Peptide isoelectric point and net charge calculator
Estimates net charge across pH, the isoelectric point (pI), Kyte–Doolittle hydropathy (GRAVY) and the 280 nm extinction coefficient for a linear sequence of standard amino acids. Useful context for choosing buffers, predicting solubility trends and reading A280 measurements. Everything runs in your browser.
Result
Enter your numbers and choose Calculate.
Formulas
- Net charge: Henderson–Hasselbalch sum over the termini and ionizable side chains (K, R, H positive; D, E, C, Y negative).
- pKa values: N-terminus 9.69, C-terminus 2.34, D 3.86, E 4.25, C 8.33, Y 10.07, H 6.00, K 10.50, R 12.40 (free amino acid values).
- pI: the pH at which the estimated net charge is zero.
- GRAVY: average Kyte–Doolittle hydropathy; positive values mean a more hydrophobic sequence.
- ε280: 5500 per Trp + 1490 per Tyr (+ 125 per cystine), per Pace et al. (1995).
Estimate only. pKa values shift inside real peptides, and different pKa sets give pI values that differ by several tenths. Non-standard or D-amino acids, lipid attachments and counter-ions are not modelled. See peptide solubility and aggregation and the isoelectric point glossary entry.
About these estimates
Why does the pI matter in the lab?
Peptides tend to be least soluble near their isoelectric point, because net charge is close to zero there. Knowing the approximate pI helps when choosing a buffer pH for solutions, chromatography or electrophoresis.
Why does my pI differ from another calculator?
Calculators use different pKa sets (for example Lehninger, EMBOSS or Bjellqvist). Results usually agree within about 0.5 pH units; treat any single value as an estimate.
Can I use the extinction coefficient to measure concentration?
Only when the sequence contains Trp or Tyr. Concentration = A280 ÷ (ε × path length). Sequences without them absorb weakly at 280 nm, so A205/A214 or amino acid analysis is used instead.
Related: peptide molecular weight estimator · molarity calculator · reading an HPLC chromatogram · research overviews (sequences listed on each page)
Background reading
The arithmetic on this page is explained, with its assumptions, in these guides.
Research use only. For laboratory research use only. Not for human or veterinary use. This page performs arithmetic on the numbers you enter and gives no recommendations about amounts. Check results against your own protocol and records. All laboratory tools · Laboratory guides · Glossary
