Describe the structure of amino acids and the concept of zwitterions at physiological pH.

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Multiple Choice

Describe the structure of amino acids and the concept of zwitterions at physiological pH.

At physiological pH, amino acids exist as zwitterions—internal charges separated but the overall molecule is neutral. The amino group is protonated to NH3+, while the carboxyl group is deprotonated to COO−. These opposite charges balance, giving no net charge even though the molecule has distinct positive and negative ends. The typical pKa values explain this: the carboxyl group loses its proton well before physiological pH, and the amino group retains its proton around pH 7, so the zwitterionic form predominates. This state is why amino acids are highly water-soluble and can participate in interactions like salt bridges in proteins, and it also defines the isoelectric point, when the molecule carries no net charge. The other options describe states that don’t match what actually predominates at physiological pH: a fully deprotonated anion isn’t typical because the amino group stays protonated; a zwitterion with a net positive charge isn’t correct since the charges balance to zero; neutral molecules with both groups uncharged don’t occur because the amino group remains protonated under physiological conditions.

At physiological pH, amino acids exist as zwitterions—internal charges separated but the overall molecule is neutral. The amino group is protonated to NH3+, while the carboxyl group is deprotonated to COO−. These opposite charges balance, giving no net charge even though the molecule has distinct positive and negative ends. The typical pKa values explain this: the carboxyl group loses its proton well before physiological pH, and the amino group retains its proton around pH 7, so the zwitterionic form predominates.

This state is why amino acids are highly water-soluble and can participate in interactions like salt bridges in proteins, and it also defines the isoelectric point, when the molecule carries no net charge. The other options describe states that don’t match what actually predominates at physiological pH: a fully deprotonated anion isn’t typical because the amino group stays protonated; a zwitterion with a net positive charge isn’t correct since the charges balance to zero; neutral molecules with both groups uncharged don’t occur because the amino group remains protonated under physiological conditions.

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