Amphoteric electrolytes neutralization

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. 

The isoelectric poiat of an amphoteric electrolyte is likewise displaced by the addition of neutral salts as is instanced by the following values for chromated gelatine in the presence of various neutral salts. 

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. 

Hydrogels may be categorized into nonionic (neutral), ionic, an amphoteric electrolyte containing both acidic and basic groups and Zwitterionic containing both anionic and cationic groups in each structural repeating unit. 

The process described is referred to as ion-exclusion as discussed by Asher and Simpson 9. The resins used are normal and the non-ionic molecules are assumed to be small enough to enter the pores. When large non-ionic molecules are involved, an alternative process called ion-retardation may be used, as discussed by Hatch et al. W]. This requires a special resin of an amphoteric type known as a snake cage poly electrolyte. The polyelectrolyte consists of a cross-linked polymer physically entrapping a tangle of linear polymers. For example, an anion exchange resin which is soaked in acrylic acid becomes entrapped when the acrylic acid is polymerised. The intricacy of the interweaving is such that counter-ions cannot be easily displaced by other counter-ions. On the other hand, ionic mobility within the resin maintains the electro-neutrality. The ionic molecule as a... 

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. 

FLUORAD Surfactant FC-100 is a general purpose amphoteric surfactant characterized by outstanding surface activity and solubility in aqueous solutions over a wide pH range. It is highly effective in solutions having a neutral pH, particularly those with a high electrolyte content. 

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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