Amino Acids & Proteins (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about amino acids and the proteins they build.

The two are covered together because the same two functional groups that make an amino acid amphoteric are the ones that join amino acids into proteins.

An amino acid is an organic compound with two functional groups: a basic amino group, and an acidic carboxylic acid group. Having both makes it amphoteric, so it can act as an acid and as a base. Those same two groups also react with each other: the amino group of one amino acid and the carboxylic acid group of another join in a condensation reaction, and that is how proteins are built. Hydrolysing that link takes you straight back to the amino acids you started with.

We'll start with the amino acid itself, then zwitterions and the isoelectric point, then the peptide link, then the levels of protein structure, and finish with hydrolysis and identifying amino acids by chromatography.

Amino acids are organic compounds containing two functional groups: a basic amino group, and an acidic carboxylic acid group. Because they carry both, they are amphoteric, which means they can act as acids and as bases.

The ones that build proteins are 2-aminocarboxylic acids, where the amino group is bonded to the carbon next to the carboxylic acid group. There are twenty naturally occurring amino acids, with the general formula RCH(NH2)COOH, and the R group is what varies between them which can be acidic, basic or neutral. Those two functional groups are what the rest of this video is about, because they are what react to build proteins.

Amino acids undergo the reactions you would expect of amines and of carboxylic acids, but they can also interact within themselves. That gives a zwitterion: a single ion carrying both a positive charge and a negative charge. Those charges mean strong intermolecular forces of attraction between amino acids, which is why they are soluble crystalline solids.

In water they exist as zwitterions and act as buffer solutions, resisting small changes in pH. Add acid and the negative part accepts a hydrogen ion, so the ion becomes positively charged. Add base and the positive part donates a hydrogen ion, so it becomes negatively charged. The pH at which neither dominates, and the amino acid exists as a neutral zwitterion, is the isoelectric point.

The amino group of one amino acid reacts with the carboxylic acid group of another in a condensation reaction, and because it is a condensation, a molecule of water is eliminated. The new bond is an amide bond, also called a peptide link, and the product is a dipeptide.

A dipeptide still has an amino group at one end and a carboxylic acid group at the other, so it can condense again to give a tripeptide, and when many amino acids join together the result is a polypeptide. This is the same pair of groups that made the molecule amphoteric, now being used to build a chain.

Proteins have four levels of structure: three relate to a single polypeptide chain, and the fourth to proteins made of two or more chains. Chain lengths run from three amino acids up to more than thirty-four thousand.

The primary structure is the sequence of amino acids joined by covalent peptide bonds, and it is specific to each protein, so one change in the sequence can affect how the protein functions. The secondary structure comes from hydrogen bonds along the backbone. An alpha-helix, where bonds form between every fourth peptide bond, or a beta-pleated sheet, where two parts of the chain lie parallel to each other. The tertiary structure adds bonds between the R groups: hydrogen bonds, disulphide bonds between cysteines, ionic bonds, and hydrophobic interactions.

Hydrolysis is the reverse of condensation: water is added, the peptide bonds break, and the polypeptide is broken back down to its amino acids. It can be done chemically, using concentrated hydrochloric acid and boiling for many hours because the reaction is slow, or with an enzyme, which works at room temperature.

Once you have the amino acids, thin layer chromatography identifies them. They share the same basic structure, but the R group changes the overall polarity, so they rise up the plate at different rates. Amino acids are colourless, so the plate is sprayed with a locating agent such as ninhydrin, or viewed under UV light, and each spot's Rf value identifies it. Where two amino acids give similar values in the same solvent, two-dimensional TLC runs the plate through a second solvent at ninety degrees to the first.

You should be able to draw the peptide formed by joining up to three amino acids, so practise a dipeptide and a tripeptide until the peptide link comes automatically.

Naturally occurring amino acids are usually referred to by a traditional name and a three-letter code, because the IUPAC systematic names can be rather complicated. Aspartic acid, serine and alanine are abbreviated to Asp, Ser and Ala.

An amino acid carries a basic amino group and an acidic carboxylic acid group, which is what makes it amphoteric and what lets it exist as a zwitterion. Those same two groups condense to form the peptide link, and many links give a polypeptide. Hydrogen bonds and disulphide bonds then hold that chain in its secondary and tertiary shape. Hydrolysis breaks the links and gives the amino acids back, and thin layer chromatography identifies them. The amino group and the acid group do both jobs.

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