Deduce Amino Acid Structure at Different pH

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

deduce amino acid structure at diff pH aspartic acid

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 > pKa), the acid will react to form conjugate base (-COO-).

If the solution is more acidic than amine FG (pH < 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

deduce amino acid structure at diff pH deduce pH 3

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

Hence structure of aspartic acid at pH 3:

deduce amino acid structure at diff pH structure at pH 3

2. At pH 7

deduce amino acid structure at diff pH deduce pH 7

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

Hence structure of aspartic acid at pH 7:

deduce amino acid structure at diff pH structure at pH 7

3. At pH 11

deduce amino acid structure at diff pH deduce pH 11

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

Hence structure of aspartic acid at pH 11:

deduce amino acid structure at diff pH structure at pH 11

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

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