  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 - dense
globules of ~ 10 nm aggregated in a fractal way, results from a conformation
modification prior to the transition. Before showing this, we have widened the
parameters range for the observation of the transition. We had shown before
that the two structures can be formed depending on chain length (for a given
[PSS]/[lysozyme] ratio): gel for large chains, globules for short chains. We
show here that the crossover between these two regimes can also be reached as a
function of chains concentration or salinity of the buffer. Since all these
crossover parameters act on chains overlapping concentration c*, we reinforce
the idea of a transition from the dilute to the semi-dilute regime, but c* is
shifted compared to pure PSS solutions. In order to understand this, we have
measured by SANS the conformation of a single chain of PSS in presence of
proteins within the complexes. This is achieved by a specific labeling trick
where we take advantage of the fact that lysozyme and hydrogenated PSS chains
have the same neutron scattering length density. In the gel structure, the PSS
chains keep a wormlike structure as in pure solutions, but their persistence
length is strongly reduced, from 50 {\AA} without proteins to 20 {\AA} in
average with lysozyme. With this value of 20 {\AA}, we calculate new
overlapping thresholds (concentration, mass, ionic strength) in agreement with
observed ones. In a second stage, after the globular structure is formed, the
PSS chains get a third conformation, no longer wormlike, but more collapsed,
within the globules.
