Protein Concentration Calculator

Why Blanking With Water Inflates Protein Readings

The error I encounter most often is blanking the spectrophotometer with water instead of the protein's storage buffer. If the buffer contains imidazole, glycerol, or other UV-absorbing components, blanking with water will include that background absorbance in the protein reading and produce an inflated concentration. Always prepare the blank using the same buffer at the same concentration the protein was eluted or stored in, and measure the blank immediately before the protein sample in the same cuvette or pedestal position. This mistake turns up most often with His-tag purification eluates containing imidazole, where imidazole absorbance at 280 nm can account for 10 to 30 percent of the total A280 signal in dilute fractions. The Bio-Rad Bradford Protein Assay documentation lists buffer and additive compatibility for all common protein assay formats.

What the Protein Concentration Calculator Actually Does

This tool determines the concentration of a protein solution from a spectrophotometric absorbance reading, using either the Beer-Lambert law for direct UV absorbance at 280 nm or a standard curve for colorimetric assays such as Bradford and BCA. Molecular biologists, biochemists, and protein chemists use it to figure out how much protein is present in a sample before downstream experiments that require a precise protein input. According to the NCBI Molecular Cloning manual, accurate protein quantification is a prerequisite for reproducible biochemical experiments, since loading errors can invalidate entire datasets.

The Beer-Lambert Law and the A280 Method

The A280 method is based on the Beer-Lambert law: A = ε × c × l. For a standard 1 cm path length cuvette, concentration in mg/mL equals absorbance at 280 nm divided by the specific extinction coefficient. Proteins absorb at 280 nm because the aromatic amino acids tryptophan and tyrosine absorb UV light at that wavelength; proteins with none, such as collagen, cannot be quantified by A280 at all. The ExPASy ProtParam tool calculates the theoretical extinction coefficient from any amino acid sequence. For unknown proteins, a general approximation of 1.0 per mg/mL per A280 unit is sometimes used, but a protein rich in tryptophan may have an extinction coefficient of 2.5 or higher, meaning the approximation would underestimate concentration by 150 percent.

Common Protein Assay Methods Compared

Purity Assessment: The 260/280 Ratio

A spectrophotometer measurement at both 260 nm and 280 nm provides a purity check alongside concentration. Nucleic acids absorb strongly at 260 nm but proteins absorb at 280 nm; a pure protein sample should have a 260/280 ratio of approximately 0.57, and a ratio above 0.6 suggests nucleic acid contamination that will overestimate A280 concentration. The PubMed guide to nucleic acid and protein spectrophotometry covers interpretation of both purity ratios in detail. Samples from cell lysates often contain nucleic acid contamination that artificially inflates the A280 reading; a DNase and RNase digestion step, or switching to a colorimetric assay, gives a more accurate reading in complex mixtures.

Accuracy and Limitations

The calculator is mathematically exact for the values entered. Its practical accuracy depends on spectrophotometer calibration, cuvette or pedestal cleanliness, and whether the blank correction used the correct buffer. The A280 method is inaccurate for proteins with no tryptophan or tyrosine, proteins that aggregate and scatter light, and samples with nucleic acid contamination. Colorimetric assays are susceptible to protein-to-protein variability since different proteins bind Bradford dye or reduce BCA reagent with different efficiencies; for absolute quantification in critical experiments, amino acid analysis or quantitative mass spectrometry provides the most accurate determination.