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Amino Acids



Amino Acids, Proteins & DNA

AQA Content

Draw the structures of amino acids as zwitterions and the ions formed from amino acids in both acid & alkaline conditions.

Specification Notes

Amino acids have both acidic and basic properties, including the formation of zwitterions.


The Structure of Amino Acids

Amino acids are the building blocks of proteins, and all proteins are made up of combinations of different amino acids. Each amino acid is composed of a central carbon atom, which is a chiral carbon, to which two functional groups are attached: a carboxylic acid group (-COOH) and an amine group (-NH2). Additionally, each amino acid has a unique variable group, referred to as the 'R' group.

All amino acids, except glycine, exhibit optical isomerism, meaning they form mirror image isomers. These isomers can be distinguished by the direction in which they rotate plane-polarized light.

Amino acids exist in the form of zwitterions, which are molecular structures that have both permanent positive and negative charges. This allows them to act as both acids and bases, with the carboxylic acid group losing a proton to act as an acid and the amine group gaining a proton to act as a base. The resulting zwitterions have a neutral charge overall.

In solution, zwitterions exist at their own unique pH and have strong electrostatic forces of attraction between them. This accounts for several properties of amino acids, such as their tendency to exist in the form of a crystalline white solid at room temperature, their high melting points, and their ability to dissolve well in water.

In acidic conditions, the amine group (NH2) of the zwitterion accepts a proton, forming a positive ion. In alkaline conditions, the carboxylic acid group (COOH) loses a proton, forming a negative ion. This change in protonation state is referred to as deprotonation.


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