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How to determine the protein concentration with the BCA Protein Assay
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the bca protein assay also known as smith assay named after its inventor is one of the most used techniques for the concentration determination of protein briefly this method is a two component system belonging to the colorimetric assays alongside the lori or bradford sa the characteristic of the copper-based bca assay is a color change from green to violet in the presence of protein speaking about other assays what are the advantages of the bca assay over the bradford test the benefit of the bca protein assay is that it is compatible with detergents to a certain degree further the bca assay is independent on the protein's amino acid composition but how does it work in the first step a working solution composed of two different reagents is prepared reagent a includes the carbonate buffer with a high ph of about 11 that means that it is a highly alkaline solution with a high concentration of o h minus ions it also contains the key molecule by kinchoninic acid which the assay is named after reagent b is a cupric sulfate solution those two reagents are mixed at a ratio of 50 to 1 producing a greenish working solution the second step requires the protein sample with the unknown concentration and a so-called protein standard this standard is a serial dilution of a protein which has a very well-defined concentration most standards use bsa as a protein this protein standard is utilized to create a standard curve in the data analysis step in the next step both the protein sample and the bsa standard is pipetted onto a micro tighter plate after adding the working solution with all main ingredients for the colorimetric reaction the samples are incubated for 30 minutes at 37 degrees celsius meanwhile we can investigate which reactions inside those wells are happening in the presence of protein the reactions of the bcasa can be divided into two steps in the first one peptide bonds of the protein present in the sample reduce copper sulfate in an alkaline environment to cupros in other words copper two plus cations are reduced to copper one plus cations the higher the protein concentration the more copper one plus cations will be produced in this reaction in the second step two bca molecules gelate with one copper plus ion this complex has an absorption maximum at 562 nanometers which will be the reason why those reactions are visible when we see the green turning into purple or blue after 30 minutes of incubation at 37 degrees the plate can be analyzed one can already see that this colorimetric reaction is proportional to the protein concentration the standard without any protein is still green whereas the highest concentration is dark purple to determine the protein content in our sample of interest the plate needs to be inserted into a plate reader in which an endpoint measurement determines the absorbance at 562 nanometers the data of the bsa standard can be plotted resulting in a standard curve the absorbance at this wavelength was also measured for our protein sample of interest extrapolation is used to estimate the protein concentration of this sample in practice a formula is generated after plotting the standard curve with that formula and the absorbance at 562 nanometers the protein concentration can be easily calculated if this video was helpful subscribe to the channel and leave a like here is another video that might be interesting to check out thanks for watching
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