Final answer:
At pH 7, histidine is near its isoelectric point but still slightly acidic, meaning the majority of its molecules will have a net positive charge. Less than 50% of histidine would be in a neutral form, as pH 7 is just below the pI of 7.6 for histidine. The exact percentage would require detailed calculations using the Henderson-Hasselbalch equation.
Step-by-step explanation:
To determine what percent of histidine has a neutral charge at pH 7, it is important to understand the isoelectric point (pI) of histidine. The isoelectric point is the pH at which the amino acid exists as a zwitterion, with no net electric charge. Histidine has a side chain with an imidazole group, which can act as both an acid and a base. The pI of histidine is approximately 7.6, which means that at a pH lower than this, histidine will have a net positive charge, and at a pH higher than this, it will have a net negative charge.
At pH 7, histidine is very close to its pI but still slightly on the acidic side meaning a majority of the molecules will have a net positive charge rather than being neutral. However, there will be a significant population of histidine molecules that are neutral. To calculate the exact percentage, one would typically use the Henderson-Hasselbalch equation to determine the ratio of the neutral to charged species. As pH 7 is slightly below the pI of histidine, the neutral form would be less than 50%. Without exact calculations, it is not possible to provide a precise percentage.
It's important to understand that neutral pH is defined as 7.00, reflecting a hydrogen ion concentration of 1.0 x 10-7 M. This definition can help predict the form of an amino acid in a given solution, whether it is acidic (pH < 7), neutral (pH = 7), or basic (pH > 7).