Peptide Molecular Weight Formula
Calculate peptide mass by summing residue masses and adding one water molecule. Follow a worked example for ACD.
Add residue masses and one water molecule
Start with the mass of each residue as it occurs inside a chain. For one linear peptide with free termini, sum those masses and add one water molecule to account for the ends. If you start with free amino acid masses instead, subtract one water for each of the n − 1 peptide bonds in a chain of n residues.
Worked example
For ACD, the average mass is 71.0788 + 103.1388 + 115.0886 + 18.01524 = 307.32144 Da. Rounded to four decimal places, that gives 307.3214 Da. Using the monoisotopic column and its water correction gives 307.0838 Da.
This calculation assumes an unmodified chain with reduced cysteines. A blocked terminus, a disulfide bond, or cyclization changes the mass calculation.