Zwitterion

A zwitterion (see Ion) is a molecule having two or more functional groups, one of which is positive and the other is negatively charged. The molecule as a whole is electrically neutral. To some extent, the term " inner salt " used for a zwitterion.

Most of them are in the groups of an acid and a base function. The most common example are amino acids, which are present both in aqueous solution and in the solid phase zwitterionic. The acid group is a hydrogen, and to contribute a negative charge, a hydrogen, the amino group takes up and carries a positive charge. In contrast to betaines the charges can therefore be compensated by proton migration.

In solution, such as amino groups at a certain pH, the isoelectric point (IEP or pI) equal to load many acid groups negative positive. Then wander amino acids in the electric field no longer, but only align themselves, since the sum charge is neutral. The pH - value below the isoelectric point, the dissociation of the acid group decreases, and the amino acid bears a positive charge amount. If the pH on the dissociation of the acid increases to, and the amino group is from the hydrogen, the molecule carries a negative charge amount. This effect is exploited in electrophoresis and isoelectric focusing in the. At the isoelectric point, the water solubility of amino acids is at its lowest, since due to the resultant intra-molecular charge no stable hydrate more can be formed.

With peptides and proteins, the conditions are similar to those at amino acids. From a carboxyl group of the peptide or protein is a proton migrates to a basic amino group, which can be at the N -terminal end, but need not be mandatory.

Calculation of the isoelectric point

The pH at the isoelectric point can be calculated from the pK a values ​​of the acid group and the amino group:

Derivation:

The acidity constants are defined as:

Equation (1) Solving for C ( zwitterion ), and sets the result in equation (2).

Since the concentrations of anions and cations at the isoelectric point are equal, the equation simplifies to:

Square roots and logarithms results in:

At acidic or basic amino acids to calculate the IEP the pKa values ​​of the carboxyl and amino groups are taken into account. For the calculation of the IEP at amino acids with more than two pK values ​​only the pK values ​​of the ionizing groups are used similarly. This means that in the case of basic amino acids, the two highest pK values ​​and acidic amino acids, the two lowest pK values ​​are used in the above equation.

Lecithins

Lecithins are phospholipids derived from fatty acids, glycerol, phosphoric acid and choline. They are components of the cell membrane of animal and vegetable organisms allow the emulsification ( mixing ) of fats and water, and are thus important natural surfactants ( emulsifiers) for food and feed as well as in the pharmaceutical industry. Two hydroxyl groups of glycerol are esterified with fatty acids. However, the third - OH group of the glycerol is a phosphate ion, a di- phosphoric acid esters; on the one hand with the other hand, glycerol and with a further group, the choline. Choline is a quaternary ammonium compound, that carries a positive charge and a cation. The phosphate group is present over a wide pH range as the anion, that carries a negative charge. Thus, one can interpret lecithins as zwitterions or Internal salts.