  Small angle neutron scattering intensity distributions taken from cytochrome
C and lysozyme protein solutions show a rising intensity at very small wave
vector, Q, which can be interpreted in terms of the presence of a weak
long-range attraction between protein molecules. This interaction has a range
several times that of the diameter of the protein molecule, much greater than
the range of the screened electrostatic repulsion. We show evidence that this
long-range attraction is closely related to the type of anion present and ion
concentration in the solution.
