Sequence chemistry
Peptide Chemistry Calculator
Type or paste any amino-acid sequence and instantly see molecular weight, isoelectric point, net charge at any pH, and Hopp & Woods hydrophilicity per residue — with terminal-modification support and one-click load from the site's peptide dataset.
Inputs
Loads a known-clean linear sequence from the site's peptide dataset.
Results
Length
15 aa
Molecular weight
1419.55 Da
Isoelectric point (pI)
3.55
Net charge @ pH 7.40
-2.00
Avg HW hydrophilicity
0.51
Hopp & Woods (1981)
Hydrophilic residues
27%
Net charge vs pH
Hydrophilicity per residue (Hopp & Woods)
Calculations run entirely in your browser. pKa reference values are Lehninger's Principles of Biochemistry typicals; hydrophilicity scores are from Hopp & Woods (PNAS, 1981).
Amino-acid composition
Counts and percentages for the parsed sequence, sorted by frequency.
| Residue | Count | % | Residue mass | Side pKa | HW score |
|---|---|---|---|---|---|
| P (Pro) Proline | 4 | 26.7% | 97.12 Da | — | 0.0 |
| G (Gly) Glycine | 3 | 20.0% | 57.05 Da | — | 0.0 |
| A (Ala) Alanine | 2 | 13.3% | 71.08 Da | — | -0.5 |
| D (Asp) Aspartate | 2 | 13.3% | 115.09 Da | 3.65 | 3.0 |
| E (Glu) Glutamate | 1 | 6.7% | 129.12 Da | 4.25 | 3.0 |
| K (Lys) Lysine | 1 | 6.7% | 128.17 Da | 10.53 | 3.0 |
| L (Leu) Leucine | 1 | 6.7% | 113.16 Da | — | -1.8 |
| V (Val) Valine | 1 | 6.7% | 99.13 Da | — | -1.5 |
Formulas used
Molecular weight MW = Σ residueMass(aa) + H₂O + Δ(N-term) + Δ(C-term). Residue masses are standard average values; the water molecule accounts for the free termini, and each terminal modification carries a documented mass delta.
Net charge at pH is calculated by summing the Henderson-Hasselbalch contribution of every ionisable group: q_acid = −1 / (1 + 10^(pKa − pH)) and q_base = +1 / (1 + 10^(pH − pKa)). Side-chain pKa values follow Lehninger reference typicals (Asp 3.65, Glu 4.25, His 6.00, Cys 8.33, Tyr 10.07, Lys 10.53, Arg 12.48); α-amino 9.69 and α-carboxyl 2.34 are included only when the corresponding terminus is free.
Isoelectric point is found by bisecting [0, 14] until the net-charge magnitude is below 10⁻⁶ or the bracket width falls under 0.005 pH units — about 12 iterations in practice, well within the precision of any experimental measurement.
Hydrophilicity per residue uses the Hopp & Woods (1981) scale, which assigns each amino acid a single hydrophilicity score from −3.4 (most hydrophobic) to +3.0 (most hydrophilic). The summary shows the mean score across the sequence and the percentage of residues with a positive score.
What this tool does not handle
- Disulfide bridges — the linear MW does not subtract the 2 Da lost on bridge formation.
- D-amino acids — Ipamorelin, Hexarelin, PT-141, Melanotan II and similar peptides have D-residues or unnatural side chains and are not represented in the linear-only calculator.
- Cyclic backbone — closing a head-to-tail amide bond removes one further water; subtract 18 Da from the linear value for cyclic peptides.
- Metal coordination — GHK-Cu's copper(II) ion is not added to the MW; report the apo-peptide mass + 63.5 Da for the Cu(II) complex.
References
- Lehninger, Nelson & Cox, Principles of Biochemistry — reference pKa values used throughout.
- Hopp T.P. & Woods K.R., Prediction of protein antigenic determinants from amino acid sequences, Proc Natl Acad Sci USA 78:3824–3828 (1981).
Related tools
- Dosage Calculator — for converting peptide mass into draw volume.
- Reconstitution Calculator — for the underlying mg-to-mcg/mL maths.
- Half-Life Calculator — for pharmacokinetic modelling of the same peptide.