What is Molecular weight?
Also called: MW, Molar mass, Average mass, Formula weight
The mass of one mole of a substance in grams (g/mol), numerically equal to the average mass of one molecule in daltons; the figure that converts milligrams into moles.
By the APL Research Team · Updated
Molecular weight, more strictly molar mass, is the mass of one mole of molecules in g/mol. For a peptide it is calculated from the formula using average atomic weights, which already reflect the natural mix of isotopes. It is the number that converts a weighed amount into moles, and therefore into a molar concentration.
One peptide, several masses
MOTS-c (MRWQEMGYIFYPRKLR, C101H152N28O22S2) shows why "the mass" needs qualifying:
| Quantity | Value | Used for |
|---|---|---|
| Average molecular weight | 2174.59 g/mol | Weighing, molarity, dilutions |
| Monoisotopic mass | 2173.11 Da | High-resolution mass spectrometry |
| Tallest isotope peak | about 2174.11 Da (one ¹³C) | The peak that looks like "the" mass in a spectrum |
| Formula weight as the penta-TFA salt | 2744.70 g/mol | Weighing the salt form |
The first three differ by only about 1.5 Da here, but each belongs to a different job: average mass for anything involving a balance, monoisotopic or tallest-peak values for spectra. For typical peptides the gap between average and monoisotopic mass grows by roughly 0.6 Da per kilodalton, so mixing conventions produces apparent errors that are really just definitions.
Worked example: milligrams to moles
1 mg of MOTS-c as the free peptide is 1 × 10⁻³ g ÷ 2174.59 g/mol = 459.9 nmol. If the powder were the penta-trifluoroacetate salt (five basic sites: the N-terminus, Lys14 and three Arg), the same milligram would hold 364.3 nmol of peptide, about 21% less. Dividing by the free-peptide molecular weight when weighing the salt would overstate every downstream concentration by about 26%. The counter-ion and net peptide content entries cover how to correct for this.
The 110-dalton shortcut and where it fails
A common estimate is about 110 Da per residue. For MOTS-c it gives 16 × 110 = 1,760, 19% below the true value, because the sequence is rich in heavy residues: three Arg, two Tyr, two Met, a Trp and a Phe. The shortcut is good enough for choosing a filter cut-off, not for making solutions. The peptide molecular weight calculator gives exact average and monoisotopic values and handles acetylation, amidation and disulfide bonds, each of which changes the formula.
| Modification | Change in average mass |
|---|---|
| N-terminal acetylation | +42.04 |
| C-terminal amidation | −0.98 |
| One disulfide bond | −2.02 |
| Free acid to TFA salt, per basic site | +114.02 |
Units
Molecular weight in g/mol and molecular mass in daltons are numerically identical: 2174.59 g/mol means one MOTS-c molecule weighs 2174.59 Da. Kilodaltons (kDa) are used for larger peptides and proteins. The molarity calculator converts between mass, amount and concentration once the correct molecular weight is chosen, and the molarity calculations guide works through stock and working-solution examples.