Which buffer pH best facilitates cathodic migration of amphoteric proteins in electrophoresis, given that they carry a positive net charge at low pH?

Which buffer pH best facilitates cathodic migration of amphoteric proteins in electrophoresis, given that they carry a positive net charge at low pH?

• A. 4.6
• B. 7.5
• C. 8.6
• D. 9.5

Answer: (A)

The key idea is that a protein’s net charge—and thus its migration direction in electrophoresis—depends on the pH relative to its isoelectric point (pI). Below the pI, proteins are positively charged and migrate toward the cathode; above the pI, they’re negatively charged and migrate toward the anode. For amphoteric proteins that are positively charged at low pH, you want a buffer pH that keeps them below their pI so they stay positively charged and move toward the cathode. Among the options, the acidic buffer with pH 4.6 keeps the proteins well below their pI, ensuring a positive charge and cathodic migration. The higher pH buffers (7.5, 8.6, 9.5) would bring the pH closer to or above the pI, reducing positive charge or making the proteins negative, which would favor anodic migration instead. So the pH 4.6 buffer best facilitates cathodic migration.

The key idea is that a protein’s net charge—and thus its migration direction in electrophoresis—depends on the pH relative to its isoelectric point (pI). Below the pI, proteins are positively charged and migrate toward the cathode; above the pI, they’re negatively charged and migrate toward the anode. For amphoteric proteins that are positively charged at low pH, you want a buffer pH that keeps them below their pI so they stay positively charged and move toward the cathode. Among the options, the acidic buffer with pH 4.6 keeps the proteins well below their pI, ensuring a positive charge and cathodic migration. The higher pH buffers (7.5, 8.6, 9.5) would bring the pH closer to or above the pI, reducing positive charge or making the proteins negative, which would favor anodic migration instead. So the pH 4.6 buffer best facilitates cathodic migration.