What is the predominant form of each of the following amino...

Name: Problem 37, Chapter 29: Organic Chemistry
Uploaded: 2020-12-25T17:51:11-08:00
Duration: 1 min 48 s
Channel: David Collins
Description: What is the predominant form of each of the following amino acids at pH = 1? What is the overall charge on the amino acid at this pH? (a) threonine; (b) methionine; (c) aspartic acid; (d) arginine

What is the predominant form of each of the following amino acids at pH = 1? What is the overall charge on the amino acid at this pH? (a) threonine; (b) methionine; (c) aspartic acid; (d) arginine

Key Concepts

Acid-Base Chemistry of Amino Acids

Amino acids are molecules that contain both acidic (carboxyl) and basic (amino) functional groups. Their behavior in solution depends on the ionization of these groups, which changes with the pH of the environment. This dual nature allows amino acids to act as buffers and determines their net charge under different conditions.

pKa Values and pH Influence

The pKa of an ionizable group is the pH at which half of the molecules of that group are deprotonated. Comparing the ambient pH to these pKa values predicts whether the group is protonated or deprotonated. At pH values lower than a group's pKa, the group is predominantly protonated, which is crucial for determining the overall charge of amino acids.

Protonation and Overall Charge Determination

To determine the overall charge of an amino acid, one must consider the ionization state of the carboxyl group, amino group, and any ionizable side chains. At a low pH like 1, carboxyl groups tend to be protonated (and thus neutral), while amino groups are protonated, carrying a positive charge. The overall net charge is the sum of these individual charges.

Side Chain Ionizability

Certain amino acids have side chains that can also donate or accept protons, influencing the overall charge of the molecule. For example, acidic side chains (with carboxyl groups) and basic side chains (with guanidinium or similar groups) have their own pKa values. The protonation state of these side chains at a given pH further modulates the net charge of the amino acid.

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