The biuret reaction is a general method for the determination of proteins or peptides

The biuret reaction is a general method for the determination of proteins or peptides. This method is based on the reaction of the Cu 2+ with four NH groups of the peptide bonds, forming a violet complex with an absorption maximum at 540 nm. The intensity of the color is proportional to the number of peptide bonds present, and therefore to the amount of protein. This reaction is specific, the dipeptides and amino acids give a negative reaction, and few substances interfere. A quantity of 19.89 g of fish meat is extracted with 100 mL of glacial acetic acid. Subsequently, 3 mL of the previous solution with 3 mL of the biuret reagent to generate the reaction, after 30 minutes. To reduce errors due to other interfering substances and residual turbidity, the corrected absorbance method Ac is applied to 500 nm and 570 nm. In a study, a standard sample analyzed by the Kjeldahl method (total Nitrogen analysis) is obtained, resulting in 0.43% N (w / v), (Protein content [%] =% N x 6.38). 3 mL of the standard are mixed with 3 mL of the biuret reagent and when examined under UV together with the fish sample (all in the same experimental conditions), the following data are obtained: Sample (type) absorbance 500nm absorbance 540nm absorbance 570nm Fish 0.284 0.423 0.158 Standard 0.247 0.479 0.152 Glacial acetic acid and biuret reagent that was used for the A = 0 calibration. Assuming the validity of the Beer-Lambert law in its modified form Ac = ε b c, what% of proteins corresponds to the type of fish meat studied?

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