Zwitterionic ampholytes

This group of ampholytes is characterised by the relation pKacidic < pKbasic. The most common examples of zwitterionic ampholytes are the amino acids, peptides and proteins. There are essentially two types of zwitterionic electrolyte depending on the difference between the pKfidic and pKbasic values, ApKa.

Large ApKa. The simplest type to consider is that of compounds having two widely separated pKa values, for example glycine. The pKa values of the carboxylate and amino groups on glycine are 2.34 and 9.6, respectively and the changes in ionisation as the pH is increased are described by the following equilibria:

HOOCCH,NH3

OOCCH,NH+

e -OOCCH2NH2

Over the pH range 3-9, glycine exists in solution predominantly in the form OOCCH2NH+ Such a structure, having both positive and negative charges on the same molecule, is referred to as a zwitterion and can react both as an acid,

OOCCH2NH2 + H3O+

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