Amino acids
Amino acids
- An amino acid has both a basic –NH₂ group and an acidic –COOH group.
- Its charge depends on the pH.
- Amino acids join into proteins by peptide bonds.
Practice
An amino acid contains:
Having both groups is what makes amino acids behave as they do (zwitterions, peptide bonds).
Zwitterions and the isoelectric point
- The –COOH can give its $\text{H}^+$ to the –NH₂ in the same molecule → a zwitterion ($\text{H}_3\text{N}^+\text{–CHR–COO}^-$): both ends charged, no overall charge.
- in acid (low pH): gains $\text{H}^+$ → positive.
- in alkali (high pH): loses $\text{H}^+$ → negative.
- at the isoelectric point: mostly the zwitterion, no net charge.
Practice
A zwitterion is:
The –COOH gives H⁺ to the –NH₂, so both ends are charged but the molecule is overall neutral.
Practice
In acidic solution (low pH), an amino acid is:
In acid it gains H⁺ (positive); in alkali it loses H⁺ (negative); at the isoelectric point it is neutral.
Peptide bonds and electrophoresis
- Two amino acids join by condensation (lose water) forming a peptide bond (an amide link) → a dipeptide.
- electrophoresis separates amino acids at a chosen pH: a positive one moves to the negative electrode, a negative one to the positive, and one at its isoelectric point stays put.
Practice
Two amino acids join by a:
Condensation between –COOH and –NH₂ forms a peptide (amide) bond and releases water.
You've got it
Key idea
- an amino acid has –NH₂ (basic) and –COOH (acidic)
- zwitterion = no net charge; positive in acid, negative in alkali; neutral at the isoelectric point
- two amino acids condense to form a peptide bond (amide link), losing water
- electrophoresis separates them by charge at a chosen pH