Glossary

Molecular Weight

Q: Why is Molecular Weight important in peptide research?

Molecular Weight is a fundamental concept in chemistry as it relates to peptide science. It directly influences experimental design, compound characterization, and the reliability of research outcomes across biochemistry and molecular biology disciplines.

Chemistry

Molecular Weight — The sum of the atomic weights of all atoms in a molecule, expressed in Daltons (Da). Used to confirm peptide identity via mass spectrometry.

What Is Molecular Weight?

Molecular weight (MW) is the sum of atomic weights of all atoms in a molecule, expressed in Daltons (Da) or grams per mole (g/mol). For peptides, MW is calculated from the amino acid sequence and is the primary identity check on every Certificate of Analysis. It is confirmed experimentally by mass spectrometry.

How to Calculate Peptide MW

Sum the residue weights of all amino acids in the sequence, then add 18.02 Da for the water molecule present at the termini (one H at the N-terminus + one OH at the C-terminus). Subtract 18.02 Da for each disulfide bond (loss of 2H). Add modification masses for any post-translational or chemical modifications (acetylation +42.04 Da, amidation -0.98 Da).

Peptide Size Classifications

Small peptides (< 1000 Da): 2-8 amino acids. Examples: GHK-Cu (403 Da)
Medium peptides (1000-5000 Da): 8-40 amino acids. Examples: BPC-157 (1419 Da), Melanotan II (1024 Da)
Large peptides (5000-10000 Da): 40-90 amino acids. Examples: IGF-1 (7649 Da)

Verifying MW on a CoA

The observed mass should match the theoretical MW within 0.1% or 1 Da. A mass difference of exactly one amino acid residue (e.g., 57 Da for glycine, 131 Da for methionine) indicates a deletion or insertion error in the synthesis.

Frequently Asked Questions

What is Molecular Weight?

The sum of the atomic weights of all atoms in a molecule, expressed in Daltons (Da). Used to confirm peptide identity via mass spectrometry.

Why is Molecular Weight important in peptide research?