Enter a peptide sequence in standard 1-letter amino acid code to calculate molecular weight, elemental formula, net charge, isoelectric point, and extinction coefficient.
Adds to the sequence above
Uses residue monoisotopic masses from NIST and average atomic weights from IUPAC 2021. Charge at pH 7.4 uses Henderson-Hasselbalch with pKa values from Pace et al. (2009). Extinction coefficient uses Pace et al. (1995) method.
Average MW uses natural isotopic abundances — use this for weighing and solution preparation. Monoisotopic MW uses only the most abundant isotope of each element — this matches what high-resolution mass spectrometry reports. For peptides under ~2 kDa the monoisotopic peak is usually the most intense; above ~4 kDa the M+1 or M+2 peaks dominate.
The pI is the pH at which the peptide carries zero net charge. At its pI, solubility is at a minimum — formulation buffers should be kept at least 1–2 pH units away. The net charge at pH 7.4 governs behaviour in physiological conditions: highly positive peptides tend to bind negatively charged bacterial membranes, which explains the activity of most antimicrobial peptides.
ε₂₈₀ lets you measure peptide concentration by UV absorbance: c = A / (ε × l). The value comes from Trp (5500 M⁻¹cm⁻¹), Tyr (1490), and disulfide bonds (125 each). If your peptide contains none of these residues, ε₂₈₀ ≈ 0 — use BCA assay or amino acid analysis instead. The Abs 0.1% value shown is useful for comparing with supplier certificates of analysis.