Final answer:
An amino acid with a pI of 7.5 will have a net charge of -1 at a pH higher than 7.5, due to the deprotonation of its basic groups in more basic conditions.
Step-by-step explanation:
If the isoelectric point (pI) for a particular amino acid is 7.5, this means that the amino acid will carry no net charge at this pH, as the positive and negative charges balance each other out. To have a net charge of -1 on the amino acid, the pH must be higher than the pI.
This is because at a higher pH, the amino acid's basic groups will tend to lose protons, resulting in a negative charge. For a molecule with a pI of 7.5, if the pH is, for example, 8.5 or higher, one would expect the molecule to have a net negative charge, as the environment is more basic relative to its pI.