Guide
Monoisotopic mass versus average molecular weight
Monoisotopic mass uses only the most abundant isotope of each element (carbon-12, hydrogen-1, nitrogen-14, oxygen-16, sulfur-32) and is the first peak of a mass spectrum's isotope pattern. Average molecular weight uses standard atomic weights, which average over natural isotopes, and is the figure for weighing and molar arithmetic. For BPC-157 they are 1,418.7042 Da and 1,419.56 g/mol.
Two ways to add up the same formula
Every element occurs naturally as a mixture of isotopes. Average molecular weight multiplies each atom by its standard atomic weight — about 12.011 for carbon, reflecting roughly 1.07% carbon-13 — and is what a balance effectively measures when weighing a bulk sample.
Monoisotopic mass instead uses the exact mass of the lightest stable isotope of each element. It describes one specific molecule, the one containing no heavy isotopes at all, and it is the value a high-resolution mass spectrometer resolves as a distinct peak.
Why the tallest peak is often not the monoisotopic one
The chance that a molecule contains at least one carbon-13 grows with the number of carbons. For a small peptide the all-carbon-12 molecule is still the most common, but for larger chains the most probable species carries one or more carbon-13 atoms, each adding about 1.00335 Da.
BPC-157 (62 carbons) peaks at its monoisotopic mass. Sermorelin (149 carbons) peaks one carbon-13 higher: its monoisotopic mass is 3,355.8187 Da but its most abundant species is near 3,356.82 Da. Both are correct; they are different members of the same isotope envelope.
Mass-to-charge: what the spectrum actually shows
Electrospray ionisation adds protons, and the instrument measures mass divided by charge. A peptide of monoisotopic mass M appears at (M + n × 1.00728) / n for n added protons. For BPC-157 that is m/z 1,419.7114 for [M+H]⁺, 710.3594 for [M+2H]²⁺ and 473.9087 for [M+3H]³⁺ — the values identity is confirmed against.
Which figure to expect where
- Molecular weight on a specification or product page: average, in g/mol.
- Identity result on a certificate from mass spectrometry: an observed m/z or deconvoluted mass, compared against the monoisotopic or most-abundant value the method specifies.
- Public records such as PubChem: usually both, labelled as molecular weight and exact or monoisotopic mass.
Common questions
Which is larger, monoisotopic or average mass?
Average mass, for any peptide: heavy isotopes can only add mass, so averaging over them always gives a value above the all-light-isotope monoisotopic mass.
Which mass does mass spectrometry measure?
A mass spectrometer measures mass-to-charge ratio of individual ions. High-resolution instruments resolve the isotope pattern, whose first peak corresponds to the monoisotopic mass.
Why do two sources give different molecular weights for the same peptide?
Usually because one quotes average molecular weight and the other monoisotopic or exact mass, or because one describes the free peptide and the other a salt form.
References
- Standard Atomic Weights — Commission on Isotopic Abundances and Atomic Weights (CIAAW), IUPAC
- Atomic Weights and Isotopic Compositions for All Elements — National Institute of Standards and Technology (NIST)
- Nomenclature and symbolism for amino acids and peptides (IUPAC-IUB JCBN recommendations 1983) — IUPAC-IUB Joint Commission on Biochemical Nomenclature, Biochemical Journal, 1984
From the catalog
Compounds referenced on this page
For laboratory research use only. Not for human or veterinary consumption.