Amino Acid

An amino acid is the monomer from which peptides are built: a central alpha carbon bearing an amino group, a carboxylic acid group, a hydrogen and a side chain that distinguishes one from another. Twenty are directly encoded, with selenocysteine and pyrrolysine added by specialised recoding. Except for glycine, whose side chain is a second hydrogen, that alpha carbon carries four different substituents and is a stereocentre, and ribosomal synthesis uses the L form exclusively.

Their masses set the arithmetic of every peptide spec sheet. As residues in a chain they run from glycine at about 57 daltons to tryptophan at about 186, averaging near 110, so a rough molecular weight follows from multiplying chain length by that figure. Chemical synthesis uses not free amino acids but protected building blocks, an Fmoc group masking the alpha-amino function and an acid-labile group capping any reactive side chain.

Composition decides much of what governs formulation. Counting acidic and basic residues gives net charge and isoelectric point, counting hydrophobic ones predicts solubility and aggregation risk, and the presence of methionine, asparagine or cysteine flags specific degradation chemistries. Where proteases cut depends on sequence and neighbouring residues rather than composition, so a composition table describes the vial and not the plasma.

The confusion worth naming is between amino acids and peptides commercially. Free amino acids are food substances regulated as such, while a peptide of the same composition is a distinct entity with its own regulatory status, so a supplement listing an impressive amino acid profile is not offering the peptide. Conversely, a molecular weight summed from free amino acid masses overshoots by about 18 daltons per bond, more than 500 daltons across a 30-residue chain.