34.4 Amino Acids

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Describe the acid/base properties of amino acids and the formation of zwitterions, including the isoelectric point.
  2. Describe the formation of amide (peptide) bonds between amino acids to give di- and tripeptides.
  3. Interpret and predict the results of electrophoresis on mixtures of amino acids and dipeptides at varying pHs.

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1. What Are Amino Acids?

Amino acids are the building blocks of proteins. Every amino acid molecule contains two important functional groups:

  • An amino group (–NH₂) — this is the basic part of the molecule (it can accept a proton, H⁺).
  • A carboxylic acid group (–COOH) — this is the acidic part of the molecule (it can donate a proton, H⁺).

Because the same molecule contains both an acidic group and a basic group, amino acids are called amphoteric — meaning they can behave as either an acid or a base depending on the conditions around them.

The general structure of an amino acid looks like this:

        H   R   O
        |   |   ||
   H₂N–C–C–OH
        |
        H

Or more simply written as: H₂N–CHR–COOH

The R group (also called the side chain) is different for every amino acid. It is just a variable part of the molecule that gives each amino acid its unique identity. For example, in the simplest amino acid, glycine, R = H.

2. Acid and Base Properties of Amino Acids

Because amino acids have both –NH₂ and –COOH groups, they can react with both acids and bases.

Reacting with an acid (behaving as a base):

The –NH₂ group accepts a proton (H⁺) from the acid:

–NH₂ + H⁺ → –NH₃⁺

The amino group gains a positive charge.

Reacting with a base (behaving as an acid):

The –COOH group donates a proton (H⁺) to the base:

–COOH → –COO⁻ + H⁺

The carboxylic acid group loses its proton and becomes negatively charged.

3. Zwitterions and the Isoelectric Point

What is a Zwitterion?

In water (aqueous solution), something interesting happens inside an amino acid molecule. The –COOH group donates its proton (H⁺) to the –NH₂ group — within the same molecule. This internal proton transfer creates a molecule that has:

  • A positive charge on the –NH₃⁺ part
  • A negative charge on the –COO⁻ part

This doubly-charged form is called a zwitterion (from the German word for "twin ion"). The molecule has both charges at the same time, but the overall charge is zero because the positive and negative cancel each other out.

The zwitterion form looks like this:

⁺H₃N–CHR–COO⁻

This is the form that most amino acids exist in when dissolved in water at a specific pH.

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