Sum individual amino acid residue weights + Hâ‚‚O (18.015).
For precise MW calculation, sum individual residue weights (monoisotopic or average masses) plus one water molecule (18.015 Da for the termini). Residue masses differ significantly: Glycine 57.02 Da (smallest) to Tryptophan 186.08 Da (largest). Post-translational modifications alter MW: phosphorylation +79.97 Da, glycosylation +162+ Da per sugar, disulfide bonds -2.02 Da per bond. For mass spectrometry, monoisotopic masses (using the most abundant isotope of each element) are used for peptides, while average masses are used for intact proteins. Tools like ExPASy ProtParam compute MW along with pI, extinction coefficient, and amino acid composition from sequence data.