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
Alanine C₃H₅NO	Ala	A	71.03712	71.08
Arginine C₆H₁₂N₄O	Arg	R	156.10112	156.19
Asparagine C₄H₆N₂O₂	Asn	N	114.04293	114.10
Aspartic acid C₄H₅NO₃	Asp	D	115.02695	115.09
Asn or Asp	Asx	B
Cysteine C₃H₅NOS	Cys	C	103.00919	103.14
Glutamic acid C₅H₇NO₃	Glu	E	129.04260	129.12
Glutamine C₅H₈N₂O₂	Gln	Q	128.05858	128.13
Glu or Gln	Glx	Z
Glycine C₂H₃NO	Gly	G	57.02147	57.05
Histidine C₆H₇N₃O	His	H	137.05891	137.14
Isoleucine C₆H₁₁NO	Ile	I	113.08407	113.16
Leucine C₆H₁₁NO	Leu	L	113.08407	113.16
Lysine C₆H₁₂N₂O	Lys	K	128.09497	128.17
Methionine C₅H₉NOS	Met	M	131.04049	131.19
Phenylalanine C₉H₉NO	Phe	F	147.06842	147.18
Proline C₅H₇NO	Pro	P	97.05277	97.12
Serine C₃H₅NO₂	Ser	S	87.03203	87.08
Threonine C₄H₇NO₂	Thr	T	101.04768	101.10
Selenocysteine C₃H₅NOSe	SeC	U	150.95364	150.03
Tryptophan C₁₁H₁₀N₂O	Trp	W	186.07932	186.21
Tyrosine C₉H₉NO₂	Tyr	Y	163.06333	163.18
Unknown	Xaa	X
Valine C₅H₉NO	Val	V	99.06842	99.13

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, as in the table above.