  We present an approximate method for calculating the electrostatic free
energy of concentrated protein solutions. Our method uses a cell model and
accounts for both the coulomb energy and the entropic cost of Donnan salt
partitioning. The former term is calculated by linearizing the
Poisson-Boltzmann equation around a nonzero average potential, while the second
term is calculated using a jellium approximation that is empirically modified
to reproduce the dilute solution limit. When combined with a short-ranged
binding interaction, calculated using the mean spherical approximation, our
model reproduces osmotic pressure measurements of bovine serum albumin
solutions. We also use our free energy to calculate the salt-dependent shift in
the critical temperature of lysozyme solutions and show why the predicted salt
partitioning between the dilute and dense phases has proven experimentally
elusive.
