Each amino acid adds a known mass when it joins the chain (that of its “residue”, that is, without the water molecule lost in the peptide bond). Adding up the residues, plus one water molecule for the ends and the modifications (amidation, acetylation, fatty-acid chains, metals such as the copper in GHK-Cu) gives the theoretical mass.
The ranges are very wide: the tripeptide KPV is around 340 Da, whereas semaglutide exceeds 4,100 Da and tirzepatide 4,800 Da. That mass determines, among other things, how many molecules there are in one milligram.
Two values are distinguished: the monoisotopic mass (using the most abundant isotope of each element), useful in high-resolution spectrometry, and the average molecular weight, which is the one shown on data sheets.
Molecular weight is the reference against which mass spectrometry is compared to confirm a batch's identity, and the basis for converting between mass (mg) and amount of substance (moles) in any laboratory calculation.
Related terms
Learn it in depth at the Peptide University
- Chemical modifications: from the natural sequence to the analog · Module 1
- Mass spectrometry and identity · Module 7
- Related impurities and counterions · Module 7
- Chemical degradation: the weak points of the sequence · Module 11
Related catalogue compounds
Definition for educational and scientific purposes. It is not medical advice or a recommendation for use. NeoPeptidos products are sold labelled for research use only (RUO).