Amino acids are amphoteric, meaning they can act as both acids and bases. This dual nature arises from the presence of both an amino group (-NH₂) and a carboxyl group (-COOH) within their structure.
The Amphoteric Nature of Amino Acids
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Acidic Behavior: The carboxyl group (-COOH) can donate a proton (H+), acting as an acid. This is reflected in the low pKa (around 2) of the α-carboxyl group, indicating a relatively strong tendency to lose a proton.
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Basic Behavior: The amino group (-NH₂) can accept a proton (H+), acting as a base. The pKa of the amino group is typically between 9 and 10, showing a weaker tendency to accept a proton compared to the carboxyl group's tendency to donate one.
This amphoteric property is crucial for their role in forming proteins and participating in various biochemical processes. The side chain (R group) of an amino acid also influences its overall acidity or basicity, leading to the classification of amino acids as acidic, basic, or neutral.
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Acidic Amino Acids: Aspartic acid (Asp) and glutamic acid (Glu) possess acidic side chains at neutral pH.
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Basic Amino Acids: Lysine, arginine, and histidine have basic side chains.
The precise behavior of an amino acid (acidic or basic) depends heavily on the surrounding pH.
References:
- The provided text mentions that compounds like amino acids that can act as either an acid or a base are called amphoteric. The basic amino group typically has a pKa between 9 and 10, while the acidic α-carboxyl group has a pKa close to 2. [Britannica, NCBI Bookshelf]
- [ScienceDirect] and [Chemistry LibreTexts] further support the bifunctional nature of amino acids, highlighting the presence of both amino and carboxyl groups that enable acidic and basic behavior.
- [Vanderbilt University] provides a table showing the classification of various amino acids and their chemical structures which includes examples of both acidic and basic amino acids.
