Reference · 25 residues

Amino acid reference

The twenty standard residues and five non-standard ones, with the exact values Sequence Studio and every product page compute from.

Standard: 20
Non-standard: 5
Updated: October 16, 2026

Residue colors: Hydrophobic Polar Basic (+) Acidic (−) Non-standard

Select a column heading to sort; standard and non-standard residues sort separately. The table scrolls sideways and down.

Amino acid residues: codes, name, class, residue formula, average and monoisotopic residue mass, Kyte–Doolittle hydropathy and side-chain pKa.
Code	Name	Class	Formula	Average mass	Monoisotopic mass	Hydropathy	pKa
Standard residues20
Ala, one-letter A	Alanine	Hydrophobic	C₃H₅NO	71.08	71.0371	+1.8	none
Arg, one-letter R	Arginine	Basic	C₆H₁₂N₄O	156.19	156.1011	−4.5	12.5
Asn, one-letter N	Asparagine	Polar	C₄H₆N₂O₂	114.10	114.0429	−3.5	none
Asp, one-letter D	Aspartic acid	Acidic	C₄H₅NO₃	115.09	115.0269	−3.5	3.9
Cys, one-letter C	Cysteine	Polar	C₃H₅NOS	103.14	103.0092	+2.5	8.5
Glu, one-letter E	Glutamic acid	Acidic	C₅H₇NO₃	129.11	129.0426	−3.5	4.1
Gln, one-letter Q	Glutamine	Polar	C₅H₈N₂O₂	128.13	128.0586	−3.5	none
Gly, one-letter G	Glycine	Hydrophobic	C₂H₃NO	57.05	57.0215	−0.4	none
His, one-letter H	Histidine	Basic	C₆H₇N₃O	137.14	137.0589	−3.2	6.5
Ile, one-letter I	Isoleucine	Hydrophobic	C₆H₁₁NO	113.16	113.0841	+4.5	none
Leu, one-letter L	Leucine	Hydrophobic	C₆H₁₁NO	113.16	113.0841	+3.8	none
Lys, one-letter K	Lysine	Basic	C₆H₁₂N₂O	128.17	128.0950	−3.9	10.8
Met, one-letter M	Methionine	Hydrophobic	C₅H₉NOS	131.20	131.0405	+1.9	none
Phe, one-letter F	Phenylalanine	Hydrophobic	C₉H₉NO	147.17	147.0684	+2.8	none
Pro, one-letter P	Proline	Hydrophobic	C₅H₇NO	97.12	97.0528	−1.6	none
Ser, one-letter S	Serine	Polar	C₃H₅NO₂	87.08	87.0320	−0.8	none
Thr, one-letter T	Threonine	Polar	C₄H₇NO₂	101.10	101.0477	−0.7	none
Trp, one-letter W	Tryptophan	Hydrophobic	C₁₁H₁₀N₂O	186.21	186.0793	−0.9	none
Tyr, one-letter Y	Tyrosine	Polar	C₉H₉NO₂	163.17	163.0633	−1.3	10.1
Val, one-letter V	Valine	Hydrophobic	C₅H₉NO	99.13	99.0684	+4.2	none
Non-standard residues5
Aib	2-Aminoisobutyric acid	Non-standard	C₄H₇NO	85.10	85.0528	+1.8	none
Dmt	2',6'-Dimethyltyrosine	Non-standard	C₁₁H₁₃NO₂	191.23	191.0946	−1.3	10.1
αMeLeu	α-Methyl-L-leucine	Non-standard	C₇H₁₃NO	127.18	127.0997	+3.8	none
2Nal	3-(2-Naphthyl)alanine	Non-standard	C₁₃H₁₁NO	197.23	197.0841	+2.8	none
Nle	Norleucine	Non-standard	C₆H₁₁NO	113.16	113.0841	+3.8	none

Masses: In daltons (Da), per residue: the amino acid minus water, as it sits inside a chain. Average mass, to two decimals, weights every isotope by its natural abundance, using the IUPAC 2007 standard atomic weights; monoisotopic mass, to four, counts only the most abundant isotope of each element.
Hydropathy: Kyte & Doolittle (1982): positive is hydrophobic. Non-standard residues use the value of their nearest standard analog where no published value exists.
pKa: Side-chain pKa from the EMBOSS set, to one decimal. Dmt, the only ionizable non-standard residue here, takes tyrosine’s value; its ring methyls shift the true pKa somewhat. A dash means the side chain has no ionizable group.

Amino acid reference

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