Isoelectric and Isoionic pH

Biochemists speak of the isoelectric or isoionic pH of polyprotic molecules, such as proteins. These terms can be understood in terms of a diprotic system, such as the amino acid alanine. 

The isoionic point (or isoionic pH) is the pH obtained when the pure, neutral polyprotic acid HA (the neutral zwitterion) is dissolved in water. The only ions are H2A+, A-, H+, and OH. Most alanine is in the form HA, and the concentrations of H2A+ and A are not equal to each other. 

The isoelectric point (or isoelectric pH) is the pH at which the average charge of the polyprotic acid is 0. Most of the molecules are in the uncharged form HA, and the concentrations of H2A+ and A- are equal to each other. There is always some H2A+ and some A in equilibrium with HA. 

When alanine is dissolved in water, the pH of the solution, by definition, is the isoionic pH. Because alanine (HA) is the intermediate form of the diprotic acid, H2A+, [H+] is given by 

From [H+], A , and K2, you could calculate [H2A+] = 1.68 X 10 5M and [A ] = 1.76 X 10 5 M for pure alanine in water (the isoionic solution). There is a slight excess of A- because HA is a slightly stronger acid than it is a base. It dissociates to make A a little more than it reacts with water to make H2A+. 

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Calculate the isoelectric and isoionic pH of 0.010 M threonine. 10-39. Explain how isoelectric focusing works. 

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