  Bacteria typically have a few thousand different proteins. The number of
proteins with a given charge is a roughly Gaussian function of charge - centred
near zero, and with a width around ten (in units of the charge on the proton).
We have used the charges on E. coli's proteins to estimate the changes in the
second virial coefficients of all its proteins as the concentration of a 1:1
salt is increased. The second virial coefficient has dimensions of volume and
we find that on average it decreases by about twice the average volume of a
protein when the salt concentration is increased from 0.2 to 1 Molar. The
standard deviation of the decrease is of the same order. The consequences of
this for the complex mixture of proteins inside an E. coli cell, are briefly
discussed.
