Amino acids

• Amino acids: General information

  • Made of an amine group, carboxylic group and an R group.
  • Alpha amino acids have the amine and carboxylic group attached to the same carbon.
  • Amino acids are amphoteric, meaning has both acidic and basic properties. Acidic= donate a proton, basic= accept a proton.
  • There are 20 naturally occurring amino acids in the body.
  • Amino acids contain chiral carbons, so produce optical isomers that rotate plane, polarised light.
• How to name an amino acid:

  1. Find the longest chain, starting with carboxyl
  2. Find the position of the amine group, this will be named 'amino'
  3. Name any other functional groups
• The amino acid and carboxylic acid can interact with each other to form a zwitterion, a molecule with both positive and negative ions.

  • A proton is transferred from the COOH to the NH2.
  • COO- cancels out the NH3+, these charges increase the attractive forces between amino acids, giving them unusually high melting points.

  This is only possible when the amino acid is at its isoelectric point, the pH at which the ions have no net electrical charge.

  • Acidic R groups lower the pH of the isoelectric point.
  • Basic R groups increase the pH of the isoelectric point.
• In acidic conditions, there is an abundance of H+ ions, so the amino acid acts as a base and accepts as many H+ ions as possible.

  
  

  In alkaline conditions, there is a deficiency of H+ ions, so the amino acid acts as an acid and donates as many H+ ions as possible.
• Amino acids in...

  • pH is lower than isoelectric point= Zwitterion becomes positivly charged, NH2 becomes NH3+, COOH stays the same.
  • pH is equal to isoelectric point= Zwitterion stays the same, no overall charge, NH3+ and COO-.
  • pH is higher than isoelectric point= Zwitterion becomes negatively charged, COOH becomes COO-, NH2 stays the same.

  Remember that all NH2 and COOH groups are altered, so 2 COOH groups means both become COO-.
• Thin-layer and paper chromatography can be used to the amino acids within a protein.
• Thin-layer chromatography: Method

  1. Place a chamber in the fume cupboard and pour chromatography solvent into a depth of about 3cm, then cover with a lid.
  2. Draw a pencil line 1 cm above the bottom of a TLC plate, mark the spots of each sample on this line.
  3. Using a capillary tube, add a dot of each sample, allow to dry, repeat 8 times.
  4. Place the TLC plate into the chamber and replace the lid, leave until the solvent has reached about 1cm from the top of the plate.
  5. Allow the plate to dry and spray with ninhydrin, dry with a hairdryer.
  6. Draw circles around purple spots and use to calculate Rf.
• Paper chromatography: Method

  1. Place the gas jar in the fume cupboard and pour chromatography solvent into a depth of about 3 cm, then cover with a lid.
  2. Use a pencil to draw a line 2cm from the bottom, mark each spot where the samples will go.
  3. Using a capillary tube, add a drop of each sample, allowing it to dry. Repeat this 8 times.
  4. Place paper into the jar, leave until the solvent has reached 2cm within the top of the paper.
  5. Hang the paper in the fume cupboard to dry, then spray with ninhydrin and dry with a hairdryer.
  6. Draw a circle around purple dots and use to calculate Rf.