Zwitterion: Structure & Function

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  • 0:00 What is a Zwitterion?
  • 0:30 Amino Acids as Zwitterions
  • 1:19 Zwitterion Character…
  • 1:58 Other Examples of Zwitterions
  • 2:50 Functions of Zwitterions
  • 4:01 Lesson Summary
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Lesson Transcript
Instructor: Nancy Devino
Anions are negatively charged ions, and cations are positively charged ions. Zwitterions are ions with both a negative and a positive charge. In this lesson, we'll look at some examples of zwitterions and learn about their function.

What is a Zwitterion?

Let's take the word zwitterion apart to figure out what it means. There's the -ion part at the end, which is a chemical species with a charge. The first part of the name comes from the German word 'zwitter', meaning hermaphrodite or hybrid. In other words, this term means half anion and half cation. Zwitterions are sometimes called dipolar ions, because they have a negative end (the anion) and a positive end (the cation).

Amino Acids as Zwitterions

Amino acids are the building blocks of proteins in living cells. They are compounds that contain an amino group and a carboxyl group. Twenty different amino acids are found in proteins. They share the structure shown here, where R represents one of the 20 possible side chains on an amino acid.

General structure of an amino acid

At neutral pH values, the amino group (-NH3+) has a positive charge and the carboxyl group (COO-) has a negative charge. Here is what the simplest amino acid, glycine, looks like in its zwitterion form:

Zwitterion form of glycine

The other interesting thing about zwitterions is if there are no other charged groups in the molecule, then a zwitterion has no net charge. That's right - a positive charge (+1) and a negative charge (-1) add up to zero.

Zwitterion Character Depends on pH

Zwitterions form of a compound at neutral pH, but it turns out that a zwitterion isn't always a zwitterion. At pH values far above or far below 7, its groups can take on different charges - or become neutral. At low, or acidic, pH values, the hydrogen ions add to the carboxyl group, making it neutral. This gives the amino acid a net charge of +1. At high, or basic, pH values, a hydrogen ion on N is removed by the excess base, neutralizing the amino group. This gives the amino acid a net charge of -1.

Amino acids lose their zwitterion character at low and high pH.

Other Examples of Zwitterions

Amino acids are what most people think of first when they think of zwitterions. But other groups of compounds can also have the same dipolar ion structure as amino acids. You've probably heard of sulfuric acid (H2SO4) and sulfurous acid (H2SO3), but sulfamic acid (NH3SO3) is less common. It's a replacement for hydrochloric acid in cleaning products because it's good at removing lime and rust. Notice the similarity to amino acid zwitterions - nitrogen has a + charge, and oxygen has a - charge.

Sulfamic acid in zwitterion form
sulfamic acid

The image below shows another kind of zwitterion. Sometimes the + and - groups are at one end of a very long molecule. These are common in molecules like detergents. As we'll see in the next section, their function is very closely related to their structure.

Some detergents have zwitterions at one end.

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