タンパク質濃度計算ツール

計算の考え方

Proteins absorb at 280 nm because of aromatic residues (tryptophan, tyrosine, and to a lesser extent cystine). For a purified protein with a known molar extinction coefficient, Beer–Lambert gives an accurate concentration; for a mixed or unknown sample, the heuristic 1 A280 = 1 mg/mL is a usable estimate.

なぜ使うのか

Almost every downstream application — gel loading, dilution to a working stock, assay setup — needs a protein concentration. A280 is the fastest available measurement, but the assumption you make (1 mg/mL per A280 vs. Beer–Lambert) drives the accuracy of every result downstream.

この計算ツールが役立つ理由

A280 readings are often reported in instrument software, but converting them into a usable mg/mL number is where mistakes happen — particularly the missing division by path length or the wrong extinction coefficient. The calculator does the unit bookkeeping and shows both the 1-A280 and Beer–Lambert results side by side.

主な前提条件

• The spectrophotometer was blanked with the same buffer the sample was diluted into.
• For the 1-A280 path, the protein mixture is roughly "average" (e.g. a crude lysate); for purified proteins use the calculated path.
• Path length is 1 cm, or the instrument has been corrected for a different path length.

制限事項

• The 1 A280 = 1 mg/mL assumption fails badly for samples rich in nucleic acids (use A260/A280 subtraction).
• It cannot detect proteins that lack aromatic residues or are present at sub-A280 noise-floor concentrations.
• It does not distinguish folded from denatured protein — the extinction coefficient assumes the folded environment.