PROBLEM SET 1
AMINO ACIDS AND PEPTIDES
1. Suggest a reason why amino acids are usually more soluble at pH extremes than they are at
neutral pH. (Note: This does not mean that they are insoluble at neutral pH)
2. Write the equations to show the ionic dissociation reactions of the following amino
acids(individually): Glutamic acid, histidine, and cysteine. Calculate their pI.
3. Draw the structure of arginine indicating the charged form that exists at pH 10.
4. Give the systematic name in three letters and one letter abbreviation of the following peptide.
a.
b.
c.
d.
5. a) Which amino acid is technically not an amino acid? b) Which amino acid contains no
chiral carbon atom(s)? Briefly explain.
6. Identify the net charge and draw the structure of Tyrosine at pH=2, pH= 7, pH =10, and
pH=11
7. Given the peptide : K-R-G-V-C
a. Complete name of the pentapeptide.
b. Draw the structure of the pentapeptide at pH=9 and pH=11.
c. Calculate the pI of the peptide. Refer to Table of pK values of individual amino acids.
d. Is this peptide soluble at neutral pH Solution? Yes or No. Why?
8. Given the peptide : val-met-ser-ile-phe-arg-cys-tyr-leu
a. Complete name of this oligopeptide.
b. Draw the structure of this peptide at pH=10.
c. Calculate the pI of the peptide. Refer to Table of pK values of individual amino acids.
d. Is this peptide soluble at neutral pH Solution? Yes or No. Why?
9. What is the stereochemical basis of the observation that D-aspartyl-D-phenylalanine has
a bitter taste whereas L-aspartyl-L-phenylalanine is significantly sweeter than sugar?
10. Which would be better to eat before an exam, a glass of milk or a piece of cheese? Why?