🤓 Based on our data, we think this question is relevant for Professor Ratliff's class at USF.
The charge of proteins and polypeptides would depend on the condition of the dissociable protons along the chain. These include the protons in the C terminal, the N terminal, and the side chains.
The protons in the C terminal and in the acidic side chains dissociate at pH lower than 7. This leaves a negative charge from each of those substituents. Meanwhile, the protons in the N terminal and those of the basic side chains dissociate at pH above 7. As long as these protons are attached to those basic groups, they will have a + charge per substituent.
What is the charge of the tetrapeptide Lys-Lys-His-Glu at pH 7?