Protein MW, pI & Extinction Calculator
Molecular weight, isoelectric point by Henderson-Hasselbalch iteration, GRAVY index and Pace 1995 A280 extinction coefficient.
How it works
Molecular weight is summed from residue masses less one water per peptide bond. Isoelectric point is found by iterating pH until net charge crosses zero, using Henderson-Hasselbalch with standard side-chain pKa values. The A280 extinction coefficient follows Pace, summing tryptophan, tyrosine and cystine contributions, which is what allows concentration to be read directly from absorbance.
Frequently asked questions
How do you calculate a protein extinction coefficient?
The molar extinction coefficient at 280 nm is estimated from aromatic residue content: ε = (5500 × number of tryptophans) + (1490 × number of tyrosines) + (125 × number of disulphide-bonded cystines), in units of M⁻¹cm⁻¹. Dividing absorbance at 280 nm by this value and the path length gives molar concentration. Proteins with no tryptophan or tyrosine cannot be quantified reliably this way.
What does the isoelectric point of a protein tell you?
The isoelectric point (pI) is the pH at which a protein carries no net charge. It predicts behaviour in ion-exchange chromatography and isoelectric focusing, and it warns of solubility problems: proteins are least soluble near their pI and tend to precipitate there, so buffers are usually chosen at least one pH unit away.
Method reference
Pace CN et al. (1995) Protein Sci 4(11):2411-2423.