Gel-Filtration: Separation by Molecular Weight
This method is based on the molecular sieve effect. The core of the process involves sieving protein molecules through a specialized medium — swollen Sephadex beads.
Sephadex itself represents a complex spatial structure consisting of three-dimensional dextran polysaccharide chains with pores of a defined diameter formed within them. When a mixture is passed through the column, the migration speed of different proteins depends directly on their molecular weight.
Separation occurs according to the following principle:
- Lower-molecular-weight molecules freely enter the internal pore space of the Sephadex beads. Because of this, their pathway is lengthened, they are retained in the column, and they emerge from it significantly later.
- Higher-molecular-weight molecules, conversely, are physically unable to enter the dextran pores. They bypass the beads externally and therefore elute (wash out) from the chromatography column much faster.
Ion-Exchange Chromatography: The Role of Electrical Charge
This type of fractionation is based on electrostatic interactions. The process occurs through the binding of ionized groups on the proteins themselves with oppositely charged functional groups of the matrix. These matrices are ion-exchange resins — special insoluble polymeric materials.
The key rule of the method: the strength of adsorption (protein binding to the resin) is strictly proportional to the magnitude of the protein molecule's electrical charge. The more strongly charged the protein, the more firmly it binds to the polymer.
To recover the isolated protein from the column, an elution procedure is applied. Adsorbed proteins are washed out using sodium chloride (NaCl) solutions, with the salt concentration being gradually increased. The pattern of elution is as follows: the lower the charge of a specific protein, the lower the NaCl concentration required to break its bond with the ionogenic groups of the resin and wash it into the collection tube.
Affinity Chromatography: Absolute Specificity
Among all the approaches discussed, affinity chromatography stands out as the most specific method for isolating individual proteins. It allows for the targeted "fishing out" of the desired macromolecule from extremely complex mixtures.
The mechanism of this method involves several consecutive steps:
- Preparation of the matrix: a ligand — a molecule with which the target protein interacts under physiological conditions — is firmly attached to a chemically inert polymer via covalent bonding.
- Application of the mixture: the protein solution is loaded onto the column. At this point, strict complementary binding occurs between the target protein and the immobilized ligand.
- Final result: exclusively the protein specific to that particular ligand is adsorbed onto the polymer column. All other (contaminant) proteins pass straight through without being retained.