Analyze biochemical and physical parameters of your protein sequences
Input Protein Sequence
Only standard amino acid characters are accepted.
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Our Protein Charge Calculator is a web-based utility that computes the net electrical charge and a set of common physicochemical descriptors for any protein or peptide sequence you provide. By accepting a raw amino-acid sequence and a target pH, the tool returns the amino-acid composition, the theoretical net charge at the chosen pH (and the isoelectric point), and allied outputs such as molecular weight, extinction coefficient, instability index, aliphatic index, and GRAVY. These results help you choose the optimal buffer pH, design purification and electrophoresis experiments, predict protein behavior in solution, and provide input for downstream structural or biophysical analyses.
How to use (step-by-step)
- Paste your protein or peptide primary sequence in the sequence input box. The calculator accepts single-letter amino-acid codes; whitespace and numbers (e.g., residue positions) are ignored.
- Select the pH at which you want the net charge computed (default = 7.0). Some workflows also allow computing net charge across a pH range or returning the theoretical isoelectric point (pI).
- Click Analyze Sequence. The page validates the input and then returns a results page or table with the computed descriptors.
- Review the summary panel (length, molecular weight, pI, net charge at chosen pH) and the detailed breakdown (amino-acid counts, atomic composition, extinction coefficient, predicted half-life, instability index, aliphatic index, GRAVY).
- Copy or download the results (CSV or plain text) for record-keeping or for input to other analyses.
How is protein charge calculated?
Net protein charge depends on the protonation state of all ionizable groups in the molecule: the N-terminal amine, the C-terminal carboxyl, and side chains that can accept or donate protons (commonly: Asp, Glu, His, Cys, Tyr, Lys, Arg). Each ionizable group has an intrinsic dissociation constant (pKa) that describes how readily it donates or accepts a proton; the fraction of that group that is protonated at a given pH is obtained from the Henderson–Hasselbalch relationship. The net charge is the algebraic sum of the fractional charges of all ionizable groups at the chosen pH. This approach is the standard computational method used by sequence-based pI/charge calculators.
Practical method (what the calculator does):
- Count each ionizable residue in the sequence (for example, the number of Asp, Glu, Lys, Arg, His, Cys, Tyr). The N- and C-termini are also treated as single titratable groups.
- For each titratable group, use an adopted pKa value (a standard set of model pKa values for side chains and termini). Note: different calculators may use slightly different pKa tables; that choice affects the computed pI/charge but not the underlying summation method.
- For each group, compute its fractional protonation at the chosen pH using the Henderson–Hasselbalch relationship:
- For a basic (protonated = positively charged) group (example: Lys, Arg, N-terminus), fractional protonation =
f_prot = 1 / (1 + 10^(pH − pKa))
contribution to net charge = +1 × f_prot. - For an acidic (deprotonated = negatively charged) group (example: Asp, Glu, C-terminus), fractional deprotonation =
f_deprot = 1 / (1 + 10^(pKa − pH))
contribution to net charge = −1 × f_deprot.
- Sum all contributions to obtain the protein’s net charge at that pH. To find the isoelectric point (pI), the calculator searches (numerically) for the pH where the net charge crosses zero (commonly via a bisection or iterative root-finding method).
Usage note. Sequence-based methods are fast and informative for planning experiments (buffer choice, expected migration in electrophoresis, likely solubility near pI), but they are approximations: the local environment of a residue (burial, neighboring charges, prosthetic groups, post-translational modifications) can shift pKa values and therefore change actual experimental behavior. For highly accurate pKa/pI estimates, structure-aware physics-based methods exist (but are more computationally intensive).
Worked example (numeric, systematic): compute net charge of the peptide A-K-D-E (A = Ala, K = Lys, D = Asp, E = Glu) at pH 7.0, using commonly cited model pKa values (representative values often used in sequence calculators):
- pKa (N-term) = 8.0
- pKa (C-term) = 3.6
- Lys (K) pKa = 10.5
- Asp (D) pKa = 3.9
- Glu (E) pKa = 4.3
(References on typical model pKa values are shown below.)
Step-by-step fractional charge calculations (Henderson–Hasselbalch arithmetic shown explicitly):
- Lys (basic, pKa 10.5):
compute 10^(pH − pKa) = 10^(7.0 − 10.5) = 10^(−3.5) = 0.00031622777
fractional protonation f_prot = 1 / (1 + 0.00031622777) = 1 / 1.00031622777 = 0.9996839
Lys contribution = +1 × 0.9996839 = +0.9996839 - N-terminus (basic, pKa 8.0):
10^(pH − pKa) = 10^(7.0 − 8.0) = 10^(−1) = 0.1
f_prot = 1 / (1 + 0.1) = 1 / 1.1 = 0.9090909
N-term contribution = +0.9090909 - Asp (acidic, pKa 3.9):
10^(pH − pKa) = 10^(7.0 − 3.9) = 10^(3.1) = 1258.92541179
fractional deprotonation f_deprot = 1258.92541179 / (1 + 1258.92541179) = 1258.92541179 / 1259.92541179 = 0.9992064
Asp contribution = −0.9992064 - Glu (acidic, pKa 4.3):
10^(pH − pKa) = 10^(7.0 − 4.3) = 10^(2.7) = 501.18723363
f_deprot = 501.18723363 / (1 + 501.18723363) = 501.18723363 / 502.18723363 = 0.9980039
Glu contribution = −0.9980039 - C-terminus (acidic, pKa 3.6):
10^(pH − pKa) = 10^(7.0 − 3.6) = 10^(3.4) = 2511.88643151
f_deprot = 2511.88643151 / (1 + 2511.88643151) = 2511.88643151 / 2512.88643151 = 0.9996011
C-term contribution = −0.9996011 - Sum positives and negatives:
Positives = Lys + N-term = 0.9996839 + 0.9090909 = 1.9087748
Negatives = Asp + Glu + C-term = 0.9992064 + 0.9980039 + 0.9996011 = 2.9968114
Net charge = Positives − Negatives = 1.9087748 − 2.9968114 = −1.0880366
Result: the peptide A-K-D-E has a net charge of approximately −1.09 at pH 7.0 (fractional net charges are normal because the calculation sums fractional protonation states). This worked example shows the precise arithmetic used by sequence-based charge calculators; the same method is applied to all titratable residues when analyzing longer sequences.
Features
This Protein Charge Calculator provides the following analyses and downloadable results:
- Amino-acid composition: counts and mole-percent of each residue type (useful for composition-based screening and plotting).
- Molecular weight (MW): total sequence mass in Daltons/kDa calculated from atomic composition and residue masses.
- Theoretical isoelectric point (pI): numerical estimate of the pH where net charge = 0 (search over pH to find the root).
- Atomic composition: total counts of C, H, N, O, and S atoms derived from residue formulas (needed for exact mass/stoichiometry calculations).
- Extinction coefficient (280 nm): estimated from counts of Tyr, Trp, and cystine (disulfide pairs) to predict UV absorbance per concentration.
- Estimated half-life (in vivo): quick prediction based on the N-end rule (model organisms: human, yeast, E. coli).
- Aliphatic index: measure of the relative volume of aliphatic side chains; associated with thermostability.
- GRAVY (Grand Average of Hydropathicity): average hydropathy score per residue (useful for solubility and membrane-association inference).
Contact and feedback
If you encounter unexpected results, have questions about pKa choices, or want to request batch processing / API access, please contact us — include your sequence and the pH(s) used so we can reproduce and help troubleshoot.
Reference
Gasteiger, E., Hoogland, C., Gattiker, A., Duvaud, S., Wilkins, M. R., Appel, R. D., & Bairoch, A. (2005). Protein Identification and Analysis Tools on the ExPASy Server. In The Proteomics Protocols Handbook (pp. 571–607). Humana Press. https://doi.org/10.1385/1-59259-890-0:571
Nicolau, D. V., Jr., Fulga, F., & Nicolau, D. V., Sr. (2003). The Protein Surface Properties Calculator. Asia-Pacific Biotech News, 07(03), 116–122. https://doi.org/10.1142/s0219030303000296
Meet the Authors
Mahdi Morshedi Yekta
Founder & Bioinformatics Developer
Mahdi is the founder of ScienceCodons and a Medical Biotechnologist with a deep passion for computational biology. Holding an M.Sc. in Medical Biotechnology, he specializes in transforming complex biological algorithms into accessible, high-performance web tools, bridging the gap between laboratory sciences and software engineering.
Fatemeh Faryadras
Medical Biotechnologist & Researcher
Fatemeh is a Medical Biotechnologist and researcher. With extensive expertise in genetic engineering, molecular cloning, and cancer biology, she combines her rigorous laboratory background with intuitive design principles to create reliable, user-centered scientific calculators and tools.
Dr. Mosayeb Rostamian
Assistant Professor, Infectious Diseases Research Center
Certified Scientific Reviewer
Google ScholarExpert Review Process
To ensure the highest quality and scientific accuracy, this tool's underlying algorithms, formulas, and edge-case handling have been thoroughly reviewed and validated by Dr. Mosayeb Rostamian.
The "Reviewed By" badge signifies that an independent expert has evaluated the tool's logic to guarantee accuracy and alignment with the current consensus in the scientific community.