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 levels, we need to consider the pKa values of the amino acid residues in the sequence. The pKa values determine the ionization of the functional groups in the amino acids.
Here are the pKa values relevant to this peptide sequence:
- Glutamate (Glu): pKa1 = 2.2, pKa2 = 9.7
- Histidine (His): pKa1 = 1.8, pKa2 = 9.3, pKa3 = 6.0
- Tryptophan (Trp): pKa = ~11 (negligible ionization at pH 3, 8, and 11)
- Serine (Ser): pKa = ~14 (fully ionized at pH 3, 8, and 11)
- Glycine (Gly): pKa = ~14 (fully ionized at pH 3, 8, and 11)
- Leucine (Leu): pKa = ~14 (fully ionized at pH 3, 8, and 11)
- Arginine (Arg): pKa1 = 2.2, pKa2 = 9.0, pKa3 = 12.5
- Proline (Pro): pKa = ~14 (fully ionized at pH 3, 8, and 11)
Now let's calculate the net charge at each pH level:
At pH 3:
- Glutamate (Glu) would be protonated: +1 charge
- Histidine (His) would also be protonated: +1 charge for the extra proton
- Arginine (Arg) would retain its positive charge: +1 charge
- The other residues (Ser, Gly, Leu, Pro) don't have any charges at this pH.
The net charge at pH 3 = (+1) + (+1) + (+1) = +3
At pH 8:
- Glutamate (Glu) would be deprotonated: -1 charge
- Histidine (His) would be mostly deprotonated, so it has a neutral charge
- Arginine (Arg) would still retain its positive charge: +1 charge
- The other residues (Tryptophan, Ser, Gly, Leu, Pro) don't have any charges at this pH.
The net charge at pH 8 = (-1) + (+1) = 0
At pH 11:
- Glutamate (Glu) would be further deprotonated: -2 charge
- Histidine (His) would be mostly deprotonated, so it has a neutral charge
- Arginine (Arg) would retain its positive charge: +1 charge
- The other residues (Tryptophan, Ser, Gly, Leu, Pro) don't have any charges at this pH.
The net charge at pH 11 = (-2) + (+1) = -1
Therefore, the net charge of the peptide at pH 3, 8, and 11 is +3, 0, and -1, respectively.
To determine the net charge of a peptide at different pH values, we need to consider the pKa values of the amino acids within the sequence. The pKa values of the amino acids are as follows:
Glutamate (Glu): pKa ~ 4.1
Histidine (His): pKa1 ~ 1.8, pKa2 ~ 9.2
Tryptophan (Trp): pKa ~ 5.9
Serine (Ser): pKa ~ 13.9
Glycine (Gly): pKa ~ 2.3
Leucine (Leu): neutral (pKa not applicable)
Arginine (Arg): pKa1 ~ 2.2, pKa2 ~ 9.0, pKa3 ~ 12.5
Proline (Pro): neutral (pKa not applicable)
Now, let's calculate the net charge of the peptide at different pH values:
1. At pH 3:
At pH 3, the Glu (pKa ~ 4.1) and Ser (pKa ~ 13.9) will have a positive charge, Arg (pKa1 ~ 2.2, pKa2 ~ 9.0, pKa3 ~ 12.5) will be fully protonated, and His (pKa1 ~ 1.8, pKa2 ~ 9.2) will mostly be protonated, while Trp, Gly, Leu, and Pro won't contribute any charge. Thus, the net charge at pH 3 will be:
(+1 for Glu) + (+1 for Ser) - (+1 for Arg) - (+1 for Arg) - (+1 for Arg) - (+1 for His)
= +1 + 1 - 1 - 1 - 1 - 1
= -2
Therefore, the net charge of the peptide at pH 3 is -2.
2. At pH 8:
At pH 8, Glu (pKa ~ 4.1), Ser (pKa ~ 13.9), and His (pKa1 ~ 1.8, pKa2 ~ 9.2) will have a negative charge, while Arg (pKa1 ~ 2.2, pKa2 ~ 9.0, pKa3 ~ 12.5) will have a positive charge. Trp, Gly, Leu, and Pro won't contribute any charge. Thus, the net charge at pH 8 will be:
(-1 for Glu) - (-1 for Ser) - (-1 for His) + (+1 for Arg) - (+1 for Arg) - (+1 for Arg)
= -1 + 1 + 1 - 1 - 1 - 1
= -2
Therefore, the net charge of the peptide at pH 8 is -2.
3. At pH 11:
At pH 11, Glu (pKa ~ 4.1), Ser (pKa ~ 13.9), and His (pKa1 ~ 1.8, pKa2 ~ 9.2) will have a negative charge, while Arg (pKa1 ~ 2.2, pKa2 ~ 9.0, pKa3 ~ 12.5) will be fully deprotonated. Trp, Gly, Leu, and Pro won't contribute any charge. Thus, the net charge at pH 11 will be:
(-1 for Glu) - (-1 for Ser) - (-1 for His) - (-1 for His) - (-1 for His)
= -1 + 1 + 1 + 1 + 1
= +3
Therefore, the net charge of the peptide at pH 11 is +3.
In conclusion, the net charge of the peptide at pH 3 is -2, at pH 8 is -2, and at pH 11 is +3.