Amino acids exist as zwitterions in the solid state and in aqueous solutions near neutral pH, where an internal acid-base reaction occurs.
The acidic group donates a proton () to the basic group, resulting in a molecule with a positive charge on the nitrogen () and a negative charge on the carboxylate ().
Because they contain full ionic charges, amino acids exhibit strong intermolecular forces (ionic-like attractions), leading to high melting points and existence as soluble crystalline solids.
This amphoteric nature allows them to act as both acids (by donating a proton from the group) and bases (by accepting a proton at the group).
The isoelectric point () is the specific pH at which an amino acid has no net electrical charge and exists predominantly as a neutral zwitterion.
In acidic conditions (low pH), the group accepts a proton to become , resulting in a net positively charged cation.
In alkaline conditions (high pH), the group loses a proton to become , resulting in a net negatively charged anion.
Amino acids act as buffers because they can neutralize small amounts of added acid or base, maintaining a relatively constant pH in biological systems.
Two amino acids can join together via a condensation reaction, where the amino group of one molecule reacts with the carboxylic acid group of another.
This process eliminates a water molecule () and forms a covalent peptide bond (also known as an amide link, ).
A dipeptide is formed from two amino acids, while a polypeptide is a long chain of many amino acids linked together; proteins consist of one or more polypeptide chains.
The resulting chain always has an N-terminal (free amino group) at one end and a C-terminal (free carboxyl group) at the other, allowing for further polymerisation.
Polypeptides and proteins can be broken back down into their constituent amino acids through acid hydrolysis.
This typically requires heating the protein with concentrated hydrochloric acid () for a prolonged period, which breaks the peptide bonds by adding water back across the link.
The resulting mixture of amino acids can be separated and identified using thin-layer chromatography (TLC) or paper chromatography.
Since amino acids are colorless, the chromatogram must be sprayed with a developing agent like ninhydrin to produce visible purple spots, which are then identified by calculating their values.
| R-Group Type | Characteristic | Example Behavior |
|---|---|---|
| Neutral | Non-polar or uncharged polar side chain | Isoelectric point is usually near pH 6 |
| Acidic | Contains an extra group | Lower isoelectric point; carries negative charge at neutral pH |
| Basic | Contains an extra or nitrogen group | Higher isoelectric point; carries positive charge at neutral pH |
Drawing Repeating Units: When asked to draw a polypeptide section, always ensure the peptide bond is drawn as and that the R-groups alternate or are correctly placed on the -carbons.
Identifying Monomers: To find the monomers from a protein structure, 'cut' the peptide bond between the and the , then add to the carbonyl and to the nitrogen.
pH Effects: Always check the pH specified in the question; in strongly acidic solutions, both the amino group and any basic R-groups will be protonated.
Chromatography Logic: Remember that values depend on the relative affinity for the stationary phase vs. the mobile phase, not the size of the molecule.
