Exams › JEE Advanced › Chemistry
Correct answer: 5
At pH 12.5, any group with pKa < 12.5 and acidic character will be fully deprotonated (negative), and any group with pKa < 12.5 and basic character will be fully deprotonated (neutral). Ionisable groups: N-terminal NH3+ (pKa ~8) -> NH2 (neutral); C-terminal COOH (pKa ~2) -> COO- (-1); Glu side chain COOH (pKa ~4.1) -> COO- (-1); Asp side chain COOH (pKa ~3.9) -> COO- (-1); Tyr phenol OH (pKa ~10.5) -> O- (-1). All four acidic groups deprotonated = 4 negative charges. Tyr -OH deprotonates at pH 12.5 (pKa 10.5 < 12.5) giving another -1. Total negative = 5. Positive charges: none (N-terminal NH2 is neutral at pH 12.5). Net = -5, so total negative charges = 5.