Multiple Choice
Which shows the proper structure of Leu at physiological pH?
3. Amino Acids
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At what pH would an amino acid bear both a neutral -COOH and a -NH 2 group?
A
Between pH 0-5.
B
Between pH 5-9.
C
Between pH 9-14.
D
Not likely to occur at any pH.
Verified step by step guidance1
Understand the structure of an amino acid: Amino acids have a central carbon atom (the alpha carbon) bonded to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable R group (side chain).
Recognize the ionizable groups: The amino group (-NH2) can accept a proton to become -NH3+, and the carboxyl group (-COOH) can donate a proton to become -COO-. These groups are ionizable and their charge state depends on the pH of the environment.
Consider the pKa values: The pKa of the carboxyl group is typically around 2-3, and the pKa of the amino group is around 9-10. These values indicate the pH at which each group is 50% ionized.
Analyze the pH range: At low pH (below the pKa of the carboxyl group), the -COOH group is protonated and neutral, while the -NH2 group is protonated to -NH3+. At high pH (above the pKa of the amino group), the -COOH group is deprotonated to -COO- and the -NH2 group is neutral.
Conclude the possibility: Since the -COOH group is neutral at low pH and the -NH2 group is neutral at high pH, there is no pH range where both groups are neutral simultaneously. Therefore, it is not likely to occur at any pH.
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