Question

Can you draw the peptide structure at pH 7.0 (including peptide bonds)

Peptide bond sequence : Tryptophan - Serine - Asparagine - Glycine - Cysteine - Histidine

Answer 1

See image below

Step-by-step explanation:

The image is labeled according to the sequence N'-Trp-Ser-Asg-Gly-Cys-His-COOH' which means that in the main chain of the peptide, the amino group of the Tryptophan and the carboxylic group of the Histidine are free and thus its charge depends on the pH; other groups that rely on the pH are the side groups of the Cysteine and the Histidine.

Overall, ionizable groups in this peptide are:

• Amino Group of the Tryptophan (pKa = 9.39)

• SH group of the Cysteine (pKa = 8.18)

• Secondary amine of the Histidine (pKa = 6.00)

• Carboxylic Group of the Histidine (pKa = 1.82)

Then, the amino group of Trp and SH group of Cys are protonated since the peptide is at a pH below the pKa. The secondary amine of the Histidine is deprotonated because the pH is greater than the pKa, as well with its carboxylic acid group.