Question

Calculate the charge of each ionisable group as well as the net charge of the peptide below:

His-Trp-Tyr-Asn-Ala-Glu

1. At pH 6.0?
2. At pH 8.0?

Given the pKa for some amino acid side chains are:

Histidine = 6.0, Aspartate = 3.9, Glutamate = 4.0, Lysine = 10.5, Arginine = 12

Answer 1

Ans:

Net charge on each ionizable group of following amino acids at pH 6.0

1. Histidine------------ COO^- , NH₃⁺ and NH⁺

2. Tryptophan-------- COO^-

3. Tyrosine----------- COO^-

4. Asparagine-------COO^-

5. Alanine------------COO^- and  NH₃⁺

6. Glutamate--------COO^2- and  NH₃⁺

Net charge on each ionizable group of following amino acids at pH 8.0

1. Histidine------------ COO^-

2. Tryptophan-------- COO^-

3. Tyrosine----------- COO^-

4. Asparagine-------COO^-

5. Alanine------------COO^-

6. Glutamate--------COO^2-

Net charge on peptide at pH 6.0

Since Isoelectric point of the peptide (His-Trp-Tyr-Asn-Ala-Glu) = 5.1

Therefore net chage on the peptide at pH 6 is Negative

Net charge on peptide at pH 8.0

Since Isoelectric point of the peptide (His-Trp-Tyr-Asn-Ala-Glu) = 5.1

Therefore net chage on the peptide at pH 8 is Negative