Aminoacids and proteins present a ph on which the number of pozitive charges is equal with the number of negative charges. This ph with the molecule net charge equal to zero is named isoelectric point and is notated with phi.
Because of the report between the functional free groups: - COOH and –NH2, proteins act like acids or like bases. The separation of carboxil groups is more intense as more the environment acidity is lower.
The separation degree of amino groups is conditioned of the environment alkalinity: when the concentration of H+ ions is lower, the separation of amino groups is more intense and reverse.
The isoelectric point of proteins is important in chemistry because the properties of the proteins undergo unique changes at this point.
The global electric charge of a protein is the algebraic sum of pozitive and negative charges, depending on the environment ph. In alkaline environment the protein has a negative charge, and in acid environment a pozitive charge. For each protein exist a concentration of H+ ions when the algebraic sum of the charges is equal to zero and when the protein is indifferent to electricity. This environment ph is named isoelectric point.
At this ph the protein precipitated because it has the lowest osmotic pressure and the lowest solubility. Consequently, the proteins in buffer solutions with ph corresponding phi, miscarriage the electrical charges, became neutral and precipitated. The addition of organic solvents (alcohol, acetone) encouraging precipitation. phi represent an important constant because each protein has a characteristic phi, e.g: hemoglobin = 6,8; casein = 4,7; gelatin = 4,6.