Use of density increment in assessing protein aggregation under unusual conditions.

in the form (1 -6p) where c, is the concentration of macromolecule at radius Y, R is the gas constant, T is the temperature, 6 is the partial specific volume of the macromolecule, p is the solution density and w is the angular velocity. This assumes that the partial specific volume required is that for the unsolvated macromolecule and also involves the solution density, which is frequently not known and will anyway vary down the concentration gradient of the macromolecule established by the sedimentation equilibrium. Not infrequently, the solution density is approximated to that of the solvent, or even just taken as 1.00-1.01 g/ml without further thought, and a value of around 0.72-0.73 mVg is taken for 6. Obviously, workers have often obtained acceptable estimates of fiw,app by using these values and so one might question whether this approximation matters. I would suggest that there are indeed circumstances where it does matter a great deal; I discuss them here.