Explain why cysteine (pI = 5.15) elutes before isoleucine (pI = 5.60) when their solution having pH 5 is passed through a cation-exchange column.

• A. Because cysteine has a higher amount of positive charge at this pH due to its lower pI value
• B. Because cysteine has a higher amount of negative charge at this pH due to its lower pI value
• C. Because isoleucine has a higher amount of negative charge at this pH due to its higher pI value
• D. Because isoleucine has a higher amount of positive charge at this pH due to its lower pI value

Answer: (B)