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Zwitterion quiz #1

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  • What is a zwitterion, and how does it relate to the structure of amino acids at physiological pH?
    A zwitterion is a molecule with both a positive and a negative charge on different groups. At physiological pH (~7), amino acids exist predominantly as zwitterions, with a positively charged amino group (NH3+) and a negatively charged carboxylate group (COO−).
  • How do the pKa values of the amino and carboxyl groups in amino acids influence their charge at different pH levels?
    The amino group has a pKa of about 9–10.5, and the carboxyl group has a pKa of about 2. At pH below each group's pKa, the group is protonated; above the pKa, it is deprotonated. This determines the overall charge of the amino acid at different pH values.
  • What is the net charge of an amino acid backbone at pH 1, and why?
    At pH 1, both the amino and carboxyl groups are protonated. The amino group is NH3+ (positive), and the carboxyl group is COOH (neutral), resulting in a net charge of +1.
  • Describe the predominant form and net charge of an amino acid at pH 12.
    At pH 12, both the amino and carboxyl groups are deprotonated. The amino group is NH2 (neutral), and the carboxyl group is COO− (negative), giving a net charge of −1.
  • Why do amino acids exist predominantly as zwitterions at physiological pH?
    At physiological pH (~7), the carboxyl group (pKa ~2) is deprotonated (COO−), and the amino group (pKa ~9–10.5) is protonated (NH3+), resulting in both a positive and a negative charge, making the molecule a zwitterion with a net charge of 0.
  • How can you determine the predominant ionic form of an amino acid at any given pH?
    Compare the pH to the pKa of each group: if pH < pKa, the group is protonated; if pH > pKa, it is deprotonated. This allows you to predict the charge state of the amino and carboxyl groups and thus the overall charge of the amino acid.
  • What is a zwitterion, and how does it relate to the structure of amino acids at physiological pH?
    A zwitterion is a molecule with both a positive and a negative charge on different groups. At physiological pH (~7), amino acids exist predominantly as zwitterions, with a positively charged amino group (NH3+) and a negatively charged carboxylate group (COO−).
  • How do the pKa values of the amino and carboxyl groups in amino acids influence their charge at different pH levels?
    The amino group has a pKa of about 9–10.5, and the carboxyl group has a pKa of about 2. At pH below each group's pKa, the group is protonated; above the pKa, it is deprotonated, determining the overall charge of the amino acid.
  • What is the net charge of an amino acid backbone at pH 1, and why?
    At pH 1, both the amino and carboxyl groups are protonated. The amino group is NH3+ (positive), and the carboxyl group is COOH (neutral), resulting in a net charge of +1.
  • Describe the predominant form and net charge of an amino acid at pH 12.
    At pH 12, both the amino and carboxyl groups are deprotonated. The amino group is NH2 (neutral), and the carboxyl group is COO− (negative), giving a net charge of −1.