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Protein Solubility Calculator

Chemistry

Calculate ammonium sulfate saturation percentage for protein precipitation and salting out. Find grams of (NH₄)₂SO₄ to add for target saturation at 0°C or 25°C.

0100
1100
100 mL
mL

Ammonium Sulfate to Add

39
Final (NH₄)₂SO₄ Concentration
460.2
Final Molarity
3.483

This calculator computes your Ammonium Sulfate to Add, Final (NH₄)₂SO₄ Concentration, Final Molarity from the values you enter.

Inputs
Initial SaturationTarget SaturationSolution VolumeTemperature
Outputs
Ammonium Sulfate to AddFinal (NH₄)₂SO₄ ConcentrationFinal Molarity

What is a Protein Solubility?

The Protein Solubility Calculator computes the grams of solid ammonium sulfate ((NH₄)₂SO₄) to add to a protein solution to raise it from an initial saturation (S1%) to a target saturation (S2%), using the Green & Hughes empirical formula for 0°C and 25°C. Enter initial saturation, target saturation, and solution volume.

Ammonium sulfate precipitation ('salting out') is the first step in most protein purification protocols. Different proteins precipitate at different % saturation, allowing fractionation: a 30–60% cut, for example, selectively precipitates proteins in that solubility range while leaving more soluble and less soluble proteins in solution. The Green & Hughes formula (g/L = 533 × [S2 − S1] / [100 − 0.3 × S2] at 0°C) accounts for volume changes as solid ammonium sulfate dissolves, giving accurate additions across the full saturation range from 0% to 100%.

For the next step after ammonium sulfate precipitation — tracking purification progress — the Enzyme Activity Calculator measures specific activity (U/mg) of each fraction. The Michaelis-Menten Calculator characterises the purified enzyme's kinetic parameters. For electrophoretic purity assessment, the Isoelectric Point Calculator predicts pI for IEF gel separation.

How to use this Protein Solubility calculator

  1. Enter Initial Saturation (%) — 0% if starting from a fresh extract with no ammonium sulfate.
  2. Enter Target Saturation (%) — the saturation at which you want to collect precipitate (e.g., 60%).
  3. Enter Solution Volume (mL) — volume of the protein solution.
  4. Select Temperature — 0°C for cold room work (recommended); 25°C for room temperature.
  5. Read Ammonium Sulfate to Add (g) — weigh this amount, add slowly with stirring over 20–30 min, then centrifuge at 10,000–15,000 × g for 15 min.

Formula & Methodology

Green & Hughes empirical formula:

At 0°C:  g/L = 533 × (S2 − S1) / (100 − 0.3 × S2) At 25°C: g/L = 514 × (S2 − S1) / (100 − 0.272 × S2)  Grams to add = g/L × Volume(mL) / 1000  Final [NH4)2SO4] (g/L) = S2% × saturated_conc   [Saturated conc at 0°C: 706 g/L; at 25°C: 767 g/L] Final Molarity = [final g/L] / 132.14 g/mol

Worked example — amylase purification from 500 mL wheat bran extract at 0°C:

Step 1: Add ammonium sulfate to 30% saturation (remove impurities that precipitate early):

g = 533 × (30 − 0) / (100 − 0.3×30) × 0.5 L = 533 × 30/91 × 0.5 = 87.9 g

Add 87.9 g slowly. Centrifuge. Discard pellet (unwanted proteins). Keep supernatant.

Step 2: Raise to 70% saturation to precipitate amylase:

g = 533 × (70 − 30) / (100 − 0.3×70) × 0.5 L = 533 × 40/79 × 0.5 = 134.9 g

Add 134.9 g to supernatant. Centrifuge. Keep pellet (amylase-enriched). Dissolve in 20 mL buffer.

This 30–70% ammonium sulfate fractionation is the first purification step for amylase from cereals — widely used at Indian food enzyme companies (AET, Aumgene, Maps Enzymes in Ahmedabad) to process wheat, rice, and sorghum starch for brewing, baking, and ethanol production industries.

Frequently Asked Questions

Ammonium sulfate ((NH₄)₂SO₄) precipitation is the most widely used technique for initial protein purification from cell extracts, tissue homogenates, and culture supernatants. At high ammonium sulfate concentrations, water molecules are competed away from protein surfaces ('salting out'), reducing hydration shells and causing proteins to aggregate and precipitate. Different proteins precipitate at different % saturation of ammonium sulfate — the specific range depends on the protein's surface hydrophobicity and charge. Fractionation collects proteins precipitating between two saturation levels (e.g., 30–60% saturation), achieving 3–10× purification in one step.
Enter Initial Saturation (%) — usually 0 for a fresh extract, or the current ammonium sulfate saturation if continuing fractionation. Enter Target (Final) Saturation (%) — the desired end saturation. Enter Solution Volume (mL). Select Temperature (0°C for cold room; 25°C for room temperature). The calculator returns grams of solid ammonium sulfate to add, the final ammonium sulfate concentration (g/L), and final molarity. The Green & Hughes empirical formula is used, which is the standard in biochemistry textbooks (Lehninger, Stryer) and widely cited in protein purification protocols.
The Green & Hughes (1955) formula gives grams of (NH₄)₂SO₄ to add per litre of solution to raise saturation from S1% to S2%: At 0°C: g/L = 533 × (S2 − S1) / (100 − 0.3 × S2). At 25°C: g/L = 514 × (S2 − S1) / (100 − 0.272 × S2). These empirical equations account for the volume change that occurs as ammonium sulfate dissolves — solid (NH₄)₂SO₄ has significant volume (i.e., adding solid changes total volume). The Sathmary and Bethlenfalvy tables (used in older textbooks) give essentially equivalent values. For accurate work, IUPAC recommends using these tables or an online calculator rather than the less accurate 'g/100mL' approximation.
Approximate ammonium sulfate saturation ranges for common proteins: Antibodies (IgG): 40–60% saturation precipitates most IgG — standard for rough antibody purification from antisera (used in Indian diagnostic antibody production at HLL Lifecare, Bharat Serums). Albumin (BSA/HSA): remains soluble to ~75% saturation (highly soluble protein). Haemoglobin: ~50–70% saturation. Most globular proteins: 40–80% saturation range for selective fractionation. Industrial enzyme (protease, amylase): typically 50–80% saturation for initial recovery from fermentation broth at Indian enzyme manufacturers like Advanced Enzyme Technologies (Thane) and Fermenta Biotech.
Adding solid (NH₄)₂SO₄ rather than a saturated solution (which is ~700 g/L) is preferred because: (1) It dilutes the protein solution minimally — a saturated solution would double the volume for high saturation levels. (2) Avoids large volume changes that dilute the protein. (3) Easier to add precise masses. The exception is when going from 0 to low saturation (0–20%): a concentrated solution may be added to avoid localised high-concentration zones that cause non-selective precipitation. Key practical point: add solid ammonium sulfate slowly with stirring at 0–4°C (cold room) — rapid addition creates local high-concentration zones. Wait 20–30 minutes before centrifuging to allow equilibration.
Ammonium sulfate must be removed before downstream steps (column chromatography, enzyme assays, electrophoresis): (1) Dialysis: dissolve pellet in buffer → dialyse overnight in 1000× volume of buffer (cellulose membrane, 12–14 kDa MWCO). Most common method in Indian research labs — dialysis tubing from HiMedia, SciLab. (2) Size exclusion chromatography (gel filtration): Sephadex G-25 (PD-10 column) removes small molecules (<5 kDa) instantly, retaining protein. (3) Ultrafiltration/diafiltration: Amicon Ultra centrifugal filters (10 kDa MWCO) — fast and quantitative. (4) Tangential flow filtration (TFF): industrial scale — used at Biocon, Serum Institute for biopharmaceutical processing.
Advantages: (1) Inexpensive — (NH₄)₂SO₄ costs ≈ ₹200/kg (500g from Merck India); highly affordable for large volumes. (2) Gentle — non-denaturing; preserves enzyme activity and protein conformation. (3) Scalable — works at mL to 1000-L industrial scale. (4) Concentrates protein from dilute solutions. (5) Stable storage — ammonium sulfate pellets can be stored at 4°C for months. Limitations: (1) Non-specific — proteins in similar solubility range co-precipitate. (2) (NH₄)₂SO₄ must be removed before many applications. (3) Does not work for hydrophilic proteins (antibodies, certain enzymes may be exceptions). Alternative precipitation methods: PEG (polyethylene glycol, milder), isoelectric precipitation (at pI, used for [Isoelectric Point Calculator](/isoelectric-point-calculator/)), heat denaturation.
India is a major antibody producer — Bharat Serums & Vaccines, Biological E, Haffkine Institute, and VINS Bioproducts produce anti-snake venom sera, anti-rabies immunoglobulin, and antitoxins. Production process: (1) Immunise horses (for anti-venom) with venoms. (2) Collect serum. (3) Ammonium sulfate precipitation at 40–50% saturation removes fibrinogen and most non-IgG proteins. (4) Re-dissolve pellet → 2nd precipitation at 50–60% saturation. (5) Dialyse against sodium chloride. (6) Pepsin digestion to F(ab')₂ fragments (safer, less complement activation). This purification reduces pyrogenic proteins and albumin, giving purer therapeutic immunoglobulin. India's National Snake Bite Protocol (MoHFW) specifies polyvalent antivenom purity requirements.
(NH₄)₂SO₄ solubility at 0°C: ~706 g/L (100% saturation). At 25°C: ~767 g/L (100% saturation). The difference means slightly more (NH₄)₂SO₄ is needed at 25°C to achieve the same % saturation compared to 0°C — the calculator implements both temperature formulas. Practically: most biochemistry labs perform ammonium sulfate precipitation in cold rooms (4–8°C) or on ice baths to: (1) Prevent proteolytic enzyme degradation (proteases are slowed at low temperature). (2) Maintain protein stability. (3) Slow microbial contamination. Indian research labs (CCMB, NCCS, NII) performing protein purification from plant, microbial, or animal sources use 4°C as the default precipitation temperature.
Salting out (kosmotropic effect): high concentrations of chaotropic-ordering salts like (NH₄)₂SO₄, Na₂SO₄ decrease protein solubility and cause precipitation — the salt ions compete with protein for water, disrupting the hydration shell. This is the basis of (NH₄)₂SO₄ precipitation. Salting in (Hofmeister effect): at low salt concentrations, ions stabilise the protein-water interface and increase solubility — proteins are often more soluble in 0.1 M NaCl than in pure water. The Hofmeister series ranks ions from most kosmotropic (salting out) to most chaotropic (salting in): SO₄²⁻ > F⁻ > Cl⁻ > Br⁻ > I⁻ > SCN⁻ (anions). NH₄⁺ and Na⁺ are mildly kosmotropic. This is why (NH₄)₂SO₄ (both SO₄²⁻ and NH₄⁺ are kosmotropic) is the most effective salting-out agent.