TL - Amino Acids and Proteins

Question 1

Define amphoteric

Answer 1

A compound that has both acidic and basic properties

Question 2

What functional groups make amino acids amphoteric?

Answer 2

A basic amino group (NH₂) and an acidic carboxyl group (COOH)

Question 3

Describe an amino acid

Answer 3

A central carbon atom bonded to:

• A hydrogen atom
• An amino group
• A carboxyl group
• A variable R group

Question 4

Define zwitterion

Answer 4

A dipolar ion - it has both a positive and a negative charge in different parts of the molecule
Zwitterions only exist near a that molecule’s isoelectric point

Question 5

Define isoelectric point

Answer 5

The pH at which the average overall charge on a dipolar ion is 0

Question 6

Describe the structure of an amino acid when the pH is lower than its isoelectric point

Answer 6

The -NH₂ group is protonated (-NH₃⁺)

Question 7

Describe the structure of an amino acid when the pH is at its isoelectric point

Answer 7

It exists as a zwitterion where both the amino group and the carboxyl group are ionised

Question 8

Describe the structure of an amino acid when the pH is higher than its isoelectric point

Answer 8

The -COOH group loses its proton (-COO⁻)

Question 9

Describe how you would identify a mixture of unknown amino acids and name the technique

Answer 9

Paper Chromatography

1) Draw a pencil line near the bottom of a piece of chromatography paper and put a concentrated spot of the mixture of amino acids on it
2) Dip the bottom of the paper (not the spot) into the solvent
3) As the solvent spreads up the paper, the different amino acids move with it, but at different rates, so they spread out
4) When the solvent nearly reaches the top, take the paper out and mark the solvent front with a pencil
5) Amino acids aren’t coloured, so spray ninhydrin solution on the paper to turn them purple
6) Work out the Rᶠ values and compare them to a table of know amino acid Rᶠ values to identify the amino acids

Question 10

Give the equation to calculate the Retention factor (Rᶠ) using paper chromatography

Answer 10

Rᶠ= Distance moved by spot / distance moved by solvent

Question 11

What compound is used to show amino acids on a paper chromatograph and what colour do they turn

Answer 11

Ninhydrin solution

Amino acids turn: Colourless → Purple

Question 12

What are the condensation polymers of amino acids?

Answer 12

Proteins

Question 13

In what reactions do amino acids:

i) form bonds
ii) break bonds

Answer 13

i) Condensation

ii) Hydrolysis

Question 14

What bonds are formed between the amine groups and carboxyl groups of amino acids?

Answer 14

Peptide bonds (-CO-NH-)

Question 15

Give the conditions and reactants required to hydrolyse proteins

Answer 15

Hot aqueous 6moldm⁻³ HCl

Heated under reflux for 24 hours and then neutralised

Question 16

State and describe the 4 levels of structure for proteins

Answer 16

1) Primary Structure - the sequence of amino acids in a polypetide chain
2) Secondary structure - the hydrogen bonds formed between a polypeptide chain creating Alpha helixes and Beta pleated sheets
3) Tertiary structure - The polypeptide chain is further coiled and folded by different bonds between the R groups of the amino acids
4) Quaternary structure - The structure produced when multiple polypeptide chains bond together

Question 17

Name and describe the 4 bonds that can hold a tertiary structure together

Answer 17

All these forces exist between the side chains (R-groups) of the amino acids

1) ID-ID forces - weak attractions between 2 non-polar side groups (e.g. CH₃)
2) Ionic interactions between charged side groups (e.g. CO₂⁻ and NH₃⁺)
3) Hydrogen bonding - between groups such as -OH and -NH₂
4) Disulfide bridges - a covalent bond between 2 sulfur-containing side groups (-SH). This type of bonding is stronger than the others

Question 18

Why are the bonds in the tertiary structure so important to a proteins’ function?

Answer 18

Because it’s the 3D structure that gives proteins their properties