Amino Acids

Question 1

What are some of the properties of amino acids?
(acidic/basic properties
zwitterion formation
chiral properties)

Answer 1

• An amino acid has a basic amino group (NH₂) and an acidic carboxyl group (COOH); two functional groups.
• They’re chiral molecules; the central carbon has four different functional groups attached (Glycine is an exception as its R group is just H). Thus a solution of a single amino acid enantiomer will rotate plane polarised light.
• The carboxylic acud group has a tendency to lose a proton (act as an acid), from COOH to COO^-.
• The amine group has a tendency to accept a proton (act as a base), from NH₃ to NH₄⁺.
• Amino acids exist as zwitterions (dipolar ion). Zwitterions have both a permanent postive and negative charge, though the compound is neutral overall.
• Zwitterions only exist near an amino acid’s isoelectric point; the pH where the average overall charge on the amino acid is zero; is dependent on the R-group.
• In conditions more acidic than the isoelectric point, the :NH₂ accepts a proton (H⁺) to form a positive ion (NH₃⁺); it is protonated.
• In conditions more basic than the isoelectric point, the -COOH (carboxylic acid group) loses a proton (H⁺ ion); it is deprotonated.
• Amino acids have high melting points due to their ionic nature, and dissolve well in water, but poorly in non-polar solvents. A typical amino acid is a white solid at room temperature and behaves very much like an ionic salt.

Question 2

What are proteins consisted of?

(#AMINO)

Answer 2

• Proteins are condensation polymers of amino acids; they are sequences of amino acids joined by peptide links.
• The peptide links are amide linkages; the -CONH- bond.
• Compounds formed by the linkage of amino acids are called peptides, and the amide linkage is called a peptide linkage in this context.
• Dipeptide can react further (-NH₂/-COOH groups on either side) to give tri-, tetra- peptides etc.
• Polypeptides and proteins are condensation polymers as a water molecule is eliminated as each link of the chain forms.

Question 3

How do we get amino acids from protein?

(how to break it up)

Answer 3

• Via the hydrolysis (a water molecule is added to break the peptide linkage) of the peptide link, producing the constituent amino acids.

Conditions:
* Boil with HCl of conc. 6 mol dm^-3 for 24 hours under reflux.
(all the peptide linkages are hydrolysed by the acid)

• Certain enzymes will partially hydrolyse specific proteins; e.g. trypsin will only break the peptide bonds formed by lysine and arginine.
• Detective work based on this and other techniques enables chemists to find the sequence of amino acids in different proteins.

Question 4

How are mixtures of amino acids separated?

Answer 4

• A mixture of amino acids can be separated and identified using paper chromatography.

1. Draw pencil line near the bottom of chromatography paper and put a concentrated spot of the mixture on it.
2. Dip the bottom of the paper (not the spot) into a solvent.
3. As the solvent spreads up the paper, the different amino acids move with it at different rates, so they seperate out.
4. You can identify each amino acid by comparing how far it moves to how far the solvent moves.
5. Amino acids aren’t coloured; you have to spray ninhydrin solution on the paper to turn them purple.

Question 5

What is the importance of hydrogen bonding in proteins?

Answer 5

• Hydrogen bonding gives rise to the secondary (folding/twisting), tertiary and quartenary structure of a protein.
• Hydrogen bonding holds the helix in shape, or the pleated sheet, in shape.
• The tertiary structure/3D shape is vital to how proteins function; altering the tertiary structure of an enzyme alters the active site, thus the substrate molecule can no longer bind; the enzyme stops working.
• Factors such as heat and pH can affect hydrogen bonding, thus affect the shape of proteins.