Amino Acid Table: Codes, Properties, Masses and pKa

All mass values in this table are monoisotopic masses in daltons (Da), rounded to four decimal places. They refer to unmodified residues and neutral free amino acids, not salts, adducts or measured mass-to-charge ratios. Residue masses and codes come from Unimod; free amino acid masses are calculated from the residue elemental composition plus H₂O using Unimod’s isotope masses, before rounding. [1, 2]

The charge descriptions concern the side chain, not the net charge of a free amino acid or a whole peptide. They are approximate classifications for aqueous conditions near pH 7; pH and the molecular environment can change ionization. Histidine is a basic amino acid but its side chain is mostly uncharged near pH 7 in the model-peptide example below. [3]

• Nonpolar: Gly, Ala, Val, Leu, Ile, Pro, Met, Phe and Trp.
• Polar, usually uncharged near pH 7: Ser, Thr, Asn, Gln, Cys and Tyr.
• Acidic: Asp and Glu; their side chains are usually negatively charged near pH 7.
• Basic: Lys, Arg and His. Lys and Arg are usually positively charged near pH 7; His is more sensitive to modest changes in pH.
• Some categories overlap: Tyr is both aromatic and polar; Cys and Met both contain sulfur but have different functional groups.

A neutral free amino acid includes the atoms of one H₂O molecule more than its conventional peptide residue. Forming a linear peptide with n residues produces n − 1 peptide bonds, so either add n free-amino-acid masses and subtract (n − 1) waters, or add n residue masses and add one water for the unmodified termini. These are two equivalent calculations. [1, 2]

Monoisotopic mass uses one specified isotope of each element, such as ¹²C, ¹H, ¹⁴N, ¹⁶O and ³²S here. Average mass uses isotope-abundance-weighted atomic masses. Keep one convention throughout a calculation: 18.010565 Da is monoisotopic H₂O, while approximately 18.015 Da is an average-mass value. Do not add the latter to a monoisotopic residue sum. [1, 2]

pKa describes an ionizable group, not a fixed property of an entire amino acid in every setting. The following values are for groups in alanine-based model pentapeptides, rounded as reported in Table 1 of Grimsley, Scholtz and Pace (2009). They are not free-amino-acid constants or universal values for folded proteins. [3]

Arginine is not included in that model-peptide table, so no value is inferred for it here. A dash or omission is not pKa zero. Non-ionizable side chains such as those of alanine and methionine have no side-chain pKa in the ordinary aqueous range; free amino acids still have terminal ionizable groups. [3]

Below its pKa a group is more protonated; above it, more deprotonated. For an isolated group at pH = pKa, the two forms are equally populated. Nearby charges, burial, solvent and hydrogen bonding can shift pKa substantially. The review reports measured Asp side-chain values from 0.5 to 9.2 in folded proteins, illustrating why a reference table cannot predict every peptide. [3]

Hydrophobicity values depend on the chosen scale. The Kyte–Doolittle scale is a hydropathy index: more positive values indicate greater hydrophobic character on that scale. Keep the scale name with any reported number. Sequence-average hydropathy is not by itself a measurement of solubility, purity or stability. [4]

• Glycine has hydrogen as its side chain and no chiral alpha-carbon.
• Leucine and isoleucine have the same elemental composition and mass but different side-chain structures. A mass match alone cannot distinguish them.
• Proline has a ring connected to the backbone nitrogen.
• Cysteine thiols can form disulfide bonds. A disulfide changes the formula and must be included in a mass calculation.

For a linear, unmodified peptide with a free N-terminus and a C-terminal carboxyl group: neutral monoisotopic mass = sum of monoisotopic residue masses + 18.010565 Da. Use unrounded input values and round only the final result. [1, 2]

A mass spectrometer commonly reports m/z, not the neutral molecular mass. Charge state, protonation, adducts and isotope assignment must be known before comparing an observed peak with a sequence calculation. There is no universal ±1 Da acceptance rule for all instruments and analyses.

Terminal modifications, cyclization, disulfides, isotopic labels and non-standard amino acids need explicit mass corrections. For example, monoisotopic N-terminal acetylation adds about 42.0106 Da and C-terminal amidation changes the mass by about −0.9840 Da. These are modifications to the unmodified sequence calculation, not extra residues. Consult the specific modification entry rather than mixing average and monoisotopic values. [1, 2]

57.0215 Da is the monoisotopic glycine residue mass; 75.0320 Da is the neutral free amino acid mass. The difference is H₂O. The older summary on this page incorrectly described a residue value as the free molecule’s molecular weight.

No. State whether a value is monoisotopic mass or average mass and whether it describes a free molecule, residue or ion. Mass in Da and molar mass in g/mol are different quantities; do not switch their labels silently.

By the neutral molecular masses in this table, glycine is the smallest and tryptophan is the largest of these 20 amino acids. This statement does not include modified or non-standard residues.

ChemVerify is responsible for this reference page. Revision of 4 October 2026: the 20 residue masses were checked against Unimod, the free-molecule values were recalculated, and the pKa table was tied to its model-peptide source. This is an editorial source check, not an external scientific certification. Earlier mixed-mass examples and a blanket mass-spectrometry tolerance were corrected.

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