Zwitterions — Definition & Importance

Zwitterions

What are Zwitterions?

• Zwitterions are molecules/ions that contain an equal number of both positively and negatively charged functional groups.
• Zwitterions are overall neutral molecules. This is because the equal number of positively and negatively charged groups cancel each other out, resulting in a neutrally charged molecule.
• Because of the positively and negatively charged groups, zwitterions are capable of forming strong dipole moments.
• Zwitterions are also called inner salts.

Examples of Zwitterions

• Amino Acids, which are the building blocks of proteins, are one of the most common examples of zwitterions.
• Amino acids contain both a carboxyl group (negatively charged) and a amine group (positively charged).

A Quick Introduction to Organic Chemistry

Today, we're going to look at some unique molecules. But, to understand, you need to know a smidge of organic chemistry. Organic chemistry is the study of carbon compounds. Remember, carbon is a diverse element. Because of its hybridization, it can covalently bond to many different atoms. But, chemists are excellent at recognizing patterns. They noticed that some groupings of atoms always behave similarly. The fancy term is functional groups. They are clusters of atoms that impart chemical properties. Let's look at a couple of common functional groups.

Acid and Base Functional Groups

When we studied acids and bases, we encountered acetic acid. It looked different from our typical acids. It had carbon! Do you remember its formula? It's $CH_{3}COOH$. The $-COOH$ is the carboxylic acid functional group. Carboxylic acids will always act as a weak Arrhenius acid. It doesn't matter what other atoms are attached. COO-, the conjugate base, is known as the carboxylate group.

So now we know there are acidic functional groups. It shouldn't surprise you that there are also basic functional groups. The most common is the amine group, $-NH_{2}$. Nitrogen is in group 5 of the periodic table. So, we know that it bonds three times to complete its octet. Amines are compounds with carbon-nitrogen single bonds. If it has a double or triple bond, it's a different functional group. The most common case is one carbon-nitrogen bond with two nitrogen-hydrogen bonds. But, it can have up to three carbon-nitrogen bonds. What makes an amine basic? It has a lone electron pair. So, it can act as a weak Lewis base.

Zwitterions

Now, are you wondering what happens if a carbon bonds to both an amine and carboxylic acid? You get a unique compound called a zwitterion. Zwitterion are molecules that contain both positively and negatively charged functional groups. But, they are electrically neutral. What does that have to do with acids and bases? When a carboxylic acid donates its hydrogen, it creates a negatively charged conjugate. What about the amine? It can donate its lone pair to a hydrogen ion. So, it ends up with a positive charge.

What's an example of a zwitterion? An amino acid! It's literally in the name! They have an amine group and a carboxylic acid group. In case you forgot, you probably learned about amino acids in biology. They are the building blocks of proteins! If you take advanced chemistry classes, like biochemistry, you'll have to memorize all the amino acids. You'll also learn, in great detail, about how they bond and fold into proteins.

Acid-Base Calculations for Zwitterions

So, zwitterions have both acidic and basic components. That means we should be able to do acid-base calculations. You might assume the pH is neutral. But, each functional group has different $pK_{a}$ values. So, the charge on the functional group depends on the pH. The isoelectric point is the specific pH where the molecule is neutral. Its symbol is $pH_{(I)}$. How do we calculate the isoelectric point? We use the formula, $\frac{pKa_{acid}+pKa_{base}}{2}$.

Let's look at an example. The molecular formula for alanine is $C_{3}H_{7}NO_{2}$. So it has a central carbon bonded to a methyl group, hydrogen, a carboxyl group, and an amine group. To help you visualize, I'm going to write it as $CH_{3}-CH(COOH)(NH_{2})$. The pKa's are 2.35 and 9.87. What is the isoelectric point?

First, let's get the answer by plugging it into the formula.

$\frac{pKa_{acid}+pKa_{base}}{2}\rightarrow \frac{9.87+2.35}{2}\rightarrow \frac{12.22}{2}=6.11$

So, we have our answer. When pH=6.11, the molecule is neutral. But what's happening at different pH values?

Below pH=2.35, we have very acidic conditions. So, the carboxylic acid won't lose its hydrogen. But, the amine will absorb hydrogen ions from the solution. Our molecule looks like $CH_{3}-CH(COOH)(NH^{+}_{3})$. The total charge is positive.

Above pH=2.35, the carboxylic acid begins to deprotonate. At the same time, the amine also absorbs hydrogen. If both happen, our molecule looks like $CH_{3}-CH(COO^{-})(NH^{+}_{3})$. It's neutral! This is the Zwitterion! But, let's say we're at pH=2.4. It's above 2.35 but, not by much. Very few molecules will be the Zwitterion.

At pH=6.11, the average charge of all the molecules is zero. So, $CH_{3}-CH(COO^{-})(NH^{+}_{3})$, best represents our molecule.

Above pH=9.87, we have extremely basic conditions. So, our carboxylic acid will deprotonate. But, the amine will be neutral. Our molecule looks like $CH_{3}-CH(COO^{-})(NH_{2})$. So, we have an overall negative charge.

(Video) How To Calculate The Isoelectric Point of Amino Acids and Zwitterions

by The Organic Chemistry Tutor

In this video, The Organic Chemistry Tutor explains the pH calculations of zwitterions. He works through three different amino acid examples. Each one is a little more complex. He explains why the isoelectric pH produces the zwitterion by showing each structural formula. If you're looking for a deeper understanding of zwitterions, this video can help!