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
Residue masses derive from IUPAC/CIAAW atomic weights (2021). Net charge uses Henderson-Hasselbalch with the consensus side-chain pKa set (Asp 3.65, Glu 4.25, His 6.00, Cys 8.18, Tyr 10.07, Lys 10.53, Arg 12.48; N-term 8.0, C-term 3.1). Pace et al. (2009) report a partly different set measured in folded proteins. Extinction coefficient follows Pace et al. (1995).
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.