What is an Amino Acid?
Amino acids contain two functional groups: amine (NH2) and carboxyl (COOH)
Examples of Amino Acids.
An Amino Acid must have an amine and carboxyl functional group attached to a carbon (as well as a Hydrogen and R group) This R is what varries between different amino acids (e.g. it could by a single hydrogen, methyl group, etc)
Optical Isomerism of Amino Acids
Optical isomers exist when a moles cule contains an asymmetric carbon atom.
Asymmetric carbon atoms have four different atoms or groups attached to it.
For instance an amino acid can have a carbon with a carboxyl group (COOH), amine group (NH2), and two different R groups (for instance the hydrogen atom and a different R group, e.g. methyl group). Glycine is not an optical isomer as the carbon has two hydrogens attached, same functional group.
Asymmetric carbon atoms cause two isomers to be formed.
One isomer will rotate plane polarised light to the left and the other isomer will rotate plane polarised light to the right.
More on Optical Isomers
Optical isomers are like mirror images.
A Zwitterion is a dipolar ion, meaning it has a plus and minus charge in its structure.
Zwitterions increase intramolecular forces (as the positive and minus charges attract between molecules) raising melting and boiling points.
Amino acids can exist as zwitterions.
A extra proton forms a dative bond with the lone pair on the Nitrogen atom to create a positive charge on the amine functional group. A proton is lost of the O-H bond in the carbonyl group to leave a negative charge.
Alpha (α) and Gamma (γ) Amino Acids
Amino acids are bifunctional compounds as they contain 2 functional groups (amine and carbonyl) Alpha amino acids have the amino group bonded to the carbon atom ajacent to the carbonyl group. Because the carbon atom is second from the acid, they can be called 2-amino acids. Other amino acids are referred to as beta, gamma, etc
Beta = 3-amino acid Gamma = 4-amino acid
Only alpha amino acids occur as a part of proteins, other amino acids may occur naturally or be synthesised.
Base Properties of Amino Acids
Amino acids possess acidic and basic properties due to their functional groups. They will from salts when treated with acids or alkalis.
1. react with H+
HOOCCH2NH2 + H+ --> HOOCCH2NH3+
2. react with HCl
HOOCCH2NH2 + HCl --> HOOCCH2NH3+Cl-
Glycerol Hydrochloric acid A chloride
Acid Properties of Amino Acids
Remember: An acid and a base form a salt and water
1. react with OH-
HOOCCH2NH2 + OH- ---> -OOCCH2NH2 + H2O
2. react with NaOH
HOOCCH2NH2 + NaOH ---> Na+-OOCCH2NH2 + H2O
Amino acids can join togwether to form peptides via an amide or peptide link. Peptides are a series of amino acids joined together by peptide links.
2 amino acids joined together = dipeptide
3 amino acids joined together = tripeptide
many amino acids joined together = polypeptide
Hydrolysis of Peptides
Hydrolysis is the chemical breakdown of a molecule via water.
Adding water to a peptide via hydrolisis breaks down the molecule to the amino acids that it was formed from.
Attack on Peptide
-CONH- is the peptide link
H20 = H+ + OH-
OH- is a nucleophile that attacks the slighty positive carbon of the C=O
The C-N bond next to C=O is broken
Hydrolysis must be done in alkaline/acidic condtitions, to be easier.
This then produces salts as before.