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

We present an extended structural study of globular complexes made by mixing a positively charged 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 scale range lying from 10 Å to 1000 Å are measured by SANS. Whatever the conditions, the same structure is found, based on the formation of dense ∼100 Å globules with a neutral core and a volume fraction of organic species (compacity) of ∼0.3. On the larger scale, the globules are arranged into fractal aggregates. Zetametry measurements show that the globular complexes have a total positive charge when the charge ratio of species introduced in the mixture [-]/[+]intro > 1 and a total 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 1/κ however the physicochemical parameters are modified in the system, as long as chain-protein interactions are of a simple electrostatic nature. The mean number of proteins per primary complex Nlyso_comp grows exponentially on a master curve with 1/κ. This enables to picture 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 repel themselves by electrostatic repulsion because their charge increases. When this repulsion becomes dominant in the system, the globules stop growing and behave like charged colloids: they aggregate with a RLCA process, which leads to the formation of fractal aggregates of dimension 2.1.