  Intermolecular interactions in protein solutions in general contain many
contributions. If short-range attractions dominate, the state diagram exhibits
liquid-liquid phase separation (LLPS) that is metastable with respect to
crystallization. In this case, the extended law of corresponding states (ELCS)
suggests that thermodynamic properties are insensitive to details of the
underlying interaction potential. Using lysozyme solutions, we investigate the
applicability of the ELCS to the static structure factor and in how far
effective colloidal interaction models can help to rationalize the phase
behavior and interactions of protein solutions in the vicinity of the LLPS
binodal. The (effective) structure factor has been determined by small-angle
X-ray scattering (SAXS). It can be described by Baxter's adhesive hard-sphere
model, which implies a single fit parameter from which the normalized second
virial coefficient $b_2$ is inferred and found to quantitatively agree with
previous results from static light scattering. The $b_2$ values are independent
of protein concentration, but systematically vary with temperature and solution
composition, i.e. salt and additive content. If plotted as a function of
temperature normalized by the critical temperature, the values of $b_2$ follow
a universal behaviour. These findings validate the applicability of the ELCS to
globular protein solutions and indicate that the ELCS can also be reflected in
the structure factor.
