1.
a. A standard solution of a protein containing 0.400 mg/ml had an absorbance of 0.312 at 280 nm. The pathlength of the cuvette is 1 cm (b= 1 cm). Calculate the extinction coefficient (mg-1 * cm-1 * mL) of the standard protein and write your final answer in the space below. Show all work.
b. A highly concentrated aliquot of this protein standard was diluted by adding 0.300 ml of protein to 0.900 ml of water. The diluted sample was tested by spectroscopy at 280 nm to determine the protein concentration. The absorbance of the diluted solution was 0.152.
Use the above extinction coefficient value, and b = 1 cm, to calculate the concentration of protein in the diluted sample. Write your final answer in the space below. Show all work.
c. What is the concentration of the undiluted protein sample? Write your final answer in the space below. Show all work - report answer to proper number of significant digits.
2.
a. What is the pH of a solution containing 0.30 M Acetic Acid (pka=4.7) and 0.20 M Sodium Acetate? Write your final answer in the space below. Show all work.
b. If you are preparing 750.0 mL of 0.150 M Sodium Acetate (MW=136.08 g/mol), how much solid Sodium Acetate do you need, in grams. Write your final answer in the space below. Show all work. Perform all calculations reporting to proper significant figures.
3. You are asked to calibrate a set of laboratory micropipettes. You decide to weigh five (5) aliquots of distilled water with each micropipettor and measure the masses dispensed on an analytical balance. Your mass values for each instrument are presented in the table shown below:
a.) Explain how the density of water was used to correlate the volume of water sampled using the micropipettor, and the masses weighed on the analytical balance. Write your answer in the space below.
b.) Explain how the average mass is used as a measure of accuracy for the micropipette instrument. What does accuracy mean, and how is the average mass used to gauge accuracy? Write your answer in the space below.
c.) Explain how the standard deviation of the data (not shown in table) is used as a measure of precision for our micropipetors. Write your answer in the space below.
Extra Credit:
The enzyme, glutathione peroxidase, elutes from a gel filtration column at a position corresponding to a molecular mass of 44,000 Kda. This enzyme was tested by SDS-PAGE to confirm the molecular mass; the enzyme sample was prepared without treating it to form covalent bonds between protein subunits. The SDS-PAGE gel showed a single band with a molecular mass of 22,000 Kda. What conclusions can be drawn about the structure of glutathione peroxidase?