Amino acids exist predominantly as zwitterions (⁺NH₃-CHR-COO⁻) at:
AAll pH
BStrong acid
CStrong base
DIsoelectric point (pI) — pH where net charge is zero
Answer & Solution
Correct answer: D. Isoelectric point (pI) — pH where net charge is zero
At pI: -NH₂ protonated to -NH₃⁺ and -COOH deprotonated to -COO⁻; net charge = 0. For glycine pI ≈ 6.0. At lower pH: protonated (net +); higher pH: deprotonated (net -). Amino acids have high mp (ionic-like solid).
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