Q96 · Practice questionComplex unfamiliar7 marks
QUESTION 96 (7 marks)
Two amino-acid standards have these properties: glutamic acid, and pI = 3.2; arginine, and pI = 10.8. A mixture on the same TLC plate gives spots 2.0 cm and 5.2 cm from a baseline; the solvent front travels 8.0 cm. A separate portion undergoes electrophoresis at pH 6.0.
a) Calculate retention factors and tentatively identify both spots. [3 marks]
b) Predict each identified amino acid’s net charge and electrophoresis direction. [4 marks]
WORKED SOLUTION
7 marksPractice marking scheme
Rf 0.25: glutamic acid; Rf 0.65: arginine. Glutamic acid is net negative and migrates to the anode; arginine is net positive and migrates to the cathode.
Divide by 8.0 and compare with standards. At pH 6.0 glutamic acid is above its pI and arginine below its pI. Their net charges therefore have opposite signs. Matching Rf is tentative because other species may co-migrate.
- Calculates both Rf values. [1 marks]
- Associates lower spot with glutamic acid. [1 marks]
- Associates upper spot with arginine. [1 marks]
- Identifies glutamic acid as net negative. [1 marks]
- Predicts glutamic acid to anode. [1 marks]
- Identifies arginine as net positive. [1 marks]
- Predicts arginine to cathode. [1 marks]
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