Amphoteric electrolytes Amino-acids

It is interesting to note that the amino acid side chains may be either neutral as in valine, acidic as in glutamic acid or basic as in lysine. The presence of both acidic and basic side chains leads to proteins such as casein acting as amphoteric electrolytes and their physical behaviour will depend on the pH of the environment in which the molecules exist. This is indicated by Figure 30.2, showing a simplified protein molecule with just one acidic and one basic side group. 

Substances having this dual nature are amphoteric and are often called ampholytes (from amphoteric electrolytes ). A simple monoamino monocarboxylic a-amino acid, such as alanine, is a diprotic acid when fully protonated—it has two groups, the —COOH group and the —NHj group, that can yield protons ... 

Theoretically, the precipitation of a protein, which is built up of amino-acids and functions as a typical amphoteric electrolyte, should best be accomplished at the isoelectric point, that is, at the hydrogen ion concentration at which the acidic and basic functions of the protein are equal and at a minimum. According to Soerensen, the iso-electric point of egg albumin is at pH 4.8,10 and while precipitation would undoubtedly be most complete at this point, it is not necessarily the most favorable for crystallization, as the egg albumin crystals are not isoelectric protein, but a compound of this with the sulfate ion, and perhaps the ammonium ion as well.11... 

Electrophoresis is more often used to separate proteins than amino acids. The degree of separation obtained for amphoteric electrolytes depends on the pH of the buffer and on the isoelectric point of the substances. This experiment illustrates the use of a horizontal low voltage paper method, which separates acidic, basic and neutral amino acids, but is not satisfactory for separating the members of one group from each other. 

J. Walker (see p. 681) considered that amino-acids such as glycocoll, NHa CHg-COOH, which can function as weak acids and weak bases and hence are called amphoteric electrolytes, behave as follows ... 

Hidaka et al. [77] reported that amphoteric Af-(2-hydroxyethyl)- -(2-hydroxyalkyl)-P-alanines s cmc values greatly depend on the nature of the electrolytes added to its nearly neutral aqueous solution, and that the cmc value decreased in the following order NaCl > CaClj > Na2S04. Also, their calcium stability is superior to that of A-dodecyl-P-alanines. The effect of pH on this amphoteric amino acid surfactant was smdied in the presence of 0.1 M NaCl [78] and the results showed that the cmc value increased on the acidic side below the isoelectric point pi = 6.8 and remained almost unchanged on the alkaline side. Examination of the configuration with molecular models indicates that the cationic ionization of the amino group on the acidic side probably takes place within the micelle, whereas under alkaline circumstances the anionic ionization of the carbonyl group occurs on the micellar surface. This makes the electrostatic potential for ionization different on the acidic and alkaline sides. 

Dipolai Form of the Amino Acids.—The monoamino-monocarboxy acids are neutral in solution, and are very weak electrolytes. At the same time they are able to neutralise either acids or bases. This property, termed amphotericity, is due to the presence of an acid and a basic group in the same molecule. In aqueous solutions amino acids ionise to form a dipolar or zwitter-ion, having two equal charges of opposite electric sign, and tending to migrate neither to anode nor cathode when a current is passed through the solution. 

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