A-level Chemistry: 3.3.13 Amino Acids and Proteins

∵ have basic amino group and acidic carboxyl group

Chiral

Zwitterions

Dipolar ions

Have both +ve and -ve charge in different parts of molecule

isoelectric point

This is the pH where average overall charge on amino acid is 0

At the isoelectric point, both carboxyl and amino group are likely to be ionised forming an ion called a zwitterion

COO- group is likely to gain an H

-NH3+ group is likely to lose an H

Thin-layer chromatography

1. Draw pencil line near bottom of thin-layer chromatography plate & put concentrated spot of mixture of amino acids on it

2. Dip bottom of plate (not the spot) into a solvent

3. As solvent spreads up plate, different amino acids move with it, but a different rates so they separate out

4. When solvent’s nearly reached the top, take out plate and mark solvent front with pencil
   

   • Leave plate to dry

5. Amino acids aren’t coloured so make spots visible

6. Work out Rf value of each amino acid

7. Use table of known amino acid Rf values to identify amino acids in mixture

• Spray ninhydrin solution on plate = turns spots purple

• Or use special plate which has fluorescent dye added to it
  

  • Dye glows when UV light shines on it

  • Where there’s spots of chemical on plate, cover fluorescent dye so sports appear dark

  • Can draw dark patches to show were spots are under UV lamp

Rf value = distance travelled by substance ÷ distance travelled by solvent front

Add hot aqueous 6 M HCl & heat mixture under reflux for 24 hours

Sequence of amino acids in polypeptide chain

• Peptide links form hydrogen bond sight each other
  

  • So chain isn’t straight

• Forms α helix or β-pleated sheet

• Further folding of whole polypeptide chain

• More bonds form between different parts (R groups) of polypeptide chain
  

  • Gives 3D shape

• Hydrogen bonding

• Disulfide (or sulfur-sulfur) bonding (-S-S-)

Exists between polar groups e.g. -OH and -NH2

Stabilise both secondary and tertiary structure of the protein

thiol

Thiol groups on different cysteine residues can lose their H atoms and join together by forming disulfide (or sulfur-sulfur) bond (-S-S-)

Disulfide bonds link together in different parts of protein chain & help to stabilise tertiary structure

Temperature and pH can affect hydrogen bonding and formation of disulfide bonds

Complex of platinum(II) with 2 chloride ion ligands and 2 ammonia ligands in square planar shape

Anti-cancer drug

• Cisplatin binds to DNA, causing kinks in DNA helix which stops proteins that replicate DNA from copying it properly

• Stops tumour cells reproducing

1. N atom on guanine base in DNA forms co-ordinate bond with cisplatin's platinum ion, replacing on of chloride ion ligands

   • (Ligand substitution reaction)

2. 2nd N atom from nearby guanine can bond to platinum and replace the second chloride ion

3. Presence of cisplatin complex bound to DNA strands causes strands to kink

   • Means DNA strands can't unwind and be copied properly = ∴ cell can't replicate

• Cisplatin can bind to DNA in normal cells as well as cancer cells

  • Problem for healthy cells that replicate frequently e.g. hair and blood cells ∵ cisplatin stops them from replicating

  • Means cisplatin causes hair loss and suppress immune system

• Can also cause kidney damage

• By giving patients very low dosages of cisplatin

• Or target cisplatin to tumour

  • Means using method that delivers drug only to cancer cells so it cant attack healthy cells

∵ balance of long-term positive effects outweigh negative short-term effects

• N and O both very electronegative or C=O and N–H are polar

• Lone pair on O attracted to the δ+H of N-H

• Hydrogen bond formed

• Amino acids have different polarities

• So they have different affinities for mobile and stationary phases

Strong electrostatic attraction between opposite charged ions