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Measurements of chain conformation in proteins/polyelectrolytes complexes(lysozyme and PSSNa) show that the crossover observed between an open structure-a chain network crosslinked by the proteins, and a globular one - denseglobules of ~ 10 nm aggregated in a fractal way, results from a conformationmodification prior to the transition. Before showing this, we have widened theparameters range for the observation of the transition. We had shown beforethat the two structures can be formed depending on chain length (for a given[PSS]/[lysozyme] ratio): gel for large chains, globules for short chains. Weshow here that the crossover between these two regimes can also be reached as afunction of chains concentration or salinity of the buffer. Since all thesecrossover parameters act on chains overlapping concentration c*, we reinforcethe idea of a transition from the dilute to the semi-dilute regime, but c* isshifted compared to pure PSS solutions. In order to understand this, we havemeasured by SANS the conformation of a single chain of PSS in presence ofproteins within the complexes. This is achieved by a specific labeling trickwhere we take advantage of the fact that lysozyme and hydrogenated PSS chainshave the same neutron scattering length density. In the gel structure, the PSSchains keep a wormlike structure as in pure solutions, but their persistencelength is strongly reduced, from 50 {\AA} without proteins to 20 {\AA} inaverage with lysozyme. With this value of 20 {\AA}, we calculate newoverlapping thresholds (concentration, mass, ionic strength) in agreement withobserved ones. In a second stage, after the globular structure is formed, thePSS chains get a third conformation, no longer wormlike, but more collapsed,within the globules.

Structure Transition in PSS/Lysozyme Complexes: A Chain-Conformation-Driven Process, as Directly Seen by Small Angle Neutron Scattering