The pI is the pH at which an amino acid molecule exists in an uncharged state. The pI of alanine is 6.11, while that of histidine is 7.6. The pH of the surrounding environment of a molecule affects its net charge.
The gain of protons can increase the positive charge of an amino acid molecule, whereas a loss can increase its negative charge. The pI value at a specific pH can affect the solubility of a molecule. Such a molecule is less soluble in a water or salt solution at the pH that coincides with its pI and normally precipitates away.