Titration Curve of Amino Acids
Titration curves of amino acids General structure of amino acids (amphoteric type): Zwitter ion C *: α – carbon: α – amino acid NH2: α – amino group, basic (proton acceptor) COOH: α – carboxylic group, acidic (proton donor)
R: side chain of amino acid
Classification of amino acids depending on the nature of side-chain:
Basic amino acid: with an extra amino group (NH2) in its side chain ( histidine, arginine, lysine )
Acidic amino acid: with an extra carboxylic acid group (COOH) in its side chain (aspartic acid & glutamic acid)
With phenol group in its side chain (Tyrosine)
With sulfide group in its side chain (Cysteine)
Isoelectric point (pI): It is the pH at which the net charge on the molecule in solution is zero (Zwitter ion). Zwitter ion will not migrate in the electric field, it is electrically neutral. pI is the average of the closest values of pKa’s. The pI for the acidic amino acids is the average of pKa1 and pKa2, while the pI for basic amino acids is the average of pKa2 and pKa3 pKa range for α COOH ( 1. 71 – 2. 63 )
pKa range for α – NH3+ ( 8. 8 – 10. 78 )
Acidity of a neutral amino acid α – COOH > α – NH3+
For weak acids : Henderson – Hasslbalch equation : pH = pKa + log [ A- ] [HA] When [ A- ] = [HA] pH = pKa
Acid- base titration: An experiment in which a measured amount of base ( or acid) are added to a measured amount of acid ( or base) to calculate something unknown such as molarity, pH….. etc.
Equivalence point: The point at which acid is exactly neutralized with a base.
Inflection point: The point in an acid-base titration at which 0. 5 mole of the base has been added to one mole of acid to achieve [ A- ] = [HA] and then; pKa = pH
Example: Inflection points 0. 5, 1, 1. 5, 2
* Alanine is an example of neutral amino acid which has 2 acidic protons (diprotic acids ) α – COOH and α – NH3+, these neutral amino acids have two titration curves.
Net charge +1 0 -1 Zwitter ion
Titration curve of Alanine: * aspartic acid is an example on acidic amino acid which has 3 acidic protons (triprotic acids ) α – COOH, α – NH3+ and R- COOH. Acidic amino acids have three titration curves. Acidity of acidic amino acid α – COOH > R- COOH > α – NH3+ Net charge +1 0 -1 -2 Zwitter ion
Titration curve of aspartic amino acid * Lysine is an example of basic amino acid which has 3 basic protons (tricrotic acids ) α – COOH, α – NH3+ and R- NH3+. Basic amino acids have also three titration curves. The acidity of a basic amino acid α – COOH > α – NH3+ > R- NH3+ Net charge +2 +1 0 -1 Zwitter ion
Titration curve of Lysine Method : at low pH ~ 1 – 1. 3 amino acid is fully protonated.