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

 

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).