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

TL;DR

This study investigates the structure and formation mechanisms of globular protein-polyelectrolyte complexes, revealing that their size is governed by electrostatic screening and that they form dense globules with neutral cores which aggregate into fractal structures.

Contribution

It provides a detailed analysis of how electrostatic screening controls the size and structure of protein-polyelectrolyte complexes, highlighting the role of Debye length and charge ratios.

Findings

Globular complexes have a dense ~100 Å core with neutral charge.

Complex size is determined by Debye length, independent of other parameters.

Complexes grow via counterion release and then aggregate through electrostatic repulsion.

Abstract

We 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 10{\AA} to 1000{\AA} are measured by SANS. Whatever the conditions, the same structure is found, based on the formation of dense globules of ~ 100{\AA} 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.