Molecular Weight
Notice
ALL ARTICLES AND PRODUCT INFORMATION PROVIDED ON THIS WEBSITE ARE FOR INFORMATIONAL AND EDUCATIONAL PURPOSES ONLY. The products offered on this website are furnished for in-vitro studies only. In-vitro studies (Latin: in glass) are performed outside of the body. These products are not medicines or drugs and have not been approved by the FDA to prevent, treat or cure any medical condition, ailment or disease. Bodily introduction of any kind into humans or animals is strictly forbidden by law.
What is Molecular Weight?
Molecular weight (MW), also referred to as molecular mass, is the sum of the atomic weights of all atoms in a molecule, expressed in daltons (Da) or grams per mole (g/mol). For peptides, the molecular weight is calculated by summing the residue weights of each amino acid in the sequence, subtracting the water molecules lost during peptide bond formation, and accounting for any chemical modifications. Molecular weight is a fundamental physical property that is used to confirm a peptide's identity, calculate molar concentrations for experiments, and interpret mass spectrometry data.
Calculating Peptide Molecular Weight
The molecular weight of a peptide can be calculated from its amino acid sequence using well-established atomic mass values. Each amino acid residue has a known average molecular weight (ranging from approximately 57 Da for glycine to 204 Da for tryptophan). The total molecular weight is determined by summing the residue masses and adding 18.02 Da for the water molecule at the terminal ends. Most research peptides have molecular weights ranging from a few hundred daltons (for di- and tripeptides) up to several thousand daltons (for longer sequences of 30-50 amino acids).
Molecular Weight in Quality Control
In peptide quality control, the observed molecular weight obtained via mass spectrometry is compared to the theoretical molecular weight calculated from the intended sequence. Agreement between these values (typically within 0.1% or 1 Da) is strong confirmation that the correct peptide was synthesized. Discrepancies may indicate incomplete synthesis, deletion sequences, chemical modifications, or counterion effects. The molecular weight is always reported on the Certificate of Analysis alongside HPLC purity data.
Practical Applications
Researchers rely on accurate molecular weight values for preparing solutions at precise molar concentrations, which is essential for dose-response studies, binding assays, and any experiment where stoichiometric relationships matter. The molecular weight is also important for selecting appropriate analytical methods, choosing chromatography columns, and interpreting pharmacokinetic data related to renal filtration and clearance.