The isoelectric point (pI) is the theoretical pH at which the sum of all acidic and basic ionizable residue charges balances to zero. This tool solves pI via the Henderson-Hasselbalch equation and bisection (tolerance 0.001) and renders the full pH 0–14 net-charge titration curve plus the physiological pH 7.4 marker. Five pKa scales toggle: Bjellqvist, EMBOSS, DTASelect, IPC2-Protein, and IPC2-Peptide.

(1) Choose ion-exchange chromatography (IEX) buffer pH (above pI → net negative → anion exchanger) (2) Predict isoelectric-focusing (IEF) band position (3) Avoid solubility minimum near pI when designing storage buffers (4) Estimate net charge at physiological pH 7.4 for drug interactions / LNP encapsulation

① Paste protein sequence (1-letter, A–Y) — whitespace / digits auto-stripped ② Pick a pKa scale: • Bjellqvist (default, ProtParam-compatible) • EMBOSS (iep standard) • DTASelect (proteomics) • IPC2 Protein / Peptide (Kozlowski 2021 ML) ③ Click a Load Example (BSA, Lysozyme, Insulin, Polylysine K10, MKDR) ④ Output: • pI to 2 decimals • Q @ pH 7.4 (physiological marker) • Length + ionizable residue counts (D/E/H/C/Y/K/R) • Charge vs pH titration curve (self-drawn SVG with Q=0 / pI / pH 7.4 guides)

Example 1) BSA (Bovine Serum Albumin) → Bjellqvist → pI ≈ 5.60 → Q @ pH 7.4 ≈ −17.8 → strong net negative → binds Q-sepharose / anion exchanger Example 2) Lysozyme (Chicken) → EMBOSS → pI ≈ 9.21 / IPC2 Protein → pI ≈ 8.4 → Q @ pH 7.4 ≈ +7.4 (Bjellqvist) → CM-sepharose / cation exchanger Example 3) Polylysine K10 → pI ≈ 11 across scales → strong net positive at physiological pH Example 4) Insulin heterodimer → pI ≈ 5.4 → amenable to isoelectric precipitation purification