Detailed Explanation
A molecule carrying both a positive and negative charge simultaneously, resulting in zero net charge. At physiological pH (~7.4), free amino acids exist predominantly as zwitterions: the amino group is protonated (–NH₃⁺, positive) and the carboxyl group is deprotonated (–COO⁻, negative).
This doubly charged state occurs because the amino group's pKₐ (~9.0) is above physiological pH (so it holds its proton) while the carboxyl group's pKₐ (~2.0) is below physiological pH (so it releases its proton). The zwitterionic form gives amino acids high melting points, water solubility, and the ability to act as buffers. Within a peptide chain, the internal amide bonds are not ionizable — only the N-terminal amino group and C-terminal carboxyl group remain as titratable groups, along with ionizable side chains.
Key Facts
- A molecule carrying both a positive and negative charge simultaneously, resulting in zero net charge.
- At physiological pH (~7.4), free amino acids exist predominantly as zwitterions: the amino group is protonated (–NH₃⁺, positive) and the carboxyl group is deprotonated (–COO⁻, negative).
- This doubly charged state occurs because the amino group's pKₐ (~9.0) is above physiological pH (so it holds its proton) while the carboxyl group's pKₐ (~2.0) is below physiological pH (so it releases its proton).
- The zwitterionic form gives amino acids high melting points, water solubility, and the ability to act as buffers.
- Within a peptide chain, the internal amide bonds are not ionizable — only the N-terminal amino group and C-terminal carboxyl group remain as titratable groups, along with ionizable side chains.
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