Deduce Amino Acid Structure at Different pH

Let's learn how to deduce the structure of amino acid at different pH using aspartic acid as an example.

pKa1 for alpha acid = 1.88
pKa2 for R group acid = 3.65
pKa3 for alpha amine = 9.60

At a certain pH, we compare the pH of solution against pKa of each functional group.

If the solution is more alkaline than acid FG (pH greater than pKa), the acid will react to form conjugate base (-COO-).

If the solution is more acidic than amine FG (pH less than pKa), the amine will react to form conjugate acid (-NH3+).

Let's deduce the structure of aspartic acid at pH 3, 7 and 11.

1. At pH 3

Alpha acid, pH greater than pKa1, solution alkaline hence acid FG reacts to form conjugate base
R group acid, pH less than pKa2, solution acidic hence acid FG no reaction
Alpha amine, pH less than pKa3, solution acidic hence amine FG reacts to form conjugate acid

2. At pH 7

Alpha acid, pH greater than pKa1, solution alkaline hence acid FG reacts to form conjugate base
R group acid, pH greater than pKa2, solution alkaline hence acid FG reacts to form conjugate base
Alpha amine, pH less than pKa3, solution acidic hence amine FG reacts to form conjugate acid

3. At pH 11

Alpha acid, pH greater than pKa1, solution alkaline hence acid FG reacts to form conjugate base
R group acid, pH greater than pKa2, solution alkaline hence acid FG reacts to form conjugate base
Alpha amine, pH greater than pKa3, solution alkaline hence amine FG no reaction

Topic: Nitrogen Compounds and Proteins, Organic Chemistry, A Level Chemistry, Singapore

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