Molecular weight is usually expressed in daltons (Da) or kilodaltons (kDa). For most laboratory peptides, the value is commonly given in Da because peptide molecules are relatively small.
How molecular weight is calculated
A peptide’s molecular weight depends mainly on its amino acid sequence. Each amino acid contributes a known atomic mass, but the final peptide mass is not simply the sum of the individual free amino acids. Amino acids are joined by peptide bonds, with the elimination of water molecules.
Chemical changes can also change the expected molecular weight. Acetylation, amidation, phosphorylation or other modifications add or remove specific atoms and, for example, change the mass calculated.
When verifying a reported molecular weight, the full sequence and any modifications to it must be considered.
The importance of molecular weight
The molecular weight helps researchers confirm that a peptide sample is the compound they expected. If the measured mass is significantly different from the calculated mass, it may indicate sequence difference, modification, degradation or some other problem with the sample.
It is also useful in preparation of solutions, calculation of concentrations and comparison of analytical results between batches.
How to Check Molecular Weight
Mass spectrometry (MS) is a commonly utilised method for the identification of peptide molecules’ mass. The theoretical molecular weight can be determined from the given sequence and compared with the experimental value.
The Certificate of Analysis (CoA) may show both the expected molecular weight and analytical results from mass spectrometry. HPLC, meanwhile, is generally used to assess purity rather than directly measure molecular weight.
When checking a peptide product, look at the full sequence, stated modifications, theoretical molecular weight and mass spectrometry result. These details should make sense together.
Related terms: Amino Acid · Peptide Sequence · Peptide Bond · Molecular Mass · Daltons · Kilodaltons · Mass Spectrometry · HPLC · Purity · Certificate of Analysis
All products supplied by Prime Peptides UK are intended for laboratory research use only and are not for human consumption.
Research Use Only. This glossary explains terminology used in published research. It is not medical advice and does not describe any product for human or animal use.