Protein concentration is determined by measuring how a sample interacts with light, dyes, or chemical reagents, then comparing that measurement against a set of known standards. The most common approaches fall into two categories: direct methods that use UV light absorption and colorimetric or fluorometric assays that rely on a chemical reaction to produce a measurable signal. The right choice depends on how much protein you have, what else is in your sample, and how precise you need to be.
UV Absorbance at 280 nm
The fastest way to estimate protein concentration is to measure how much ultraviolet light your sample absorbs at a wavelength of 280 nm. Proteins absorb at this wavelength because of their aromatic amino acids, particularly tryptophan and tyrosine. You place the sample in a spectrophotometer, record the absorbance reading, and calculate concentration using the Beer-Lambert law:
Concentration (mg/mL) = Absorbance at 280 nm ÷ (extinction coefficient × path length in cm)
The extinction coefficient is specific to each protein. If you know the identity of your protein, you can look up or calculate its coefficient from the amino acid sequence. This method requires no reagents, takes seconds, and works well for purified proteins at moderate concentrations. Micro-volume spectrophotometers need as little as 1–2 microliters of sample.
The limitation is accuracy. A sample containing nucleic acids, other UV-absorbing compounds, or a mixture of unknown proteins will give unreliable results. UV absorbance also varies significantly between proteins depending on how many aromatic amino acids they contain. For crude cell lysates or mixed samples, a chemical assay is a better option.
The Bradford Assay
The Bradford assay is one of the most popular colorimetric methods because it is fast and simple. It uses a dye called Coomassie Brilliant Blue that shifts color when it binds to protein. In its unbound form the dye is reddish-brown, but when it binds protein the solution turns blue, with peak absorbance around 590–595 nm. The more protein in the sample, the more intense the blue color.
You prepare a series of standards at known concentrations, add the dye reagent, wait a few minutes, and read the absorbance. The whole process takes about 10 minutes. The Bradford assay is roughly twice as sensitive to some proteins as others. It responds about twice as strongly to bovine serum albumin (BSA) as to immunoglobulin G (IgG), for example, so the choice of standard protein matters. Ideally, you use the same protein as your standard that you are measuring in the unknown sample. If that is not possible, you can apply a correction factor.
One well-known drawback is interference from detergents. Many protein samples are prepared using detergents to break open cells, and these can reduce or distort the Bradford signal. Modified versions of the assay have been developed to handle detergent-containing buffers more reliably.
The BCA Assay
The bicinchoninic acid (BCA) assay works through a two-step chemical reaction. First, protein in your sample reduces copper ions from their oxidized form to a reduced form in an alkaline solution. Then bicinchoninic acid reacts with the reduced copper to form a deep purple complex that absorbs light at 562 nm. The detection range spans roughly 0.2 to 50 micrograms per milliliter, making it suitable for dilute samples.
Certain amino acids, especially cysteine, tryptophan, and tyrosine, drive the first step of the reaction at lower temperatures (around 37°C). At higher temperatures (around 60°C), the peptide bonds themselves contribute, which makes the color development less dependent on amino acid composition and more uniform across different proteins. This is an advantage over the Bradford assay when you are measuring a protein mixture.
The BCA assay is more tolerant of detergents than the Bradford, but it is sensitive to reducing agents like DTT. If your sample buffer contains a reducing agent, it will artificially inflate your reading by generating extra reduced copper. In those situations, you either need to remove the reducing agent first or choose a compatible assay format.
The Lowry Assay
The Lowry assay is one of the oldest colorimetric protein methods and uses a similar copper-based chemistry. Protein first forms a complex with copper in an alkaline solution, then a second reagent (Folin-Ciocalteu) is added. The Folin reagent is reduced by the copper-protein complex, producing a blue color that absorbs maximally at 750 nm. The color develops over about 30 minutes.
The Lowry method is sensitive and works at low protein concentrations, but it involves more steps than the BCA or Bradford assays and is more susceptible to interference from common lab chemicals, including reducing agents and high concentrations of urea. For most routine applications, the BCA assay has largely replaced the Lowry because it is simpler and more tolerant of buffer components.
Fluorometric Methods
Fluorescence-based assays use dyes that emit light only when bound to protein, which makes them highly selective. The Qubit Protein Assay, one of the most widely used fluorometric options, can detect protein at concentrations as low as 0.025 mg/mL in the sample. In direct comparisons with the BCA, Bradford, and Lowry assays, fluorometric methods show lower protein-to-protein variation, meaning the result is more consistent regardless of which protein you are measuring.
Because the dyes bind selectively to protein rather than to nucleic acids or other molecules, fluorometric assays outperform UV absorbance in samples that contain DNA or RNA contamination. The trade-off is cost: the reagents and dedicated fluorometer are more expensive than a standard spectrophotometer and colorimetric kit.
Building a Standard Curve
Every colorimetric and fluorometric assay requires a standard curve. You prepare a series of dilutions of a protein with a known concentration, run them through the same assay as your unknown sample, and plot absorbance (or fluorescence) against concentration. Your unknown sample’s reading is then mapped onto this curve to determine its concentration.
BSA is the most common standard protein because it is inexpensive and widely available. IgG is also used, particularly in immunology labs. The important thing to understand is that different proteins produce different signal intensities in the same assay. The Bradford assay, for example, is about twice as sensitive to BSA as to IgG. If you calibrate with BSA but measure an IgG-rich sample, your result will be off unless you apply a correction factor. Whenever possible, match the standard to the protein you are actually measuring.
A typical dilution series for a Bradford assay in a microplate might range from 0.005 to 0.05 mg/mL, prepared in triplicate to account for pipetting variability. You also include a blank (water or buffer only) to subtract background absorbance from all readings. Run standards on the same plate and at the same time as your unknowns so that temperature and timing are identical.
Common Sources of Interference
The biggest practical challenge in protein quantification is interference from other chemicals in your sample buffer. Here is a quick guide to the most common problem substances:
- Detergents (SDS, Triton X-100): These interfere strongly with the standard Bradford assay by disrupting dye-protein binding. The BCA assay is more tolerant, and detergent-compatible Bradford formulations are available.
- Reducing agents (DTT, beta-mercaptoethanol): These interfere with the BCA, Lowry, and Micro BCA assays because they reduce copper ions independently of protein, producing a falsely high signal.
- Urea at high concentrations: Commonly found in 2D gel sample buffers at 8 M, urea can interfere with both BCA and Lowry assays.
If your buffer contains a problematic compound, you have a few options: dilute the sample until the interfering substance is below the threshold, use a desalting column to exchange the buffer, or choose an assay that tolerates the compound. Manufacturers publish compatibility tables listing the maximum tolerable concentration of dozens of common reagents for each assay format.
Choosing the Right Method
Your decision comes down to four questions: what is in your buffer, how much protein you expect, how much sample you can spare, and how much accuracy you need.
For a quick check of a purified protein at moderate concentration, UV absorbance at 280 nm is the simplest option. No reagents, no incubation, and only a microliter or two of sample. For crude lysates or mixed samples, a colorimetric assay is more reliable. The Bradford is the fastest colorimetric choice and works well when your buffer is free of detergents. The BCA assay handles detergents better and gives more uniform results across different proteins, but fails with reducing agents. When you need the highest sensitivity or the lowest protein-to-protein variation, a fluorometric assay is the strongest performer.
Keep in mind that conventional colorimetric assays tend to overestimate protein concentration in complex samples. A study comparing the Lowry, BCA, and Bradford against a protein-specific immunoassay found that all three colorimetric methods reported significantly higher concentrations than the targeted method, and all three agreed closely with each other. This happens because colorimetric assays measure total protein, including contaminants, not just your protein of interest. If absolute accuracy for a specific protein matters, you may need a targeted approach like an ELISA or a quantitative Western blot rather than a total-protein assay.