Final answer:
At pH=0, histidine will be in its fully protonated form, with the amine group in the side chain and the amino group in the main chain as -NH3+, the carboxyl group as -COOH, and the imidazole ring also protonated, leading to an overall positive charge on the molecule.
Step-by-step explanation:
The student has asked to draw the predominant form of histidine at pH=0. At a pH of 0, histidine will exist in its fully protonated form. This means that both the amine group in the side chain and the amino group in the main chain will have an extra hydrogen ion attached, making them –NH₃⁺. Similarly, the carboxyl group will be in its protonated form as –COOH rather than –COO⁻. The imidazole ring in the side chain of histidine, which can pick up a hydrogen ion, will also be protonated. The resulting structure will have a positively charged imidazole ring, leading to an overall positive charge on the molecule.
Understanding the electric polarity of an amino acid in aqueous solution and its relation to pH is important in biochemistry. Amino acids can exist in different ionic forms depending on the pH of their environment, and this has crucial implications for their behavior and function in biological systems. As histidine is a basic amino acid with a basic side chain, its side chain amine group will be protonated (as –NH₃⁺) at low pH values.