Amino acids amphiprotic behavior

The doubly ionized form, called a zwitterion, usually predominates at near-neutral pH values. This form is a result of the transfer of a proton from the carbox)1ic acid group to the amine group. (Section 16.10 Chemistry and Life The Amphiprotic Behavior of Amino Acids )... 

An amphoteric species is a molecule or ion that can react as an acid as well as a base. One type of amphoteric species are amphiprotic molecules, which can either donate or accept a proton examples are amino acids and proteins, which have amine and carboxylic groups. The competition between the acid-base equilibria of these groups leads to additional complications in their physical behavior but can also present some advantages due to the variation of their charge as a function of pH. 

This type of behavior has been observed also for proteins. The solubility of a protein built out of amino acids is minimum when pH = pi, the isoelectric point. The minimum is a point not a band. As the pH moves away from the isoelectric point, the protein solubility increases. The presence of a net charge on the protein molecule increases its solubilization due to its interaction with dipolar water molecules (H+ as well as OH ). Kirwan and Orella (1993) have illustrated broadly a similar solubility behavior for other biological molecules of smaller molecular weight which produce zwitterions. Observe the solubility VcUiation with pH of amphiprotic solutes such as the p-lactam antibiotics, ampidllin, amoxicillin, etc., as determined by Tsujiet aZ. (1978) (Figure 5.2.11(b)). If the species is susceptible to hydrolysis at high pH, then pH effects are observed only at lower pH in the range of pH < pKi < pi. 

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