BCA and Bradford Protein Assay Standard Curve Interpolator
Fit colorimetric protein assay standards and interpolate unknown sample dilutions within the supported BSA calibration span. This reference opens a self-generated Bradford-style BSA series with a linear calibration, diluted lysates and out-of-range unknowns, illustrating why background absorbance, curvature and amino-acid composition govern colorimetric protein estimates.
Five BSA standards from 125 to 1,000 µg/mL in duplicate (A595), two lysates (lysate B read at 1:5) and one unknown on each side of the tested range, from the independently checked linear calibration fixture. The fixture expects about 389 µg/mL for lysate A and 3,123 µg/mL for undiluted lysate B, and no value for the two out-of-range wells. These are not readings from a commercial kit lot.
Account for buffer interference and blank wells on the same plate
Colorimetric protein assays are sensitive to buffer components, incubation time and temperature. BCA colour development continues slowly even after room-temperature cooling, so standards and unknown samples must be incubated together on the same plate and read within the same run. Prepare the BSA standards and the zero-protein blank wells in the exact lysis or storage buffer present in the diluted unknowns so that background copper reduction or detergent dye shift is matched.
This example applies no blank subtraction: the standards and unknowns are raw A595 readings, and the fitted intercept carries the zero-protein background. Subtracting one blank mean from every well would move the intercept towards zero without changing the slope or the interpolated concentrations of an unweighted straight-line fit, whereas adding a blank as an extra standard point at zero concentration changes the regression leverage.
Compare colorimetric mass estimates with sequence-based A280
For unfractionated cell lysates or complex mixtures where no single extinction coefficient exists, BCA and Bradford standard curves provide a practical relative mass normalization for western blots or activity assays. For a purified recombinant protein of known sequence, compare the colorimetric BSA-equivalent value against a direct Beer–Lambert A280 measurement to uncover dye-binding bias or nucleic-acid contamination.
Equations and variables
Straight-line calibration with an intercept
y = m × c + b
- y
- Measured well absorbance as read; this example subtracts no blank (absorbance units)
- c
- Standard BSA-equivalent mass concentration (µg/mL)
- m
- Fitted slope on the calibrated interval (absorbance units per µg/mL)
- b
- Fitted intercept, which carries the zero-protein background absorbance (absorbance units)
Pre-dilution sample concentration
csample = cinterpolated × d
- csample
- Estimated BSA-equivalent concentration in the undiluted sample (µg/mL)
- cinterpolated
- Concentration read from the calibration curve for the assayed well (µg/mL)
- d
- Volumetric dilution factor applied prior to plate loading
Scope and limitations
- Both Bradford Coomassie dye binding and BCA copper reduction respond to specific amino-acid side chains and peptide bonds, so the result is a BSA-equivalent mass concentration rather than an absolute gravimetric weight.
- Colorimetric protein assays curve downward at high protein concentrations as dye or copper reagent is depleted; forcing a straight line across a saturating span biases both low and high unknowns.
- Reducing agents such as DTT or TCEP interfere strongly with BCA copper reduction, whereas high detergent concentrations interfere with Bradford dye binding.
Sources and provenance
- Published numerical verification inputs and independent referencesOriginal synthetic inputs; see individual fixture notices
The generated inputs are demonstrations, not experimental measurements. Existing versioned fixtures are retained unchanged.
- Independent linear calibration fixture (exact arithmetic)MIT; self-generated readings, not kit data
- Bradford: a rapid and sensitive method for the quantitation of microgram quantities of proteinCited method; Coomassie dye-binding assay
- Smith et al.: measurement of protein using bicinchoninic acidCited method; BCA copper-reduction assay
Technical questions
Why is a Bradford or BCA result described as a BSA-equivalent concentration?
Bovine serum albumin or gamma globulin is typically used to construct the standard curve, but different proteins contain different proportions of basic, aromatic or cysteine/tyrosine/tryptophan residues. A purified protein with few basic residues can bind less Coomassie dye than BSA at the same true mass concentration.
Should I use a linear fit or a nonlinear curve for BCA and Bradford assays?
Over a narrow concentration range, BCA responses are approximately linear, whereas Bradford assays are noticeably nonlinear because free and bound dye species overlap spectrally. Always inspect the residual plot across the standards; if systematic curvature is visible, restrict the linear range or use an appropriate nonlinear calibration model.
What should I do when an unknown sample absorbance exceeds the highest BSA standard?
Do not extrapolate beyond the top standard, where dye depletion flattens the response. Dilute the original protein sample further in matching buffer and re-measure it alongside a fresh standard curve on the same plate.
Connected workflows
- 4PL standard-curve interpolation
Inspect the shared 4PL inverse-prediction and range-withholding mechanics.
- Protein A280 concentration calculation
Compare colorimetric BSA-equivalent concentration with direct A280 extinction.
- Viable-cell seeding volumes
Use quantified lysate or suspension preparations consistently across downstream plates.