Molecular Weight

Definition

Molecular weight (MW) is the mass of a single molecule of a compound, expressed in atomic mass units (daltons, Da). For peptides, molecular weight is calculated by summing the atomic masses of all constituent atoms in the peptide chain, accounting for water molecules lost during peptide bond formation. Molecular weight is a key physical property that influences membrane permeability, volume of distribution, renal clearance rates, and immunogenicity. Peptides typically range from a few hundred daltons (dipeptides) to tens of thousands of daltons (large proteins), with most research peptides falling in the range of 500 to 5000 Da.

Research Context

Molecular weight is one of the fundamental parameters reported on a peptide certificate of analysis and is used to verify compound identity by mass spectrometry. In pharmacokinetic research, molecular weight influences how a peptide distributes across tissue compartments, crosses biological barriers such as the blood-brain barrier, and is cleared by renal filtration. Compounds with molecular weight below approximately 1000 Da generally have greater membrane permeability and different distribution profiles than larger peptides.