List many different worked example of finding isoelectric points based on greater than three given Pka
1. Amino acid: Glutamic acid
Given pKa values: α-carboxyl group (2.1), α-amino group (9.5), α-carboxyl group of side chain (4.1)
To find the isoelectric point, we look for the pH at which the amino acid has equal concentrations of positively and negatively charged forms.
Step 1: Determine the dominant form of the amino acid at low pH (below pKa values): In this case, the α-carboxyl group (pKa 2.1) and the α-carboxyl group of side chain (pKa 4.1) will be protonated, while the α-amino group (pKa 9.5) will be deprotonated.
Step 2: Determine the dominant form of the amino acid at high pH (above pKa values): In this case, the α-carboxyl group (pKa 2.1) and the α-carboxyl group of side chain (pKa 4.1) will be deprotonated, while the α-amino group (pKa 9.5) will be protonated.
Step 3: Find the pH at which the amino acid becomes neutral: This will occur when all three groups are in their zwitterionic (neutral) form. In this example, it will happen when the pH is between the pKa values of the α-carboxyl group (2.1) and the α-amino group (9.5). Thus, the isoelectric point of glutamic acid is approximately 5.8.
2. Amino acid: Histidine
Given pKa values: α-carboxyl group (2.1), α-amino group (9.6), imidazole side chain (6.0)
Step 1: At low pH (below pKa values), the α-carboxyl group (pKa 2.1) and the imidazole side chain (pKa 6.0) will be protonated, while the α-amino group (pKa 9.6) will be deprotonated.
Step 2: At high pH (above pKa values), the α-carboxyl group (pKa 2.1) and the imidazole side chain (pKa 6.0) will be deprotonated, while the α-amino group (pKa 9.6) will be protonated.
Step 3: The isoelectric point will be between the pKa values of the imidazole side chain and the α-amino group. In this case, it will be approximately 7.8.
These are just two examples of finding isoelectric points based on given pKa values. Each amino acid has its own unique pKa values, which determine its isoelectric point.