Answer:
At pH 7.40, the predominant protonation state and charge of the histidine (His) side chain would be positively charged, while the aspartic acid (Asp) side chain would be negatively charged. If the pH were to change from 7.40 to 5.00, the His side chain would become neutral, while the Asp side chain would remain negatively charged.
Explanation:
The protonation states of amino acid side chains are affected by the pH of their environment. At a given pH, some amino acid side chains will be positively charged, some will be negatively charged, and some will be neutral.
Histidine (His) has a side chain that can be protonated or deprotonated depending on the pH of its environment. At pH 7.40, the predominant protonation state of the His side chain is positively charged, as it is more likely to have a proton attached to it than not. At pH 5.00, however, the protonation state of the His side chain will shift to a neutral state, as it is less likely to have a proton attached to it than at pH 7.40.
Aspartic acid (Asp) has a negatively charged side chain that is stable at pH 7.40. If the pH were to change to 5.00, the Asp side chain would remain negatively charged, as it is already at its lowest pKa value and will not be affected by further changes in pH.
Therefore, the predominant protonation state and charge of the His and Asp side chains would be different if the pH changed from 7.40 to 5.00.