nounIntermediate
Definition
Unit of molecular mass equal to approximately 1.66 × 10⁻²⁴ grams; named after John Dalton. Peptides are measured in daltons (Da) or kilodaltons (kDa).

Detailed Explanation

The dalton is the standard unit for expressing molecular weight in biochemistry. One dalton equals one-twelfth the mass of a carbon-12 atom, approximately the mass of a single hydrogen atom. A typical amino acid residue adds about 110–130 Da to a peptide's mass. Peptides are generally under 5,000 Da (5 kDa); proteins are larger. This distinction matters practically: molecules under ~500 Da can often cross cell membranes (Lipinski's Rule of Five for oral drugs); peptides at 500–5,000 Da occupy a middle space; and proteins above 5 kDa typically require injection. Molecular weight is measured by mass spectrometry and confirmed on Certificates of Analysis (COAs) for commercial peptides.

Key Facts

  • The dalton is the standard unit for expressing molecular weight in biochemistry
  • One dalton equals one-twelfth the mass of a carbon-12 atom, approximately the mass of a single hydrogen atom
  • A typical amino acid residue adds about 110–130 Da to a peptide's mass
  • Peptides are generally under 5,000 Da (5 kDa); proteins are larger
  • This distinction matters practically: molecules under ~500 Da can often cross cell membranes (Lipinski's Rule of Five for oral drugs); peptides at 500–5,000 Da occupy a middle space; and proteins above 5 kDa typically require injection
Related Terms Molecular Weight Mass Spectrometry HPLC Peptide Protein

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