  Though often considered as one of the main driving process of the
complexation of species of opposite charges, the release of counterions has
never been experimentally directly measured on polyelectrolyte/proteins
complexes. We present here the first structural determination of such a release
by Small Angle Neutron Scattering in complexes made of lysozyme, a positively
charged protein and of PSS, a negatively charged polyelectrolyte. Both
components have the same neutron density length, so their scattering can be
switched off simultaneously in an appropriate "matching" solvent; this enables
determination of the spatial distribution of the single counterions within the
complexes. The counterions (including the one subjected to Manning
condensation) are expelled from the cores where the species are at
electrostatic stoichiometry.
