  We computed the phase-separation behavior and effective interactions of
colloid-polymer mixtures in the "protein limit", where the polymer radius of
gyration is much larger than the colloid radius. For ideal polymers, the
critical colloidal packing fraction tends to zero, whereas for interacting
polymers in a good solvent the behavior is governed by a universal binodal,
implying a constant critical colloid packing fraction. In both systems the
depletion interaction is not well described by effective pair potentials but
requires the incorporation of many-body contributions.
