Separation of Peptides and Proteins by Molecular Sieving

Distribution coefficients obviously cannot exceed unity when they are solely determined by molecular-sieve equilibria. This fact does not seriously detract from the value of the gel-filtration method. Even substances with K D ratios close to unity can often be separated in long columns or by recycling procedures (Bennich and Porath, 1962). 

When filtrations are performed under appropriate conditions Sephadex and similar gel substances act as almost ideal molecular sieves. These gels. 

Craig and collaborators have extensively supplemented countercurrent extraction with gel filtration (Hill and Konigsberg, 1960 Rasmussen and Craig, 1961). Occasionally the more laborious countercurrent extraction method can be replaced by Sephadex filtration (Rasmussen and Craig, 1962). 

Procedures Based on the Interplay between Several Sorption Factors 

Aromatic and basic substances are adsorbed on Sephadex columns. Aromatic sorption is usually completely reversible and if electrolyte concentration is high enough this is also the case for adsorption due to electrostatic interaction. Negative adsorption or ion exclusion is encountered for acidic substances in sufficiently weak electrolyte solutions. 

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