Symbols and Mass Values
Symbols
The following symbols are recognised in Unimod composition and neutral loss fields. Some represent elements
while others represent molecular sub-units. Note that C, F, H, etc. are symbols for elements, not one letter
codes for amino acids.
| Symbol |
Description |
|---|
| 13C | Carbon 13 |
| 15N | Nitrogen 15 |
| 18O | Oxygen 18 |
| 2H | Deuterium |
| Ac | Acetate |
| Ag | Silver |
| Al | Aluminium |
| As | Arsenic |
| Au | Gold |
| B | Boron |
| Br | Bromine |
| C | Carbon |
| Ca | Calcium |
| Cd | Cadmium |
| Cl | Chlorine |
| Co | Cobalt |
| Cr | Chromium |
| Cu | Copper |
| dHex | Deoxy-hexose |
| F | Fluorine |
| Fe | Iron |
| H | Hydrogen |
| Hep | Heptose |
| Hex | Hexose |
| HexA | Hexuronic acid |
| HexN | Hexosamine |
| HexNAc | N-Acetyl Hexosamine |
| Hg | Mercury |
| I | Iodine |
| K | Potassium |
| Kdn | 3-deoxy-d-glycero-D-galacto-nonulosonic acid |
| Kdo | 2-keto-3-deoxyoctulosonic acid |
| Li | Lithium |
| Me | Methyl |
| Mg | Magnesium |
| Mn | Manganese |
| Mo | Molybdenum |
| N | Nitrogen |
| Na | Sodium |
| NeuAc | N-acetyl neuraminic acid |
| NeuGc | N-glycoyl neuraminic acid |
| Ni | Nickel |
| O | Oxygen |
| P | Phosphorous |
| Pd | Palladium |
| Pent | Pentose |
| Phos | Phosphate |
| Pt | Platinum |
| Ru | Ruthenium |
| S | Sulphur |
| Se | Selenium |
| Sulf | Sulphate |
| Water | Water |
| Zn | Zinc |
Mass values
All mass values in Unimod are calculated from the IUPAC atomic weights and isotopic abundances tabulated by
WebElements:
| Element |
Name |
Monoisotopic |
Average |
|---|
| H | Hydrogen | 1.007825035 | 1.00794 |
| 2H | Deuterium | 2.014101779 | 2.014101779 |
| Li | Lithium | 7.016003 | 6.941 |
| B | Boron | 11.0093055 | 10.811 |
| C | Carbon | 12 | 12.0107 |
| 13C | Carbon13 | 13.00335483 | 13.00335483 |
| N | Nitrogen | 14.003074 | 14.0067 |
| 15N | Nitrogen15 | 15.00010897 | 15.00010897 |
| O | Oxygen | 15.99491463 | 15.9994 |
| 18O | Oxygen18 | 17.9991603 | 17.9991603 |
| F | Fluorine | 18.99840322 | 18.9984032 |
| Na | Sodium | 22.9897677 | 22.98977 |
| Mg | Magnesium | 23.9850423 | 24.305 |
| Al | Aluminium | 26.9815386 | 26.9815386 |
| P | Phosphorous | 30.973762 | 30.973761 |
| S | Sulfur | 31.9720707 | 32.065 |
| Cl | Chlorine | 34.96885272 | 35.453 |
| K | Potassium | 38.9637074 | 39.0983 |
| Ca | Calcium | 39.9625906 | 40.078 |
| Cr | Chromium | 51.9405098 | 51.9961 |
| Mn | Manganese | 54.9380471 | 54.938045 |
| Fe | Iron | 55.9349393 | 55.845 |
| Ni | Nickel | 57.9353462 | 58.6934 |
| Co | Cobalt | 58.9331976 | 58.933195 |
| Cu | Copper | 62.9295989 | 63.546 |
| Zn | Zinc | 63.9291448 | 65.409 |
| As | Arsenic | 74.9215942 | 74.9215942 |
| Br | Bromine | 78.9183361 | 79.904 |
| Se | Selenium | 79.9165196 | 78.96 |
| Mo | Molybdenum | 97.9054073 | 95.94 |
| Ru | Ruthenium | 101.9043485 | 101.07 |
| Pd | Palladium | 105.903478 | 106.42 |
| Ag | Silver | 106.905092 | 107.8682 |
| Cd | Cadmium | 113.903357 | 112.411 |
| I | Iodine | 126.904473 | 126.90447 |
| Pt | Platinum | 194.964766 | 195.084 |
| Au | Gold | 196.966543 | 196.96655 |
| Hg | Mercury | 201.970617 | 200.59 |
Monoisotopic mass is calculated using the mass of the most abundant natural isotope
of each constituent element. An average mass is calculated using the "atomic weight" of each constituent element,
which is the weighted average of all its natural isotopes. This definition of monoisotopic mass conforms to
that given in Standard Definitions of Terms Relating to Mass Spectrometry, Phil. Price, J. Am. Soc.
Mass Spectrom. (1991) 2 336-348. Some sources suggest that monoisotopic mass is calculated from the
lightest natural isotope of each element, but this is wrong.
The compositions and mass values in Unimod define the differences (or "deltas") between the
modified and unmodified residues. Although monoisotopic values are displayed with 6 decimal places, actual accuracies
will usually be 7 to 8 significant figures. In the case of average masses, accuracies will usually be
5 to 6 significant figures.
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