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Lowry Protein Assay Calculator

Calculate protein concentrations from a Lowry (Folin) assay: fit the standard curve as a line or a curve, read the unknowns off it and apply dilution factors.

Formula

A750=ac2+bc+d(or A750=bc+d)A_{750} = a c^2 + b c + d\quad(\text{or } A_{750} = b c + d)
c=root of the fitted curve for the measured Ac = \text{root of the fitted curve for the measured } A
coriginal=c×DFc_{\text{original}} = c \times \mathrm{DF}
cc
protein concentration in the tube
a, b, da,\ b,\ d
coefficients of the fitted curve

How it works

The Lowry assay has two steps. Protein first reacts with copper in alkaline solution, as in the biuret reaction, and the copper-protein complex then reduces the Folin-Ciocalteu reagent, largely through tyrosine and tryptophan, to give a blue colour read near 750 nm. It was the standard method for decades and is still used.

It is sensitive, but its colour depends on the protein's composition, its range is narrow and its curve bends, and many common reagents interfere. The tool fits the standards with either model and shows the residuals.

Worked example

Eight BSA standards from 0 to 400 µg/mL (illustrative absorbance values) and two fractions diluted 2- and 1-fold.

  1. The second-order fit is A = −6.04 × 10⁻⁷ c² + 0.001748 c − 0.0013, R² = 0.9999.
  2. Fraction 1 (A = 0.210) gives 126.4 µg/mL in the tube, times 2 = 252.8 µg/mL.

Fraction 1 is about 253 µg/mL and fraction 2 about 217 µg/mL.

These are the values the calculator opens with, so you can check its output against this example.

Assumptions

  • The unknown proteins have a tyrosine and tryptophan content like the standard's.
  • The same volumes, timing and reagents are used for every tube.
  • The standards' absorbances here are illustrative; use your own measured values.

Common mistakes

  • Working outside the narrow range of the assay.
  • Using samples with detergents, reducing agents or high sugar, which interfere.
  • Timing the tubes differently, which changes the colour.

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Related equipment

Service documentation, failure modes and parts for the instruments this calculation is used with.