Key facts
- Definition
- Sum of the standard atomic weights of all atoms in a molecular formula
- Usual unit
- g/mol (numerically equal to daltons per molecule)
- Also called
- Relative molecular mass (Mr) or molar mass
- Example
- BPC-157, C62H98N16O22: 1419.5 g/mol (PubChem CID 9941957)
- Water lost per peptide bond
- 18.015 g/mol
- Related value
- Monoisotopic mass (BPC-157: 1418.70 Da)
What is molecular weight?
Molecular weight is the mass of a molecule, worked out from the atoms it contains. Chemists are fussy about the names. The unitless number is properly the relative molecular mass (Mr), and the same number in grams per mole is the molar mass. Product records, certificates of analysis and databases use all three terms interchangeably, and the number does not change.
One point trips people up: a molecular weight is an average. Most elements occur as a natural mixture of isotopes. Carbon, for instance, is roughly 98.9% carbon-12 and 1.1% carbon-13. Standard atomic weights build in that natural mix, so the figure describes the average mass across a huge population of molecules, not any single one [2]. The single-molecule figure is the monoisotopic mass, and we compare the two in molecular weight vs. molecular mass.
On a peptide record, this value sits with the other core identifiers: the peptide sequence, the molecular formula and the CAS number.
How is molecular weight calculated?
Take the molecular formula, which gives you the atom counts. Multiply the count for each element by that element's standard atomic weight, then add everything up. Here it is for BPC-157:
| Element | Atoms | Atomic weight (g/mol) | Contribution (g/mol) |
|---|---|---|---|
| Carbon (C) | 62 | 12.011 | 744.68 |
| Hydrogen (H) | 98 | 1.008 | 98.78 |
| Nitrogen (N) | 16 | 14.007 | 224.11 |
| Oxygen (O) | 22 | 15.999 | 351.98 |
| Total | 198 | 1419.56 |
PubChem gives 1419.5 g/mol for the same formula [1]. The small gap in the last decimal place comes from the atomic weight values and rounding the database uses. It is the same molecule. As a rule of thumb, a gap of about 0.1 g/mol for a peptide this size is rounding. Once the gap reaches several g/mol, something real is usually going on, such as a salt form, a modification or a different sequence.
How do you calculate the molecular weight of a peptide?
Add up the free amino acids, then take away one water molecule (18.015 g/mol) for every peptide bond formed. The result is the weight of the peptide. Each peptide bond comes from a condensation reaction that releases water, which means a chain of n amino acids loses n minus 1 waters.
| Step | Value (g/mol) |
|---|---|
| L-Lysine | 146.19 |
| L-Proline | 115.13 |
| L-Valine | 117.15 |
| Sum of free amino acids | 378.47 |
| Minus 2 waters (2 peptide bonds) | -36.03 |
| Calculated molecular weight | 342.44 |
| PubChem value for KPV (CID 125672) | 342.4 |
That agrees with the PubChem record for KPV [1]. Modifications shift the number in predictable ways:
- C-terminal amide (written -NH2): replaces OH with NH2 and lowers the weight by about 0.98 g/mol. Sermorelin is amidated.
- N-terminal acetylation: adds C2H2O, about 42.04 g/mol.
- Disulfide bond: removes two hydrogen atoms, about 2.02 g/mol.
- Metal complex: GHK-Cu includes a copper ion, so its value (402.9 g/mol) differs from the peptide GHK alone.
Why does molecular weight matter for peptides?
It does two jobs. It is the expected value that analytical results get checked against, and it lets you convert between mass and amount of substance.
- Identity checks. Mass spectrometry measures ions, and the analyst compares the observed mass with the one expected from the formula. With large molecules you have to be careful which peak you compare against, since isotopes spread the signal over several peaks [3].
- Moles from milligrams. Amount in moles equals mass divided by molecular weight. One milligram of a substance weighing 1419.5 g/mol is about 0.704 micromoles. Labs rely on this when they prepare analytical standards at defined concentrations.
- Spotting the wrong form. When the measured value does not fit the expected sequence, look for a salt form, a truncated chain or a modification.
Does the listed molecular weight include salts and water?
Usually not. The figure on a compound page normally refers to the free peptide, but the solid material can also hold counter-ions and water. Synthetic peptides are commonly isolated as salts with acetate or trifluoroacetate (TFA), and specifications for peptide substances treat counter-ion and water content as separate test items [4].
Each acetic acid adds 60.05 g/mol and each trifluoroacetic acid adds 114.02 g/mol. Isolate a peptide with three basic sites as a tris-TFA salt, and each molecule weighs about 342 g/mol more than the free form. It is one reason the weight of powder in a vial and the weight of peptide are different numbers, as explained in peptide purity and on the peptide testing page.
Before comparing two molecular weights, check whether both refer to the same form: free peptide, acetate salt, TFA salt or metal complex. When two correct values disagree, mismatched forms are the usual culprit.
Molecular weight values for Vinnix compounds
Here are the molecular weights for compounds in the Vinnix compound library, taken from PubChem records for the forms shown [1].
| Compound | Molecular formula | Molecular weight (g/mol) | PubChem CID |
|---|---|---|---|
| KPV | C16H30N4O4 | 342.4 | 125672 |
| GHK-Cu (copper complex) | C14H23CuN6O4+ | 402.9 | 71587328 |
| SS-31 | C32H49N9O5 | 639.8 | 11764719 |
| Ipamorelin | C38H49N9O5 | 711.9 | 9831659 |
| BPC-157 | C62H98N16O22 | 1419.5 | 9941957 |
| MOTS-c | C101H152N28O22S2 | 2174.6 | 146675088 |
| Sermorelin | C149H246N44O42S | 3357.9 | 16132413 |
Common mistakes when reading molecular weight
Nearly every mix-up here comes from comparing two values that describe different things.
- Comparing an average molecular weight with a monoisotopic mass from a high-resolution mass spectrum. For BPC-157 these differ by about 0.8 units.
- Comparing a free-peptide value with a salt-form value.
- Mixing units: proteins are often quoted in kilodaltons (kDa), where 1 kDa equals 1,000 g/mol.
- Treating a matching molecular weight as proof of identity. Leucine and isoleucine both weigh 131.17 g/mol, so swapping one for the other in a sequence leaves the weight unchanged.
FAQFrequently asked questions
What is the unit of molecular weight?
Grams per mole (g/mol), most of the time. The same number can be written in daltons (Da) when it describes the mass of a single molecule, because one dalton per molecule corresponds to one gram per mole. Strictly, relative molecular mass has no unit at all, but product documents and databases almost always show g/mol.
Is molecular weight the same as molar mass?
In practice, yes. Molar mass is the mass of one mole of a substance in g/mol, and molecular weight is that same number treated as a relative value. Both are averages based on the natural isotope mix of each element. The value that differs is the monoisotopic mass, which uses only the most abundant isotope of each element and is slightly lower.
What is the molecular weight of BPC-157?
PubChem lists the molecular weight of BPC-157 as 1419.5 g/mol for the molecular formula C62H98N16O22 (CID 9941957). That value refers to the free peptide. Salt forms such as an acetate salt have a different formula and a higher weight, so a product record should state which form it describes. The monoisotopic mass of the free peptide is 1418.70 Da.
Why do different sources list slightly different molecular weights?
Small differences are usually down to which atomic weight table a source uses and how it rounds. For BPC-157, adding standard atomic weights gives about 1419.56 g/mol while PubChem shows 1419.5 g/mol. Larger differences of several g/mol or more usually mean the sources describe different forms, such as a free peptide versus a salt, or a modified sequence.
Does molecular weight confirm a peptide's identity?
Not on its own. A match tells you the result is consistent with the expected compound, but different molecules can share a formula and a weight. Leucine and isoleucine are a simple example: both are C6H13NO2 and 131.17 g/mol. Real identity work pairs mass measurement with sequence information and chromatographic behavior, as described in the Vinnix peptide testing pages.
How is the molecular weight of a peptide calculated?
Add the molecular weights of the free amino acids in the sequence, then subtract 18.015 g/mol for each peptide bond, because each bond releases one water molecule. For KPV, lysine, proline and valine sum to 378.47 g/mol; subtracting two waters gives 342.44 g/mol, which matches the PubChem value of 342.4 g/mol. Modifications such as amidation adjust the result further.
REFScientific references
-
Kim S, Chen J, Cheng T, et al. PubChem 2025 update. Nucleic Acids Res. 2025;53(D1):D1516-D1525. PubMed 39558165
database (PubChem description) -
Yergey JA. A general approach to calculating isotopic distributions for mass spectrometry. J Mass Spectrom. 2020;55(8):e4498. PubMed 31957110
methods paper (isotope calculation) -
Senko MW, Beu SC, McLafferty FW. Determination of monoisotopic masses and ion populations for large biomolecules from resolved isotopic distributions. J Am Soc Mass Spectrom. 1995;6(4):229-233. PubMed 24214167
analytical methods study (mass spectrometry) -
Vergote V, Burvenich C, Van de Wiele C, De Spiegeleer B. Quality specifications for peptide drugs: a regulatory-pharmaceutical approach. J Pept Sci. 2009;15(11):697-710. PubMed 19750489
review (pharmacopeial specifications)
Research Library
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Testing & Analysis
Compound Library