Amino Acids Practice Problems with Answers

Problem 1
1.812 g of a crystallized α-amino acid (pKa1: 2.4; pKa2; 9.7) has a pH of 10.4 when dissolved in 100 mL of 0.1M NaOH. Calculate the molecular mass of this amino acid.

Problem 2
Calculate the pI of histidine and draw its titration curve. Indicate the position of all pKas and the pI as well as the percentages of each ionic form at the start and finish of the titration and at all pKas.

Problem 3
What is the net charge (+, 0, -) of the amino acids glycine, serine, aspartic acid, glutamine and arginine at:
a) pH 2.01 b) pH 3.96 c) pH 5.68 d) pH 10.76

Problem 4
A mixture of lysine, glycine, alanine, isoleucine and glutamic acid are separated by ionic exchange chromatography. What is the order of elution of these amino acids if you use gradient buffer system from pH 10 to pH 2:
a) with a cation exchange resin?
b) with an anion exchange resin?
Which column would give the best separation?

Problem 5
What amino acids can be converted into another amino acid with gentle hydrolysis, resulting in release of ammonia?

Problem 6
Phosphoserine is found after enzymatic hydrolysis of casein, a milk protein. However, it does not belong to the 20 amino acids coded during protein synthesis. Give a plausible explanation.

Problem 7
Glycine, alanine, valine and leucine can be successfully separated by ionic exchange
chromatography even though their pKas are almost identical. Explain the behaviour of these amino acids.

Problem 8
A peptide is hydrolyzed and its amino acid content analyzed. Hydrolysis destroys the amino acid tryptophan, therefore the content of tryptophan can be estimated with spectrophotometry. Establish the empirical formula of the peptide with the following information.

Problem 9
Draw the structure of the following peptide GWYQR. Indicate the ionic form of the peptide at the following pH:
a) pH 2.0 b) pH 7.0 c) pH 10.5

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