Alanine (Ala) is a diprotic acid that can undergo two dissociation reactions wit

Question

Alanine (Ala) is a diprotic acid that can undergo two dissociation reactions with pKas, (2.34 for COOH and 9.69 for NH3 ).

a)Of the three possible forms of Ala, which form would be present at the highest concentration in a solution of
pH = 1.0, pH = 6.2, pH = 8.02,pH = 11.9

Explain/show your reasoning in terms of pH relative to the two pKas.

Explanation / Answer

At pHpKa, we have deprotonation. When a base is protonated (NH2-->NH3+), it has a +1 charge. No charge when deprotonated. When an acid is deprotonated (COOH to COO-), it has -1 charge. No charge when protonated. At pH1: Amino group is protonated (NH3+), the carboxyl group is also protonated (COOH). At pH6.2: Amino group is protonated (NH3+), the carboxyl group is deprotonated (COO-) At pH 8.02: Amino group is protonated (NH3+), the carboxyl group is deprotonated (COO-) At pH 11.9: Amino group is now deprotonated (NH2), the carboxyl group is deprotonated (COO-).

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