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Precipitation with organic solvents

Addition of miscible solvents such as ethanol or methanol to a solution may cause proteins in the solution to precipitate. The solvation layer around the protein will decrease as the organic solvent progressively displaces water from the protein surface and binds it in hydration layers around the organic solvent molecules. With smaller hydration layers, the proteins can aggregate by attractive electrostatic and dipole forces. Important parameters to consider are temperature, which should be less than 0 °C to avoid denaturation, pH and protein concentration in solution. Miscible organic solvents decrease the dielectric constant of water, which in effect allows two proteins to come close together. At the isoelectric the relationship between the dielectric constant and protein solubility is given by:

S0 is an extrapolated value of Se is the dielectric constant of the mixture and k is a constant that relates to the dielectric constant of water. The Cohn process for plasma protein fractionation relies on solvent precipitation with ethanol to isolate individual plasma proteins.

a clinical application for the use of methanol as a protein precipitating agent is in the estimation of bilirubin.

Non-ionic hydrophilic polymers

Polymers, such as dextran and polyethylene glycols, are frequently used to precipitate proteins because they have low flammability and are less likely to denature biomaterials than isoelectric precipitation. These polymers in solution attract water molecules away from the solvation layer around the protein. This increases the protein-protein interactions and enhances precipitation.

9) Concept about complex proteins.

multiprotein complex (or protein complex) is a group of two or more associated polypeptide chains. If the different polypeptide chains contain different protein domain, the resulting multiprotein complex can have multiple catalytic functions. This is distinct from a multienzyme polypeptide, in which multiple catalytic domains are found in a single polypeptide chain.

Haemoglobin as the most important chromoprotein.

Haemoglobin of adult person – HbA – is the compound protein of chromoprotein’s group, takes place in blood. It’s main function is transport of oxygen and carbon acid. Haemoglobin consists of 4 molecules of haeme and protein – globin. The last one contains two a- and two в- spirals. Every a- chain contains 141,в- chain contains 146 amino acids. Thus, in the globin of haemoglobin takes place 574 amino acids. Haeme has the following structure:

In the presence of “sickleoidcellularanemy” in the blood of person HbS takes place instead of HbA, which differs from HbS by primary structure of globin: in the every в- chain one GLU substituted for VAL. This decease often takes place in “malary zone” of tropical countries. Haemoglobin of this persons weakly bonded with oxygen, as a result tissues of organs badly provides in oxygen. Now it is well known above 100 pathologic forms of haemoglobin. Hereditary deceases, which is result of alteration of structure of substance on the molecular level, are called by Poling as “molecular deceases”.

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