Amino Acids
- Amino acids are organic compounds that contain two functional groups:
- A basic amino (-NH2) group
- An acidic carboxylic acid (-COOH) group
- Due to the presence of both a basic and acidic group in amino acids, they are said to be amphoteric
- They can act as both acids and bases
Naturally occurring amino acids
- 2-aminocarboxylic acids are a type of amino acids in which the amine (-NH2) group is bonded to the carbon atom next to the -COOH group
- These type of amino acids form the ‘building blocks’ that make up proteins
- There are 20 naturally occurring amino acids with the general structural formula of RCH(NH2)COOH
General structural formula of amino acids
- The R group varies in different amino acids and can be:
- Acidic
- Basic
- Neutral
The R group varies in different amino acids
Acid / base properties of amino acids
- Amino acids will undergo most reactions of amines and carboxylic acids including acid-base reactions of:
- Amines with acids
- Carboxylic acids with bases
- However, they can also interact intramolecularly (within themselves) to form a zwitterion
- A zwitterion is an ion with both a positive (-NH3+) and a negative (-COO-) charge
- Because of these charges in a zwitterion, there are strong intermolecular forces of attraction between amino acids
- Amino acids are therefore soluble crystalline solids
An amino acid molecule can interact within itself to form a zwitterion
Isoelectric point
- A solution of amino acids in water will exist as zwitterions with both acidic and basic properties
- They act as buffer solutions as they resist any changes in pH when small amounts of acids or alkali are added
- If an acid is added (and thus the pH is lowered):
- The -COO- part of the zwitterion will accept an H+ ion to reform the -COOH group
- This causes the zwitterion to become a positively charged ion
- If a base is added (and thus the pH is raised):
- The -NH3+ part of the zwitterion will donate an H+ ion to reform the -NH2 group
- This causes the zwitterion to become a negatively charged ion
A solution of amino acids can act as a buffer solution by resisting any small changes in pH
- The pH can be slightly adjusted to reach a point at which neither the negatively charged or positively charged ions dominate and the amino acid exists as a neutral zwitterion
- This is called the isoelectric point of the amino acid
The isoelectric point of amino acids is the pH at which the amino acid exists as a neutral zwitterion
Examiner Tip
Naturally occurring amino acids are usually referred to by a traditional name and a three letter code as the IUPAC systematic names can be rather complicated. For example, aspartic acid, serine and alanine are abbreviated to Asp, Ser and Ala, respectively.