Look up molecular weights, residue masses, pKa values, and physicochemical properties for all 20 standard amino acids. Click any amino acid for full details.
| 1-Letter | 3-Letter | Name | Free MW (Da) | Residue MW (Da) | Monoisotopic (Da) | pKa (side chain) | Group |
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Free form MW is the molecular weight of the amino acid as a standalone molecule (with both free amino -NH₂ and carboxyl -COOH groups intact). This is the value you would look up in a chemical catalogue.
Residue MW is the molecular weight of the amino acid when it is incorporated into a peptide chain. During peptide bond formation, one water molecule (18.015 Da) is lost between each amino acid. Therefore: Residue MW = Free MW − 18.015 Da.
When calculating the molecular weight of a protein or peptide from sequence, you should sum the residue MWs of all amino acids and then add one water molecule (18.015 Da) for the free N- and C-termini.
Average mass (shown as Free MW and Residue MW) is calculated using average atomic weights — the weighted average of all naturally occurring isotopes. This is the value used for most biochemical calculations and protein purification work.
Monoisotopic mass is calculated using only the most abundant isotope of each element (¹H, ¹²C, ¹⁴N, ¹⁶O, ³²S). It is used in mass spectrometry (MS) for small peptides, where individual isotope peaks can be resolved.
For proteins larger than ~2 kDa, the monoisotopic peak is no longer the most abundant peak, so average mass becomes more relevant for MS analysis of large proteins.