Amino Acid
An amino acid is the fundamental molecular building block composing all peptides and proteins. Its universal structure centers on a central carbon atom (Calpha) bearing four substituents: an amine group (-NH2), a carboxyl group (-COOH), a hydrogen atom (-H), and a variable side chain denoted R. It is this R chain that gives each amino acid its unique physicochemical properties: hydrophobicity, charge, size, aromaticity, interaction capacity.
The genetic code specifies 20 standard (proteinogenic) amino acids, classified into four major families based on side-chain nature. Non-polar amino acids (Glycine, Alanine, Valine, Leucine, Isoleucine, Methionine, Proline, Phenylalanine, Tryptophan) are hydrophobic and typically reside in the protein core. Polar uncharged residues (Serine, Threonine, Cysteine, Asparagine, Glutamine, Tyrosine) form hydrogen bonds with water. Acidic ones (Aspartate, Glutamate) carry a negative charge at physiological pH. Basic ones (Lysine, Arginine, Histidine) carry a positive charge.
All proteinogenic amino acids (except glycine) possess a chiral center at the Calpha, existing theoretically in two enantiomeric forms L and D. Biology uses exclusively the L form in ribosomal protein synthesis. D-amino acids, sometimes incorporated in research peptides (e.g., D-Arginine, D-Lysine), resist proteases and substantially extend plasma half-life. This is a major peptide design lever, exploited for long-acting therapeutic peptides.
Beyond the 20 standards, non-proteinogenic amino acids are highly useful in peptide research. Alpha-aminoisobutyric acid (Aib) with its double methyl group blocks rotation and stabilizes the alpha-helix — found at positions 2 and 20 of semaglutide and tirzepatide to resist DPP-4. Norleucine (Nle) replaces methionine to prevent oxidation. Ornithine (Orn) and diaminobutyric acid (Dab) adjust lysine chain length. Pyroglutamic acid cyclizes the N-terminus and protects against aminopeptidases.
Understanding amino acid chemistry is essential for peptide analysis: reading sequence, predicting solubility (hydrophilic dominant = water, hydrophobic dominant = DMSO or dilute ethanol), identifying vulnerability points (Met → oxidation, Asn → deamidation, Cys → spurious disulfide bridges), and interpreting HPLC chromatographic behavior. Each residue matters: a single mutation can abolish biological activity entirely or, conversely, multiply peptide potency.