Protein Molecular Weight Calculator (pI, Extinction & Charge vs pH)
Compute protein average and monoisotopic molecular weight from amino acid sequence or FASTA input. Generates theoretical isoelectric point (pI), molar extinction coefficient (ε280), absorbance at 280 nm (1 mg/mL), empirical formula, dynamic Net Charge vs. pH titration curves with IEX chromatography guidance, and Kyte-Doolittle hydropathy profile plots.
Amino Acid Sequence & Presets
Post-Translational Modifications & Engineered Tags
Net Charge vs. pH Titration Curve
At pH 7.4, the protein carries a net negative charge (-19.96 e). It will bind tightly to positively charged anion exchange resins (e.g., Q-Sepharose, DEAE, Capto Q). Positively charged host impurities will flow through. Elute using a 0 to 1.0 M NaCl gradient.
Kyte-Doolittle Hydropathy Profile Plot
Amino Acid Composition & Classification
Downstream Wet-Lab Workflows
The MW, extinction coefficient, and pI generated here directly feed into your downstream assays:
Feed ε280 (42,925) into A280 Beer-Lambert or BCA/Bradford standard curves.
Select optimal resolving gel % (6% to 18%) scaled for 66.4 kDa protein.
Select 3-fold smaller MWCO (< 22.1 kDa) to desalt with >95% recovery.
Protein Molecular Weight Calculator Principles: Biophysical Parameters
Predicting protein molecular weight from primary sequence is foundational to molecular biology and mass spectrometry. When amino acids condense to form a polypeptide backbone, the alpha-amino and alpha-carboxyl groups form a peptide bond (-CO-NH-), expelling one water molecule (18.015 Da) per bond.
Average Molecular Weight (MWavg)
Calculated using isotope abundance-weighted atomic weights. Standard for wet-lab benchwork such as SDS-PAGE migration, molarity conversions, and lyophilized mass measurements.
Monoisotopic Mass (MWmono)
Calculated using the mass of the most abundant stable isotope for each element. The gold standard for high-resolution mass spectrometry (LC-MS/MS, Orbitrap, Q-TOF intact mass verification).
The molar extinction coefficient at 280 nm is determined via the Pace (1995) model: Trp contributes 5,500 M⁻¹cm⁻¹, Tyr contributes 1,490 M⁻¹cm⁻¹, and cystine (disulfide bridge) contributes 125 M⁻¹cm⁻¹. Under reducing conditions (DTT, TCEP), cystine contributions are eliminated.
Ion Exchange Chromatography (IEX) & Protein Net Charge Optimization
Ion Exchange Chromatography (IEX) is the primary charge-based separation method in downstream bioprocessing. A protein’s net surface charge is dynamically governed by the buffer pH:
Anion Exchange (AEX, pH > pI)
When buffer pH > pI, acidic residues deprotonate, conferring net negative charge. Target protein binds to positively charged quaternary amine resins (Q-Sepharose, Capto Q).
Cation Exchange (CEX, pH < pI)
When buffer pH < pI, basic residues protonate, conferring net positive charge. Target protein binds to negatively charged sulfopropyl resins (SP-Sepharose, Capto S).
Aggregation Zone (pH ≈ pI)
Near pI (±0.5 pH units), net charge approaches zero. Absence of electrostatic repulsion allows hydrophobic patches to coalesce into insoluble aggregates. Always park formulation buffers ≥1 unit away from pI.
Standard 20 Amino Acids Physicochemical & Mass Matrix
The molecular mass, theoretical pI, extinction coefficient (ε280), and hydropathy profile of any polypeptide derive fundamentally from the summation of its 20 constituent residue building blocks.
| Code | Amino Acid | Residue Monoisotopic Mass | Residue Average Molecular Mass | Side-Chain pKa | Charge at pH 7.4 | Hydropathy | ε280 (M⁻¹cm⁻¹) |
|---|---|---|---|---|---|---|---|
| R / Arg | Arginine | 156.101 Da | 156.19 Da | 12.48 | +1.0 | -4.5 | — |
| K / Lys | Lysine | 128.095 Da | 128.17 Da | 10.53 | +1.0 | -3.9 | — |
| H / His | Histidine | 137.059 Da | 137.14 Da | 6.00 | +0.09 | -3.2 | — |
| D / Asp | Aspartate | 115.027 Da | 115.09 Da | 3.90 | -1.0 | -3.5 | — |
| E / Glu | Glutamate | 129.043 Da | 129.12 Da | 4.07 | -1.0 | -3.5 | — |
| W / Trp | Tryptophan | 186.079 Da | 186.21 Da | — | 0.0 | -0.9 | 5,500 |
| Y / Tyr | Tyrosine | 163.063 Da | 163.18 Da | 10.07 | 0.0 | -1.3 | 1,490 |
| F / Phe | Phenylalanine | 147.068 Da | 147.18 Da | — | 0.0 | +2.8 | ~200 |
| C / Cys | Cysteine | 103.009 Da | 103.14 Da | 8.33 | -0.1 | +2.5 | 125 |
| M / Met | Methionine | 131.040 Da | 131.20 Da | — | 0.0 | +1.9 | — |
| I / Ile | Isoleucine | 113.084 Da | 113.16 Da | — | 0.0 | +4.5 | — |
| V / Val | Valine | 99.068 Da | 99.13 Da | — | 0.0 | +4.2 | — |
| L / Leu | Leucine | 113.084 Da | 113.16 Da | — | 0.0 | +3.8 | — |
| A / Ala | Alanine | 71.037 Da | 71.08 Da | — | 0.0 | +1.8 | — |
| P / Pro | Proline | 97.053 Da | 97.12 Da | — | 0.0 | -1.6 | — |
| N / Asn | Asparagine | 114.043 Da | 114.10 Da | — | 0.0 | -3.5 | — |
| Q / Gln | Glutamine | 128.059 Da | 128.13 Da | — | 0.0 | -3.5 | — |
| S / Ser | Serine | 87.032 Da | 87.08 Da | — | 0.0 | -0.8 | — |
| T / Thr | Threonine | 101.048 Da | 101.11 Da | — | 0.0 | -0.7 | — |
| G / Gly | Glycine | 57.021 Da | 57.05 Da | — | 0.0 | -0.4 | — |
Charges are estimates. Hydropathy uses the Kyte–Doolittle scale (positive values are more hydrophobic; negative values are more hydrophilic). Extinction coefficients show the Trp, Tyr, and disulfide contributions at 280 nm.
💡 Mass Accounting: Total intact polypeptide MW = ∑(Residue Mass) + 18.015 Da (accounting for terminal hydrogen and hydroxyl caps). Post-translational modifications require addition of respective delta masses.
Protein Molecular Weight Calculator Worked Lab Example: BSA Characterization
Bovine serum albumin (BSA) is commonly used as a reference protein. The following are illustrative calculations for the mature sequence:
These are sequence-based estimates. Protein source, isoform, modifications, and buffer conditions can change measured values. Verify ion-exchange behavior and solubility with the actual sample and conditions.
Frequently Asked Questions (FAQ)
Q1How is protein molecular weight calculated from an amino acid sequence?
The calculator sums the residue masses of the 20 amino acids and adds 18.015 Da (one water molecule, H2O) to account for the free N-terminal -H and C-terminal -OH groups. Every peptide bond formation releases one molecule of H2O.
Q2What is the difference between monoisotopic mass and average molecular weight?
Monoisotopic mass uses the mass of the most abundant natural isotope of each element (12C, 1H, 14N, 16O, 32S) and is critical for high-resolution mass spectrometry (LC-MS/MS). Average molecular weight uses the abundance-weighted atomic masses and is used for bench calculations like SDS-PAGE and solution preparation.
Q3How does the Net Charge vs. pH curve guide Ion Exchange Chromatography (IEX)?
At buffer pH > pI, the protein carries a net negative charge and binds to Anion Exchange (AEX) resins like Q-Sepharose or Capto Q. At buffer pH < pI, the protein carries a net positive charge and binds to Cation Exchange (CEX) resins like SP-Sepharose or Capto S. Near pH = pI (±0.5), net charge is near zero, minimizing electrostatic repulsion and increasing aggregation risk.
Q4How is the molar extinction coefficient (ε280) calculated?
Using the Pace (1995) method: ε280 = (nTrp × 5500) + (nTyr × 1490) + (nCystine × 125) M⁻¹cm⁻¹, where nCystine is the number of disulfide bonds (floor(nCys / 2)). Under reducing conditions with DTT or TCEP, the cystine contribution is 0.
Q5Why does my protein migrate at an apparent molecular weight different from theoretical MW on SDS-PAGE?
Gel migration discrepancies commonly arise from: 1) Post-translational modifications (especially N/O-glycosylation adding bulky glycans that bind less SDS); 2) High negative charge or high proline content altering standard SDS binding ratios (nominally 1.4 g SDS per g protein); 3) Hydrophobic transmembrane helices; or 4) Incomplete disulfide bond reduction in non-reducing gels.
Q6How do signal peptides and purification tags affect recombinant protein molecular weight?
Secreted recombinant proteins typically have their N-terminal signal leader (typically 15–30 aa, ~1.5–3.5 kDa) cleaved by signal peptidase during translocation. Conversely, engineered affinity tags add mass: His6 adds ~0.84 kDa, FLAG adds ~1.01 kDa, HA adds ~1.10 kDa, and GST adds ~26.0 kDa. Use the PTM/Tags panel above to simulate these processing events.
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