A peptide has the sequence:
Glutamate - His - Trp - Ser - Gly - Leu - Arg - Pro - Gly
What is the net charge of the molecule at pH 3, 8, and 11?
To determine the net charge of a peptide at different pH values, we need to consider the pKa values of its constituent amino acids.
First, let's list the pKa values of the amino acids present in the peptide:
- Glutamate (Glu): pKa1 = 2.19, pKa2 = 4.25, pKa3 = 9.67
- Histidine (His): pKa1 = 1.8, pKa2 = 6.0, pKa3 = 9.17
- Tryptophan (Trp): pKa1 = 2.38, pKa2 = 9.39
- Serine (Ser): pKa1 = 2.2, pKa2 = 9.21
- Glycine (Gly): No ionizable groups
- Leucine (Leu): No ionizable groups
- Arginine (Arg): pKa1 = 2.17, pKa2 = 9.04, pKa3 = 12.1
- Proline (Pro): No ionizable groups
Now, let's calculate the net charge of the peptide at pH 3, 8, and 11:
1. At pH 3:
- Glutamate (Glu): Fully protonated (+1 charge)
- Histidine (His): Partially protonated (+1 charge)
- Tryptophan (Trp): Fully protonated (+1 charge)
- Serine (Ser): Fully protonated (+1 charge)
- Glycine (Gly): No charge
- Leucine (Leu): No charge
- Arginine (Arg): Partially protonated (+1 charge)
- Proline (Pro): No charge
Summing up the charges, we get:
1 + 1 + 1 + 1 + 0 + 0 + 1 + 0 = +5
Therefore, at pH 3, the net charge of the peptide is +5.
2. At pH 8:
- Glutamate (Glu): Partially deprotonated (0 charge)
- Histidine (His): Partially protonated (+1 charge)
- Tryptophan (Trp): Partially deprotonated (0 charge)
- Serine (Ser): Partially deprotonated (0 charge)
- Glycine (Gly): No charge
- Leucine (Leu): No charge
- Arginine (Arg): Partially charged (+1 charge)
- Proline (Pro): No charge
Summing up the charges, we get:
0 + 1 + 0 + 0 + 0 + 0 + 1 + 0 = +2
Therefore, at pH 8, the net charge of the peptide is +2.
3. At pH 11:
- Glutamate (Glu): Fully deprotonated (-1 charge)
- Histidine (His): Fully deprotonated (0 charge)
- Tryptophan (Trp): Fully deprotonated (0 charge)
- Serine (Ser): Fully deprotonated (0 charge)
- Glycine (Gly): No charge
- Leucine (Leu): No charge
- Arginine (Arg): Partially deprotonated (-1 charge)
- Proline (Pro): No charge
Summing up the charges, we get:
-1 + 0 + 0 + 0 + 0 + 0 - 1 + 0 = -2
Therefore, at pH 11, the net charge of the peptide is -2.
In conclusion:
- At pH 3, the net charge is +5.
- At pH 8, the net charge is +2.
- At pH 11, the net charge is -2.