Final answer:
At pH 5, histidine will have a positively charged side chain, with the amine group in its -NH3+ form, as the pH is below the pKa value of the amine group of histidine (6.04).
Step-by-step explanation:
The student is asking what is the form of histidine at pH 5. Histidine is an amino acid that has an amine group in its side chain with a pKa of 6.04. At a pH of 5, which is below the pKa value, the amine group of histidine will be protonated. This means that at pH 5, histidine will have a positively charged side chain, specifically the -NH3+ form. Amino acids like histidine can exist in different forms depending on the pH of the environment, and the form at pH 5 is part of the categorization of amino acids with basic side chains that are hydrophilic and become protonated when the pH is below their pKa.