Protein Solubility Calculator

• Choose an input method: Known pI or Sequence.
• Enter solution conditions: pH, salt, temperature (and optional concentration).
• Click Calculate to see a solubility score and aggregation risk.
• If the score is poor, try moving the pH further away from pI or lowering temperature.

• Compute ΔpH = pH − pI (pI known or estimated from sequence).
• Convert salt and temperature into normalized “effects” (simple heuristics).
• Combine factors into a Solubility Score (0–100) and an Aggregation Risk (Low/Medium/High).
• Show callouts explaining what’s driving the score (e.g., “near pI” or “high salt”).

Example 1 — Known pI (typical buffer)

A protein has pI = 6.2. Your buffer is pH 7.4, 150 mM NaCl, 25°C.

1. Compute ΔpH = 7.4 − 6.2 = +1.2 (farther from pI → typically more charge).
2. Moderate salt (150 mM) is often compatible for many proteins (heuristic).
3. Combine factors → score trends moderate-to-high with lower aggregation risk than near pI.

Example 2 — Near pI (risk)

A protein has pI = 6.0 and the buffer is pH 6.0.

1. ΔpH = 6.0 − 6.0 = 0 → minimal net charge.
2. Lower charge often means weaker electrostatic repulsion → higher aggregation risk.
3. Try moving pH to 5.0 or 7.0 (and/or reduce temperature) to improve solubility.

Example 3 — High salt, moderate pH, room temperature

A protein has pI = 5.8. The solution conditions are pH 7.0, 500 mM NaCl, and 25°C.

1. Compute ΔpH = 7.0 − 5.8 = +1.2, which is reasonably far from the pI and usually favorable for solubility.
2. Salt at 0.5 M is relatively high. While moderate salt can stabilize some proteins, higher ionic strength may begin to promote salting-out (protein-dependent).
3. Temperature at 25°C is common but can still allow partial unfolding or aggregation for sensitive proteins.
4. Overall, the score trends moderate: pH helps solubility, but high salt and room temperature increase aggregation risk.
5. Practical adjustments: reduce salt to 100–200 mM and/or lower temperature to 4°C to improve stability.

Example 4 — Estimate pI from sequence (approx.)

You don’t know the protein’s pI, but you have the sequence: MKWVTFISLLFLFSSAYS. You’re preparing a buffer at pH 7.4 with 150 mM NaCl at 25°C.

1. Switch the calculator method to “Estimate pI from amino-acid sequence” and paste the sequence.
2. Choose a pKa preset (e.g., EMBOSS-like) and calculate to get an estimated pI. (This is an approximation — real pI can shift due to folding and modifications.)
3. Compute the charge-distance proxy: ΔpH = pH − pI. If the estimated pI is far from 7.4 (for example, ~5–6 or ~9–10), the protein is likely more charged and often more soluble than near pI.
4. With 150 mM NaCl and 25°C, the calculator typically reports a moderate solubility score unless the estimated pI is very close to 7.4.
5. Practical takeaway: if the score is low, your easiest “knob” is to move pH by ±1 pH unit away from the estimated pI (and/or chill to 4°C).

Tip: If you later measure the pI experimentally, switch to “I know the protein’s pI” for more reliable guidance.