Open Question
Draw in the R-groups from memory for each of the charged amino acids at physiological pH.
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3. Amino Acids
Charged Amino Acids
Multiple Choice
In the context of charged amino acids, a basic amino acid has a side chain that contains which functional group that can accept a proton and become positively charged at physiological pH?
A
A thioether group that is nonpolar and uncharged (e.g., )
B
An amine or guanidinium-containing group that can be protonated (e.g., )
C
A carboxylate group that donates a proton to become negatively charged (e.g., )
D
A hydroxyl group that is typically uncharged at physiological pH (e.g., )
Verified step by step guidance1
Understand that basic amino acids have side chains capable of accepting protons, which makes them positively charged at physiological pH (~7.4).
Recall the common functional groups in amino acid side chains: thioether (nonpolar, uncharged), carboxylate (negatively charged), hydroxyl (usually uncharged), and amine or guanidinium groups (can accept protons).
Recognize that the thioether group (−S−CH₃) is nonpolar and does not accept protons, so it does not confer basic properties.
Identify that the carboxylate group (−COO⁻) actually donates protons and becomes negatively charged, characteristic of acidic amino acids, not basic ones.
Conclude that the amine (−NH₃⁺) or guanidinium groups are the functional groups in basic amino acids that accept protons and become positively charged at physiological pH.
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