Titration Curve of Amino Acids

tion Experiment 1
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:

* Neutral * Basic amino acid : with extra amino group (NH2) in its side chain ( histidine, arginine, lysine ) * Acidic amino acid : * with 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 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 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 an acid is exactly neutralized with a base.