A researcher is studying the properties of a novel protein. They determine its isoelectric point (pI) to be 8.5. At which of the following pH values would this protein have a net positive charge?
- ApH 10.0
- BpH 9.0
- CpH 8.5
- DpH 7.0
Show answer & explanationAnswer & explanation
Correct answer: D. pH 7.0
The isoelectric point (pI) is the pH at which a protein has a net zero charge. If the pH of the solution is below the pI, the protein will gain protons (become protonated) on its basic groups and zwitterionic form, resulting in a net positive charge. If the pH is above the pI, the protein will lose protons (become deprotonated) on its acidic groups, resulting in a net negative charge. Since the pI is 8.5, at pH 7.0 (which is below 8.5), the protein will have a net positive charge.
Why the other options are wrong
- A. At pH 10.0 (above pI 8.5), the protein would have a net negative charge.
- B. At pH 9.0 (above pI 8.5), the protein would have a net negative charge.
- C. At pH 8.5 (at the pI), the protein would have a net zero charge.
Isoelectric Point (pI) of Proteins
The isoelectric point (pI) is the specific pH at which a molecule, particularly a protein or amino acid, carries no net electrical charge. At this pH, the number of positive charges equals the number of negative charges.
- Net charge is zero at pI.
- Below pI, protein has net positive charge.
- Above pI, protein has net negative charge.
- Determined by the pKa values of ionizable groups.
Memory trick: Below pI, 'P'ositive; Above pI, 'N'egative.