Amino acid reference data

The data in this table are for amino acid residues. To calculate the mass of a neutral peptide or protein, sum the residue masses plus the masses of the terminating groups (e.g. H at the N-terminus and OH at the C-terminus).

Residue	3- letter code	1- letter code	Mono- isotopic mass	Average mass	Structure
Alanine C3H5NO	Ala	A	71.037114	71.0779
Arginine C6H12N4O	Arg	R	156.101111	156.1857
Asparagine C4H6N2O2	Asn	N	114.042927	114.1026
Aspartic acid C4H5NO3	Asp	D	115.026943	115.0874
Asn or Asp	Asx	B
Cysteine C3H5NOS	Cys	C	103.009185	103.1429
Glutamic acid C5H7NO3	Glu	E	129.042593	129.114
Glutamine C5H8N2O2	Gln	Q	128.058578	128.1292
Glu or Gln	Glx	Z
Glycine C2H3NO	Gly	G	57.021464	57.0513
Histidine C6H7N3O	His	H	137.058912	137.1393
Isoleucine C6H11NO	Ile	I	113.084064	113.1576
Leucine C6H11NO	Leu	L	113.084064	113.1576
Lysine C6H12N2O	Lys	K	128.094963	128.1723
Methionine C5H9NOS	Met	M	131.040485	131.1961
Phenylalanine C9H9NO	Phe	F	147.068414	147.1739
Proline C5H7NO	Pro	P	97.052764	97.1152
Serine C3H5NO2	Ser	S	87.032028	87.0773
Threonine C4H7NO2	Thr	T	101.047679	101.1039
Selenocysteine C3H5NOSe	Sec	U	150.95363	150.0379
Tryptophan C11H10N2O	Trp	W	186.079313	186.2099
Tyrosine C9H9NO2	Tyr	Y	163.06332	163.1733
Unknown	Xaa	X
Valine C5H9NO	Val	V	99.068414	99.1311

When Mascot tests a peptide containing ambiguous residue codes, it expands and tests all possibilities. For example, a peptide containing a single B will be tested twice, once with N at that position and once with D. A peptide containing a single X will generate 20 peptides to be tested. To avoid excessive search times, Mascot will test all possibilities up to a maximum of 3 B or Z residues and 1 X residue per peptide. Above this limit, the weighted average mass will be used (~ 110 Da).