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Theoretical net peptide content (calculated assuming that counterions are the only non-peptide components present in your peptide sample) can be estimated by dividing molecular weight (MW) of the peptide by a sum of this molecular weight and a number of trifluoroacetate (TFA) or acetate (AcO-) counterions that are required to neutralize the peptide multiplied by the molecular weight of the TFA counterion (MW= 114) and the AcO– counterion is (MW= 59).
For example, a synthetic peptide with a TFA salt and a MW= 1000 with a free N-terminal amino group and one Lys has theoretical net peptide content of 1000 / (1000 + (2 x 114)) = 1000/1228 =0.81 or 81%. This example peptide has 2 positions for the TFA salt to bind, hence the 2×114.
Theoretical net peptide content formula: (peptide MW)/(peptide MW + (#bound salts x salt MW))
Counterions are not the only potential non-peptide components in the peptide sample. It can also contain residual water, adsorbed solvents and traces of other substances. As a result, the actual net peptide content is usually determined by either elemental analysis (N2 content) or quantitative amino acid analysis.
Read more about net peptide content here.
Need help determining the molecular weight of your peptide? Here is a useful peptide calculator: https://cem.com/en/peptide-calculator
For other useful tools, read this FAQ: Do you have any useful tools for the peptide researcher?
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