After purification, the His-tagged protein is quantified using BCA. At what imidazole concentration will it interfere?

The most effective way to determine which value is accurate is spike recovery. Take a portion of the eluate sample, add a known concentration of BSA standard so that the theoretical increase falls in the middle range of the standard curve, for example, add 25–50 ug/mL, and then observe the increase measured by BCA. If the recovery is significantly higher than 110–120%, it basically indicates positive interference caused by imidazole; if the recovery is low, it may be due to protein loss or a mismatch of the standard. Run an additional unspiked matrix blank. Do not directly compare the two values before and after dialysis, because the dialysis process may change the volume and protein concentration. Spike recovery can help you determine whether the sample needs to be processed before measurement.

Imidazole-containing eluates are also not recommended for direct Bradford measurement, as their interference is more complex. In practice, if the samples are only intended for Western blot loading, a small amount of sample can first be dialyzed or desalted, and the change in BCA readings after treatment can be compared. Alternatively, a standard curve can be prepared using imidazole elution buffer and quantification performed directly in a 300 mM imidazole matrix, which minimizes sample consumption. However, if downstream experiments are sensitive to imidazole, the buffer should still be exchanged before measurement. Do not repeatedly dilute simply because the BCA reading is high; dilution reduces imidazole but may also bring the protein below the detection limit. Identifying the source of interference first is more important than blindly changing methods.

The sample volume is only one to two hundred microliters. It is recommended to use a 0.5 mL or 2 mL desalting column for buffer exchange, which is the most convenient option—faster than dialysis and with controllable volume. However, note that the desalting column itself may cause dilution, so the dilution factor must be taken into account during quantification. Ultrafiltration tubes can also be used, but low-concentration proteins tend to adsorb to the membrane, resulting in unstable recovery. Although dialysis is thorough, it takes a long time and the sample volume may change, making it more prone to loss at low concentrations. If enzyme activity or structural experiments are to be performed subsequently, imidazole removal is required anyway, and quantification after buffer exchange is more reliable. If the measured value is somewhat low after buffer exchange, it may be due to protein loss, and recovery validation is needed.

For trace samples, micro BCA or enhanced BCA protocols are recommended, with sample volumes reduced to 10–25 uL; some kits can be adapted for test tube or 96-well plate micro reactions. When selecting a kit, focus on asking the manufacturer about the tolerated concentrations of imidazole, urea, and reducing agents. Even with a micro protocol, standards must be prepared in the same elution buffer matrix. For reading, measure at 562 nm in a 96-well plate, and note that the ratio of working reagent to sample must not be altered arbitrarily, as this affects the linearity of color development. If imidazole interference persists, a small amount of sample can be pretreated by TCA/DOC precipitation or desalting before the reaction.

Directly measuring BCA with 300 mM imidazole does tend to give falsely high readings. Many BCA kits tolerate imidazole only at the tens-of-mM level, depending on the manufacturer's specifications. 300 mM usually exceeds the recommended range. Besides imidazole itself possibly reacting with the reagent, the falsely high readings are also affected by nickel ions in the elution buffer, buffer pH, and reducing agents. The most direct approach is to prepare the standard curve using the elution buffer as the diluent and set up a matrix-matched blank. If that is not feasible, at least dilute the sample to below 50 mM imidazole and then check how the reading changes. Do not read directly against a water-based standard curve.