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Mascot database search > Help > Amino acid reference data

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).

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