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Amino acids
Amino acids are molecules with both acidic and basic parts, allowing them to act as buffers and link together to form proteins. Their unique side chains (R groups) give proteins their diverse functions.
Need to know
What you need to know
- The α-carbon is a chiral centre for 19 of the 20 common amino acids, as it is bonded to four different groups (–H, –NH₂, –COOH, and –R).
- This chirality means that amino acids (except glycine) can exist as two non-superimposable mirror images, or enantiomers (L- and D-forms).
- The exception is glycine, where the R group is a hydrogen atom. Since the α-carbon is bonded to two hydrogen atoms, it is achiral and does not exhibit optical isomerism.
Explanation
Life's Versatile Building Blocks
- The general structure is H₂N–CH(R)–COOH. All are chiral and optically active except glycine (where R = H), which has no chiral centre. In solution, they exist as zwitterions: H₃N⁺–CH(R)–COO⁻.
- The isoelectric point (pI) is the pH where the amino acid exists as a zwitterion with a net charge of zero. This property is used to separate amino acids using electrophoresis.
- Amino acids join via condensation reactions to form a dipeptide, eliminating water. The resulting amide link, –CO–NH–, is called a peptide bond and forms the primary structure of proteins.
- There are 20 common amino acids found in proteins, each with a different R group. This R group determines if the amino acid is acidic, basic, polar, or non-polar, dictating the protein's final structure and function.