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Amino Acids

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Chemistry

Our genome is amazing. It is made up of just four subunits: bases called A, C, T, and G. In fact, these four bases make up all of the DNA on Earth. The bases are arranged in groups of three called codons, and each codon instructs the cell to bring over one particular molecule. These molecules are called amino acids and our DNA can code for just 20 of them.

Amino acids are organic molecules that contain both the amine () and the carboxyl () functional groups. They are the building blocks of proteins.

Amino acids are joined together in long chains to make proteins. Think of the huge array of proteins on Earth - from structural proteins to hormones and enzymes. They're all coded for by DNA. This means that every single protein on Earth was coded for by just these four bases and made from just 20 amino acids. In this article, we're going to find out more about amino acids, from their structure to their bonding and their types.

  • This article is about amino acids in chemistry.
  • We'll start by looking at the general structure of amino acids before exploring how they can act as both acids and bases.
  • We'll then move on to identifying amino acids using thin-layer chromatography.
  • Next, we'll look at bonding between amino acids to form polypeptides and proteins.
  • Finally, we'll explore different types of amino acids and you'll learn about proteinogenic, standard, and essential amino acids.

The structure of amino acids

As we mentioned above, amino acids contain both the amine () and carboxyl () functional groups. In fact, all of the amino acids we'll be looking at today have the same basic structure, shown below:

Amino Acids structure of amino acids StudySmarter

The structure of amino acids. Anna Brewer, StudySmarter Originals

Let's look more closely at the structure.

  • The amine group and the carboxyl group are bonded to the same carbon, highlighted in green. This carbon is sometimes called the central carbon. Because the amine group is also bonded to the first carbon atom that is joined to the carboxyl group, these particular amino acids are alpha-amino acids.
  • There is also a hydrogen atom and an R group attached to the central carbon. The R group can vary from a simple methyl group to a benzene ring, and is what differentiates the amino acids - different amino acids have different R groups.

Amino Acids examples of amino acids StudySmarterExamples of amino acids. Their R groups are highlighted. Created using images from commons.wikimedia.org

Naming amino acids

When it comes to naming amino acids, we tend to ignore IUPAC nomenclature. Instead, we call them by their common names. We've already showed alanine and lysine above, but some more examples include threonine and cysteine. Using IUPAC nomenclature, these are respectively 2-amino-3-hydroxybutanoic acid, and 2-amino-3-sulfhydrylpropanoic acid.

Further examples of amino acids with their R groups highlighted. Created using images from commons.wikimedia.org

Properties of amino acids

Let's now move on to exploring some of the properties of amino acids. In order to fully understand them, we first need to look at zwitterions.

Zwitterions

Zwitterions are molecules that contain both a positively charged part and a negatively charged part but are neutral overall.

In most states, amino acids form zwitterions. Why is this the case? They don't seem to have any charged parts!

Take a look back at their general structure again. As we know, amino acids contain both the amine group and the carboxyl group. This makes amino acids amphoteric.

Amphoteric substances are substances that can act as both an acid and a base.

The carboxyl group acts as an acid by losing a hydrogen atom, which is really just a proton. The amine group acts as a base by gaining this proton. The resulting structure is shown below:

Amino Acids zwitterion StudySmarterA zwitterion. Anna Brewer, StudySmarter Originals

Now the amino acid has a positively charged group and a negatively charged group. It is a zwitterion ion.

Because they form zwitterions, amino acids have some slightly unexpected properties. We'll focus on their melting and boiling points, solubility, behaviour as an acid, and behaviour as a base. We'll also look at their chirality.

Melting and boiling points

Amino acids have high melting and boiling points. Can you guess why?

You guessed it - it's because they form zwitterions. This means that instead of simply experiencing weak intermolecular forces between neighbouring molecules, amino acids actually experience strong ionic attraction. This holds them together in a lattice and requires a lot of energy to overcome.

Solubility

Amino acids are soluble in polar solvents such as water, but insoluble in nonpolar solvents such as alkanes. Once again, this is because they form zwitterions. There are strong attractions between polar solvent molecules and the ionic zwitterions, which are able to overcome the ionic attraction holding the zwitterions together in a lattice. In contrast, the weak attractions between nonpolar solvent molecules and zwitterions aren't strong enough to pull the lattice apart. Amino acids are therefore insoluble in nonpolar solvents.

Behaviour as an acid

In basic solutions, amino acid zwitterions act as an acid by donating a proton from their group. This lowers the pH of the surrounding solution and turns the amino acid into a negative ion:

Amino Acids zwitterion basic solution StudySmarterA zwitterion in basic solution. Note that the molecule now forms a negative ion. Anna Brewer, StudySmarter Originals

Behaviour as a base

In acidic solution, the opposite happens - amino acid zwitterions act as a base. The negative group gains a proton, forming a positive ion:

Amino Acids zwitterion acidic solution StudySmarterA zwitterion in acidic solution. Anna Brewer, StudySmarter Originals

Isoelectric point

We now know that if you put amino acids in an acidic solution, they'll form positive ions. If you put them in a basic solution, they'll form negative ions. However, in a solution somewhere in the middle of the two, the amino acids will all form zwitterions - they'll have no overall charge. The pH at which this happens is known as the isoelectric point.

The isoelectric point is the pH at which an amino acid has no net electrical charge.

Different amino acids have different isoelectric points depending on their R groups.

Optical isomerism

All of the common amino acids, with the exception of glycine, show stereoisomerism. More specifically, they show optical isomerism.

Take a look at the central carbon in an amino acid. It is bonded to four different groups - an amine group, a carboxyl group, a hydrogen atom and an R group. This means that it is a chiral centre. It can form two non-superimposable, mirror-image molecules called enantiomers which differ in their arrangement of the groups around that central carbon.

Amino Acids stereoisomers StudySmarterTwo general amino acid stereoisomers. Anna Brewer, StudySmarter Originals

We name these isomers using the letters L- and D-. All naturally occuring amino acids have the L- form, which is the left-hand configuration shown above.

Glycine doesn't show optical isomerism. This is because its R group is just a hydrogen atom. Therefore, it doesn't have four different groups attached to its central carbon atom and so doesn't have a chiral centre.

Find out more about chirality in Optical Isomerism.

Identifying amino acids

Imagine that you have a solution containing an unknown mixture of amino acids. They're colourless and seemingly impossible to distinguish. How could you find out which amino acids are present? For this, you could use thin-layer chromatography.

Thin-layer chromatography, also known as TLC, is a chromatography technique used to separate and analyse soluble mixtures.

To identify the amino acids present in your solution, follow these steps.

  1. Draw a line in pencil across the bottom of a plate covered in a thin layer of silica gel.
  2. Take your unknown solution, and other solutions containing a known amino acid to use as references. Place a small spot of each along the pencil line.
  3. Place the plate in a beaker partially filled with a solvent, so the solvent level is below the pencil line. Cover the beaker with a lid and leave the setup alone until the solvent has travelled almost all of the way to the top of the plate.
  4. Remove the plate from the beaker. Mark the position of the solvent front with pencil and leave the plate to dry.

This plate is now your chromatogram. You'll use it to find out which amino acids are present in your solution. Each amino acid in your solution will have travelled a different distance up the plate and formed a spot. You can compare these spots to the spots produced by your reference solutions that contain known amino acids. If any of the spots are in the same position, that means they are caused by the same amino acid. However, you may have noticed a problem - the amino acid spots are colourless. To view them, you need to spray the plate with a substance such as ninhydrin. This dyes the spots brown.

Amino Acids TLC identification StudySmarterThe setup for amino acid identification TLC. The solutions containing known amino acids are numbered for ease of reference. Anna Brewer, StudySmarter Originals

Amino Acids a finished chromatogram StudySmarterThe finished chromatogram, sprayed with ninhydrin. Anna Brewer, StudySmarter Originals

You can see that the unknown solution has produced spots that match those given by amino acids 1 and 3. The solution must therefore contain these amino acids. The unknown solution also contains another substance, which doesn't match any of the four amino acid spots. It must be caused by a different amino acid. To find out which amino acid this is, you could run the experiment again, using different amino acid solutions as references.

For a more detailed look at TLC, check out Thin-Layer Chromatography, where you'll explore its underlying principles and some uses of the technique.

Bonding between amino acids

Let's move on to look at bonding between amino acids. This is perhaps more important than the amino acids themselves, as it is through this bonding that amino acids form proteins.

Proteins are long chains of amino acids joined together by peptide bonds.

When just two amino acids join together, they form a molecule called a dipeptide. But when lots of amino acids join together in a long chain, they form a polypeptide. They join together using peptide bonds. Peptide bonds are formed in a condensation reaction between the carboxyl group of one amino acid and the amine group of another. Because this is a condensation reaction, it releases water. Take a look at the diagram below.

Amino Acids peptide bonding in amino acidsStudySmarter

Here, the atoms that are eliminated are circled in blue and the atoms that bond together are circled in red. You can see that the carbon atom from the carboxyl group and the nitrogen atom from the amine group join together to form a peptide bond. This peptide bond is an example of an amide linkage, .

Have a go at drawing the dipeptide formed between alanine and valine. Their R groups are and respectively. There are two different possibilities, depending on which amino acid you draw on the left and which amino acid you draw on the right. For example, the top dipeptide shown down below features alanine on the left and valine on the right. But the bottom dipeptide has valine on the left and alanine on the right! We've highlighted the functional groups and peptide bond to make them clear for you.

Amino Acids dipeptide StudySmarterThe two dipeptides formed from alanine and valine. Anna Brewer, StudySmarter Originals

Hydrolysis of peptide bonds

You'll have noticed that when two amino acids join together, they release water. In order to break the bond between two amino acids in a dipeptide or a polypeptide, we need to add water back in. This is an example of a hydrolysis reaction and requires an acid catalyst. It reforms the two amino acids.

You'll learn more about polypeptides in Proteins Biochemistry.

Types of amino acids

There are a few different ways of grouping amino acids. We'll explore some of them below.

Learn whether your exam board wants you to know any of these types of amino acids. Even if this knowledge isn't required, it is still interesting to know!

Proteinogenic amino acids

Proteinogenic amino acids are amino acids that are made into proteins during DNA translation.

At the start of the article, we explored how awesome DNA is. Take any known life, unravel its DNA, and you'll find that it encodes for just 20 different amino acids. These 20 amino acids are the proteinogenic amino acids. All of life is based on this meagre handful of molecules.

OK, this isn't the whole story. In actual fact, there are 22 proteinogenic proteins, but DNA only codes for 20 of them. The other two are made and incorporated into proteins by special translation mechanisms.

The first of these rarities is selenocysteine. The codon UGA usually acts as a stop codon but under certain conditions, a special mRNA sequence called the SECIS element makes the codon UGA encode selenocysteine. Selenocysteine is just like the amino acid cysteine, but with a selenium atom instead of a sulfur atom.

Amino Acids Cysteine and selenocysteine StudySmarterCysteine and selenocysteine. Created using images from commons.wikimedia.org

The other proteinogenic amino acid not coded for by DNA is pyrrolysine. Pyrrolysine is encoded for under certain conditions by the stop codon UAG. Only specific methanogenic archaea (microorganisms that produce methane) and some bacteria make pyrrolysine, so you won't find it in humans.

Amino Acids pyrrolysine StudySmarterPyrrolysine. Image credits: commons.wikimedia.org

We call the 20 amino acids coded for in the DNA standard amino acids, and all other amino acids nonstandard amino acids. Selenocysteine and pyrrolysine are the only two proteinogenic, nonstandard amino acids.

When representing proteinogenic amino acids, we can give them either single-letter or three-letter abbreviations. Here's a handy table.

A table of amino acids and their abbreviations. The two nonstandard amino acids are highlighted in red. Anna Brewer, StudySmarter Originals

Essential amino acids

Although our DNA codes for all 20 standard amino acids, there are nine that we can't synthesise fast enough to meet our body's demands. Instead, we must get them by breaking down protein from our diet. These nine amino acids are called essential amino acids - it is essential that we eat enough of them in order to properly support our body.

Essential amino acids are amino acids that can't be synthesised by the body fast enough to meet their demand and must instead come from the diet.

The 9 essential amino acids are:

  • Histidine (His)
  • Isoleucine (Ile)
  • Leucine (Leu)
  • Lysine (Lys)
  • Methionine (Met)
  • Phenylalanine (Phe)
  • Threonine (Thr)
  • Tryptophan (Trp)
  • Valine (Val)

Foods that contain all nine essential amino acids are called complete proteins. These include not only animal proteins such as all types of meat and dairy, but some plant proteins like soy beans, quinoa, hemp seeds, and buckwheat.

However, you don't have to worry about having complete proteins with every meal. Eating certain foods in combination with each other will provide you with all the essential amino acids as well. Pairing any bean or legume with either a nut, seed, or bread will give you all nine essential amino acids. For example, you could have hummus and pitta bread, a bean chilli with rice, or a stir-fry scattered with peanuts.

Amino Acids A stir-fry with all the essential amino acids StudySmarterThis stir-fry contains all the essential amino acids you need. Image credits: pixabay.com

Amino Acids - Key takeaways

  • Amino acids are organic molecules that contain both the amine () and the carboxyl () functional groups. They are the building blocks of proteins.
  • Amino acids all have the same general structure.
  • In most states, amino acids form zwitterions. These are neutral molecules with a positively charged part and a negatively charged part.
  • Amino acids have high melting and boiling points and are soluble in water.
  • In acidic solution, amino acids act as a base by accepting a proton. In basic solution, they act as an acid by donating a proton.
  • Amino acids show optical isomerism.
  • We can identify amino acids using thin-layer chromatography.
  • Amino acids join together using a peptide bond to form polypeptides, also known as proteins.
  • Amino acids can be classified in different ways. Types of amino acids include proteinogenic, standard, essential and alpha amino acids.

Amino Acids

The simplest amino acid is glycine. Other examples of amino acids are valine, leucine, and glutamine.

There are hundreds of different amino acids, but only 22 are found in living organisms and only 20 are coded for by DNA. For humans, nine of these are essential amino acids, meaning we can't make them in large enough quantities and must get them from our diet.

Amino acids are organic molecules that contain both the amine and the carboxyl functional groups. They are the building blocks of proteins.

Essential amino acids are amino acids that the body can't make in large enough quantities to meet demand. This means that we have to get them from our diet.

Amino acids are the building blocks of proteins. Proteins have a variety of different roles, from structural proteins in your muscles to hormones and enzymes.

Final Amino Acids Quiz

Question

What are amino acids?

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Answer

Organic molecules that contain both the amine and the carboxyl functional groups.

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Question

What is a polypeptide?

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Answer

A long chain of amino acids joined together by peptide bonds.

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Question

What is a polypeptide?

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Answer

A long chain of amino acids joined together by peptide bonds.

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Question

What is a dipeptide?

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Answer

2 amino acids joined by a peptide bond.

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Question

Different amino acids can have the same R group. True or false?

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Answer

False

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Question

True or false? Amino acids only form zwitterions in aqueous solution.

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Answer

False

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Question

What is a zwitterion?

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Answer

A molecule that contains both a positively charged part and a negatively charged part but is neutral overall. 

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Question

True or false? Amino acids are amphoteric.

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Answer

True

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Question

Amino acids have relatively _____ melting and boiling points.

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Answer

High

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Question

Amino acids are _____ in water and other polar solvents.

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Answer

Soluble

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Question

Amino acids are _____ in nonpolar solvents.

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Answer

Insoluble

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Question

All amino acids show chirality. True or false?

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Answer

False

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Question

Which technique can we use to identify amino acids?

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Answer

Thin-layer chromatography

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Question

What substance do we use to show up amino acids on a chromatogram?

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Answer

Ninhydrin

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Question

Name the bond formed between two amino acids.

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Answer

Amine bond

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Question

Two amino acids react together in a _____ reaction.

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Answer

Condensation

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Question

Breaking a peptide bond requires ____ and _____.

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Answer

Water, an acid catalyst

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Question

What are proteinogenic amino acids?

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Answer

Amino acids that are made into proteins during DNA translation. 

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Question

What are essential amino acids?

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Answer

Amino acids that can't be synthesised by the body fast enough to meet their demand and must instead come from the diet. 

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Question

How many essential amino acids are there?

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Answer

9

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Question

Amino acids are ______.

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Answer

Positively charged

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