Chromatography is often used for the analysis of the mixture of amino acids that is formed when proteins are broken down. The small protein met-enkephalin is known for its pain killing activity. An aqueous solution of methionine enkephalin is broken down into its constituent amino acids and the resultant solution of amino acids is subjected to paper chromatography. A strip from such a chromatogram is shown below. Amino acids are colourless, but the position of an amino acid spot on the strip can be seen by spraying the strip with a solution of ninhydrin, a substance that reacts with amino acids to produce an intense purple colour. a. The identities of the four amino acids in this particular mixture have been determined by measuring their R_f values. i. Explain how an R_f value is calculated. ii. Calculate the R_f of the methionine spot. b. Describe how the sample containing the mixture of the four amino acids was added to the paper in order to begin the procedure. c. What factors determine the different R_f values of the different amino acids?

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Chromatography is often used for the analysis of the mixture of amino acids that is formed when proteins are broken down - VCE - SSCE Chemistry - Question 5 - 2003 - Paper 1

Question 5

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Chromatography is often used for the analysis of the mixture of amino acids that is formed when proteins are broken down. The small protein met-enkephalin is known f... show full transcript

Worked Solution & Example Answer:Chromatography is often used for the analysis of the mixture of amino acids that is formed when proteins are broken down - VCE - SSCE Chemistry - Question 5 - 2003 - Paper 1

Step 1

Explain how an R_f value is calculated.

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Answer

The R_f value, or retention factor, is calculated by taking the ratio of the distance traveled by the amino acid to the distance traveled by the solvent front from the original starting point. The formula is:

$R_f = \frac{\text{Distance moved by the substance}}{\text{Distance moved by the solvent}}$

In this case, if the starting point (origin) is at 2.7 cm and the distance moved by methionine is measured at 17.4 cm and the solvent front at 28.8 cm, the calculation would be:

$R_f = \frac{(17.4 - 2.7)}{(28.8 - 2.7)} = \frac{14.7}{26.1} = 0.56$ .

This R_f value indicates the relative rate at which the substance travels compared to the solvent.

Step 2

Calculate the R_f of the methionine spot.

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To calculate the R_f of the methionine spot, we can use the previously mentioned distances. From the first explanation:

Distance moved by the methionine spot: 17.4 cm - 2.7 cm = 14.7 cm
Distance moved by the solvent front: 28.8 cm - 2.7 cm = 26.1 cm

Thus, the calculation is:

$R_f = \frac{14.7}{26.1} \approx 0.56$

It is essential to ensure that both measurements are taken from the same starting point for accuracy.

Step 3

Describe how the sample containing the mixture of the four amino acids was added to the paper.

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The sample containing the mixture of the four amino acids was applied to the chromatography paper using a capillary tube or a dropping pipette to create a small spot at the origin (zero point). Care should be taken to ensure minimal disturbance to maintain the integrity of the spot for accurate separation.

Step 4

What factors determine the different R_f values of the different amino acids?

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The R_f values of different amino acids are influenced by several factors, including:

Polarity: More polar amino acids will interact differently with the stationary phase (the paper) compared to non-polar amino acids.
Solubility: The solubility of the amino acids in the mobile phase (the solvent) plays a significant role; more soluble amino acids will travel further.
Structure: Variations in side chains and molecular structures of the amino acids affect their interaction with the stationary and mobile phases.
Environmental factors: Temperature and the composition of the solvent can also impact the R_f values.

Chromatography is often used for the analysis of the mixture of amino acids that is formed when proteins are broken down - VCE - SSCE Chemistry - Question 5 - 2003 - Paper 1

Worked Solution & Example Answer:Chromatography is often used for the analysis of the mixture of amino acids that is formed when proteins are broken down - VCE - SSCE Chemistry - Question 5 - 2003 - Paper 1

Explain how an R_f value is calculated.

Calculate the R_f of the methionine spot.

Describe how the sample containing the mixture of the four amino acids was added to the paper.

What factors determine the different R_f values of the different amino acids?

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