Question

You are provided with a high protein shake. One serving size is 240 ml and the protein content is labeled as 16 gm. You decided to perform the biuret test to accurately determine the protein concentration in this shake. The linear range of the Biuret assay is 40-1000 ug/ml standard protein. How will you dilute the protein sample? using (Protein Assay Lab)

Answer 1

Ans. Given,

            [Protein] in protein shake = 16.0 g / 240.0 mL

                                                            = 0.0666666667 g / mL                   ; [1 g = 10³ mg]

                                                            = 66.6666667 mg/ mL                     ; [1 mg = 10³ ug]

                                                            = 66666.6667 ug/ mL         

# Desired [Protein] at lower limit = 40 ug/ mL

Since the original [Protein] in protein shake is extremely concentrated, around 15000 times more concentrated, than the lower limit of desired [Protein], the best approach is to make a stock solution of the upper limit (1000 ug/mL) protein concertation. The volume of stock solution shall preferably be 100.0 mL or more so that the dilution can be made appropriately.

So,

            Prepare a stock solution of 1000 ug/mL from original protein shake.

Using              C1V1 (protein shake) = C2V2 (desired stock, 1000 ug/ mL)

            Or, (66666.6667 ug/ mL) x V1 = (1000.0 ug / mL) x 100.0 mL

            Or, V1 = (1000.0 ug / mL) x 100.0 mL / (66666.6667 ug/ mL)

            Hence, V1 = 1.499999 mL

Preparation of stock solution: Transfer 1.499 mL of original protein shaken into a 100-mL standard volumetric flask using suitable micropipette. Make the final volume upto the mark with distilled water. The resultant stock solution is 1000.0 ug/ mL.

# Preparation of standard aliquots using stock solution: Take different volumes of the stock solution in clean test tubes. Add sufficient water to make the final volume to 1.0 mL (or, as required) for all standard aliquots. A sample calculation is shown below-