Key points
- Average mass uses natural isotope abundances. Use it for weighing, molarity and anything done with bulk material.
- Monoisotopic mass uses only the lightest stable isotope of each element (¹²C, ¹H, ¹⁴N, ¹⁶O, ³²S). Use it to match peaks in high-resolution mass spectrometry.
- The gap between the two grows with size, roughly 0.6–0.7 Da per 1000 Da: 0.36 Da for a pentapeptide, 2.7 Da for a 37-residue peptide.
- A calculation from sequence alone assumes a linear chain with free termini. Disulfides, C-terminal amides and counterions must be accounted for separately.
How a peptide's mass is built
Each peptide bond forms by condensation: the carboxyl group of one amino acid and the amino group of the next join and release one molecule of water. A chain of n residues contains n − 1 peptide bonds, so it has lost n − 1 waters compared with the free amino acids it was made from.
That is why peptide masses are calculated from residue masses, not from free amino acid masses. A residue mass is the mass of an amino acid minus one water, which is the unit that actually repeats in the chain. Summing residue masses and then adding back a single water, representing the H on the N-terminus and the OH on the C-terminus, gives the mass of the linear peptide:
The same result comes from summing the free amino acid masses and subtracting (n − 1) waters. Residue masses are the convention because they make modifications easy to handle: a phosphate or an acetyl group is simply added to the residue it sits on.
Worked example: Leu-enkephalin (YGGFL)
Leu-enkephalin is one of the brain's endogenous opioid peptides, five residues long. Its residue masses are:
| Residue | Average (Da) | Monoisotopic (Da) |
|---|---|---|
| Tyr (Y) | 163.1741 | 163.06333 |
| Gly (G) | 57.0516 | 57.02146 |
| Gly (G) | 57.0516 | 57.02146 |
| Phe (F) | 147.1747 | 147.06841 |
| Leu (L) | 113.1583 | 113.08406 |
| + H₂O (termini) | 18.0153 | 18.01056 |
| Peptide | 555.63 | 555.2693 |
The formula is C₂₈H₃₇N₅O₇. Entering YGGFL in the MW calculator returns the same numbers. Even for a molecule this small, the two masses already differ by 0.36 Da, which is more than enough to confuse a mass assignment if the wrong one is used.
Average vs monoisotopic: where the difference comes from
Most elements in a peptide exist as several stable isotopes. Carbon is about 98.9% ¹²C and 1.1% ¹³C; nitrogen is 99.6% ¹⁴N and 0.4% ¹⁵N; sulfur carries about 4.3% ³⁴S. The standard atomic weights published by IUPAC, such as 12.011 for carbon, are averages over these natural abundances.
Average mass is calculated with those atomic weights. It describes the mean mass of the molecules in a real sample: some contain one ¹³C atom, some two, most none. It is the correct number whenever you work with a population of molecules as a whole, for example when converting milligrams to moles.
Monoisotopic mass is calculated using only the lightest stable isotope of each element: ¹²C = 12 exactly, ¹H = 1.00783, ¹⁴N = 14.00307, ¹⁶O = 15.99491, ³²S = 31.97207. It is the exact mass of one specific molecular species, the one containing no heavy isotopes. A mass spectrometer that resolves individual isotope peaks sees exactly this species as the first peak of the cluster.
Because every carbon atom has a 1.1% chance of being ¹³C, the probability that a molecule contains at least one heavy isotope rises with the number of atoms. Average mass therefore drifts above monoisotopic mass as peptides get larger:
| Peptide | Length | Average (Da) | Monoisotopic (Da) | Difference |
|---|---|---|---|---|
| Leu-enkephalin | 5 | 555.63 | 555.2693 | 0.36 |
| Oxytocin (linear, free acid) | 9 | 1010.19 | 1009.4361 | 0.76 |
| Substance P (free acid) | 11 | 1348.62 | 1347.7121 | 0.91 |
| Melittin (free acid) | 26 | 2847.46 | 2845.7381 | 1.72 |
| LL-37 | 37 | 4493.29 | 4490.5753 | 2.71 |
All values are for unmodified linear sequences as returned by the calculator. Oxytocin, substance P and melittin are C-terminally amidated in nature, which is covered below.
Which one should you use?
| Task | Use |
|---|---|
| Weighing a peptide, preparing a stock solution, calculating molarity | Average |
| Comparing with the MW on a supplier's certificate of analysis | Average, usually |
| Assigning peaks in high-resolution MS (Orbitrap, FT-ICR, modern Q-TOF) | Monoisotopic |
| Database searching of MS/MS data in proteomics | Monoisotopic |
| Low-resolution MS, or large peptides where isotope peaks are not resolved | Average |
| Linear-mode MALDI of peptides above ~5 kDa | Average |
Isotope patterns: what the spectrum actually shows
In a resolved mass spectrum a peptide does not appear as a single line but as a cluster of peaks spaced about 1 Da apart (divided by the charge state). The first peak, M, is the monoisotopic species. M+1 contains one ¹³C or ¹⁵N; M+2 contains two heavy atoms or one ³⁴S, and so on.
The crossover, where M and M+1 are about equal, lies around 1.5–2 kDa for typical peptides. Above that the monoisotopic peak is no longer the tallest, and for proteins above roughly 10–15 kDa it may be too weak to detect at all. A common mistake is to take the tallest peak of a large peptide and compare it with the calculated monoisotopic mass. For LL-37 that produces an apparent error of about 2 Da, which looks like a modification but is not.
When the monoisotopic peak cannot be seen, software estimates it by fitting the observed cluster to a model distribution. The best-known model is the "averagine" residue introduced by Senko and colleagues in 1995, a hypothetical average amino acid with a fixed elemental composition.
Try it: enter any sequence to get both masses and the elemental formula.
Open MW Calculator →Modifications: disulfides, amides and other changes
A calculation from sequence alone describes a linear chain with a free amine at the N-terminus and a free carboxylic acid at the C-terminus. Many natural and synthetic peptides differ from this, and each difference has a precise mass shift that can be added to the base value.
Worked example: oxytocin
Oxytocin has the sequence CYIQNCPLG, but the native hormone differs from that linear chain in two ways. Cys1 and Cys6 are joined by a disulfide bond, which closes a six-residue ring. The C-terminal glycine is amidated, ending in CONH₂ rather than COOH.
The final values match the literature figures for oxytocin. Using the uncorrected linear value would put the answer 3 Da too high, easily enough to misidentify a mass spectrum or to question the purity of a perfectly good sample.
C-terminal amidation is common among peptide hormones and neuropeptides. Oxytocin, vasopressin, substance P, calcitonin, melittin and many insect and amphibian peptides are amidated. It is worth checking the literature before assuming a free acid.
Mass shifts for common modifications
| Modification | Change | Monoisotopic Δ (Da) | Average Δ (Da) |
|---|---|---|---|
| Disulfide bond (per bond) | −2H | −2.0157 | −2.0159 |
| C-terminal amidation | −O +NH | −0.9840 | −0.9848 |
| N-terminal acetylation | +C₂H₂O | +42.0106 | +42.0367 |
| Phosphorylation (Ser, Thr, Tyr) | +HPO₃ | +79.9663 | +79.9799 |
| Methionine oxidation | +O | +15.9949 | +15.9994 |
| Proline hydroxylation | +O | +15.9949 | +15.9994 |
| Pyroglutamate from N-terminal Gln | −NH₃ | −17.0265 | −17.0305 |
| Pyroglutamate from N-terminal Glu | −H₂O | −18.0106 | −18.0153 |
| Deamidation (Asn→Asp, Gln→Glu) | −NH +O | +0.9840 | +0.9848 |
| Carbamidomethyl Cys (iodoacetamide) | +C₂H₃NO | +57.0215 | +57.0513 |
Two of these deserve attention because they happen by accident. Methionine oxidation occurs during storage and handling, so an unexpected +16 Da peak in a Met-containing peptide is usually an artefact rather than a new species. Deamidation shifts the mass by less than 1 Da, which low-resolution instruments may not separate from the M+1 isotope peak. It is fastest at Asn–Gly sequences.
From mass to m/z: reading charge states
Mass spectrometers measure mass-to-charge ratio (m/z), not mass. In positive-mode electrospray, a peptide picks up z protons, and each proton adds 1.00728 Da:
For native oxytocin (monoisotopic M = 1006.4364) the singly protonated ion [M+H]⁺ appears at m/z 1007.4437 and the doubly protonated ion [M+2H]²⁺ at m/z 504.2255. In a doubly charged ion the isotope peaks are spaced 0.5 m/z apart instead of 1.0, which is the standard way to read the charge state directly from the spectrum.
Larger peptides with many basic residues carry more charges. LL-37 contains six lysines and five arginines, and in electrospray it typically appears as a series of ions from about 3+ to 7+. Software combines this series back into a single neutral mass by deconvolution.
Weighing peptides: salts and net peptide content
The molecular weight of the peptide is not the same as the mass of the powder in the vial. Synthetic peptides are usually purified by reversed-phase HPLC with trifluoroacetic acid (TFA) in the mobile phase, and after lyophilisation the basic groups are left as TFA salts. The powder also retains some water.
Suppliers report this as net peptide content: the fraction of the powder mass that is actually peptide. Values in the range 60–90% are common, and the lower end is typical for peptides rich in Lys and Arg, since each basic group can hold a counterion.
Worked example: LL-37 as a TFA salt
LL-37 has eleven basic side chains (six Lys, five Arg) plus the N-terminal amine: twelve potential sites for a trifluoroacetate counterion. Each TFA adds 114.02 Da. If all twelve sites carry TFA, the salt form weighs 4493.29 + 12 × 114.02 ≈ 5861 Da, so the peptide makes up only about 77% of the mass, before any water is counted.
Ignoring the salt overestimates the concentration by a third. For quantitative work such as binding constants, MIC assays or dose–response curves, use the net peptide content from the certificate of analysis. Better still, measure the concentration directly: by UV absorbance at 280 nm if the peptide contains Trp or Tyr, or by amino acid analysis if it does not.
Where it matters, TFA can be exchanged for acetate (60.05 Da) or chloride (36.46 Da as HCl). Residual TFA is itself known to interfere with some cell-based assays.
Common mistakes
- Using free amino acid masses without subtracting water. Five free amino acids summed directly overshoot the pentapeptide by 4 × 18.02 = 72 Da.
- Forgetting disulfide bonds. Each one removes 2.016 Da. Insulin, with three disulfides, is 6 Da lighter than its two reduced chains combined.
- Assuming a free C-terminus for a peptide that is amidated in nature or was synthesised on an amide resin.
- Comparing an average mass with a monoisotopic peak, or the tallest peak of a large peptide with its monoisotopic mass.
- Treating powder mass as peptide mass when preparing solutions, which ignores counterions and water.
- Expecting mass to distinguish Leu from Ile. They are exact isomers (113.08406 Da). Lys (128.09496) and Gln (128.05858) differ by only 0.036 Da and need high resolution to separate.
Frequently asked questions
Is a dalton the same as g/mol?
Numerically, yes. The dalton (Da, also written u) is a unit of mass for a single molecule. g/mol is molar mass, the mass of one mole of molecules. A peptide of 1007.19 Da has a molar mass of 1007.19 g/mol. Biochemists often use kDa for larger peptides and proteins.
Why does my calculated MW not match the supplier's certificate?
Check four things in order: whether the supplier's value is average or monoisotopic; whether it includes disulfide bonds; whether the C-terminus is an acid or an amide; and whether the figure refers to the free peptide or to a salt form. Almost every discrepancy of a few daltons is explained by one of these.
Does the calculator on this site account for modifications?
No. It calculates the linear peptide with a free N-terminus and a free C-terminal acid. Apply the mass shifts from the table above to account for disulfides, amidation, acetylation and other modifications.
What is "nominal mass"?
Nominal mass uses integer masses for each element (C = 12, H = 1, N = 14, O = 16, S = 32). It is useful for quick mental arithmetic and for low-resolution instruments, but it ignores the mass defect, which becomes significant for larger molecules. Hydrogen-rich peptides in the 1–3 kDa range have monoisotopic masses about 0.5–1.5 Da above their nominal mass.
Why do different calculators give slightly different average masses?
Atomic weights are revised periodically by IUPAC, and different tools use different editions or round residue masses to different precision. Differences in the second decimal place are normal and have no practical consequence. Differences of a whole dalton or more point to a modification or terminus mismatch.
Related: net charge and isoelectric point are covered in the pI calculator.
Open pI Calculator →References
- Prohaska T, Irrgeher J, Benefield J, et al. (2022) Standard atomic weights of the elements 2021 (IUPAC Technical Report). Pure and Applied Chemistry 94(5):573–600.
- Senko MW, Beu SC, McLafferty FW (1995) Determination of monoisotopic masses and ion populations for large biomolecules from resolved isotopic distributions. Journal of the American Society for Mass Spectrometry 6:229–233.
- Mann M, Hendrickson RC, Pandey A (2001) Analysis of proteins and proteomes by mass spectrometry. Annual Review of Biochemistry 70:437–473.
- Creasy DM, Cottrell JS (2004) Unimod: protein modifications for mass spectrometry. Proteomics 4:1534–1536.