Protein Solubility Calculator

Protein solubility in salt solutions is commonly modelled with the Cohn equation, in which the logarithm of solubility equals beta minus the product of the salting-out constant Ks and the ionic strength. Beta is the extrapolated solubility at zero ionic strength and reflects how well the protein surface interacts with water and how much net charge it carries, so it rises as the solution pH moves away from the isoelectric point. Ks reflects exposed hydrophobic surface and rises with hydrophobicity, which is why a Kyte-Doolittle GRAVY score serves as a useful proxy. At low salt, added ions screen surface charges and can raise solubility slightly, an effect called salting-in. Beyond that point, competition for hydration water dominates and solubility falls approximately log-linearly, which is exactly what ammonium sulfate fractionation exploits, typically cutting between 30 and 60 percent saturation. Reference points help calibrate expectations: serum albumin exceeds 100 mg/mL at physiological pH and ionic strength, lysozyme reaches tens of mg/mL, and many recombinant proteins fall below 1 mg/mL near their pI. Temperature adds a further term, with solubility usually rising modestly up to about 40 degrees C before denaturation reverses the trend, and some proteins showing cold denaturation at low temperature. The Cohn form is empirical and treats the protein as a single homogeneous particle, so it ignores specific ion effects following the Hofmeister series, buffer identity, glycosylation, disulfide state, and the reality that observed solubility is often limited kinetically by aggregation rather than by a true thermodynamic equilibrium. Numbers from this kind of model should be read as an order-of-magnitude guide for planning a precipitation step.

Systematic study of salt effects on proteins began with Franz Hofmeister in Prague, whose 1888 studies on the precipitation of egg white proteins produced a ranking of anions and cations by their ability to salt out, with sulfate and phosphate at the strong end and thiocyanate and perchlorate at the other. The series still bears his name and is still not fully explained at a molecular level. Ammonium sulfate became the workhorse precipitant precisely because it sits at the salting-out end of that series and is highly soluble in water. Edwin Joseph Cohn at Harvard Medical School turned these observations into quantitative physical chemistry through the 1920s and 1930s, studying how solubility varies with ionic strength, pH, dielectric constant, and temperature, and summarising the work with John Edsall in the 1943 monograph Proteins, Amino Acids and Peptides as Ions and Dipolar Ions. The practical payoff came during the Second World War, when Cohn and his group developed cold ethanol fractionation of human blood plasma. The Method 6 protocol published in 1946 supplied albumin for treating shock in wounded servicemen and later yielded immune globulin and fibrinogen fractions. The hydropathy scale used to estimate the hydrophobic contribution came later, from Jack Kyte and Russell Doolittle in 1982.