a volume fraction of organic species (compacity) of ~ 0.3. At higher scale, the globules are arranged in fractal aggregates. Zetametry measurements show that globular complexes have a total positive charge when the charge ratio of species introduced in the mixture [-]/[+]intro > 1 and a total 20 negative charge when [-]/[+]intro < 1. This comes from the presence of charged species in slight excess in a layer at the surface of the globules. The globule finite size is determined by the Debye length k-1 whatever the way the physicochemical parameters are modified in the system, as long as chain-protein interactions are of simple electrostatics nature. The mean number of proteins per primary complex Nlyso_comp grows exponentially on a master curve with k-1. This enables to picture 25 the mechanisms of formation of the complexes. There is an initial stage of formation where the growth of the complexes is only driven by attractions between opposite species associated with counterion release. During the growth of the complexes, the globules progressively repell themselves by electrostatic repulsion because their charge increases. When this repulsion becomes dominent in the system, globules stop growing and behave like charged colloids: they aggregate 30 with a RLCA process, which leads to the formation of fractal aggregates of dimension Df 2.1

Generic, phenomenological, on-the-fly renormalized repulsion model for self-limited organization of terminal supraparticle assemblies

“…Because SPs of size V max SP after equilibration occur with the highest frequency (compare Fig. 3B), it is reasonable to deduce that the terminal finite size should correspond to a free-energy minimum, as predicted by previous studies (8,16). The uniform SPs are thermodynamically stable, rather than long-lived, kinetically arrested, clusters.…”

“…An enormous body of theoretical studies that followed has been successful in characterizing the thermodynamic stability of finite-sized aggregates in various systems such as polyelectrolytes (5,7,11,(15)(16)(17)(18)(19), colloidal suspensions (20-24), and block copolymer and protein solutions (4,8,(25)(26)(27)(28)(29)(30). The optimal size of the assembled clusters is often attributed to the balance of short-ranged attraction and longer-ranged repulsion between the primary building blocks.…”

“…self-assembly | terminal assemblies | self-limited aggregation T he spontaneous formation of uniformly sized aggregates observed in inorganic, biological, and colloidal systems (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14) is of both fundamental and practical interest. Their formation suggests that a generic mechanism is applicable to those chemically distinct systems, whereas their ability to serve as building blocks for engineering nanostructures at larger length scales is of practical interest.…”

“…Their formation suggests that a generic mechanism is applicable to those chemically distinct systems, whereas their ability to serve as building blocks for engineering nanostructures at larger length scales is of practical interest. Examples of uniform aggregates include filaments, bundles, and toroids with uniform diameters formed by macromolecules (6)(7)(8)(9), and regular domains by like-charged macroions on planar and cylindrical surfaces (5,11). In a recent study, we observed the assembly of positively charged polydisperse CdSe, PbS, and PbSe nanoparticles (NPs) into monodisperse supraparticles (SPs), which form colloidal crystals at sufficiently high density (12).…”

Internal structure and colloidal behaviour of covalent whey protein microgels obtained by heat treatment

“…Under these conditions of charge neutrality, attractive interactions between the building blocks are dominating as already reported for lysozyme/polystyrene sulfonate microgels. 46 This led to a situation where the average number of nearest neighbours is higher compared to the case where the building blocks are highly charged. Thus, the structure peak was most pronounced close to the IEP.…”