Organic Chemistry

996 CHAPTER 23 Amino Acids, Peptides, and Proteins

43. Explain the difference in the values of the carboxyl groups of alanine, serine, and cysteine.


44. Which would be a more effective buffer at physiological pH, a solution of 0.1 M glycylglycylglycylglycine or a solution of 0.2M
    glycine?


45. Identify the location and type of charge on the hexapeptide Lys-Ser-Asp-Cys-His-Tyr at
    a. b. c.


46. The following polypeptide was treated with 2-mercaptoethanol and then with iodoacetic acid. After reacting with maleic
    anhydride, the peptide was hydrolyzed by trypsin. (Treatment with maleic anhydride causes trypsin to cleave a peptide only at
    arginine residues.)
    Gly-Ser-Asp-Ala-Leu-Pro-Gly-Ile-Thr-Ser-Arg-Asp-Val-Ser-Lys-Val-Glu-Tyr-Phe-Glu-Ala-Gly-Arg-Ser-Glu-Phe-Lys-Glu-Pro-
    Arg-Leu-Tyr-Met-Lys-Val-Glu-Gly-Arg-Pro-Val-Ser-Ala-Gly-Leu-Trp
    a. Why, after a peptide is treated with maleic anhydride, does trypsin no longer cleave it at lysine residues?
    b. How many fragments are obtained from the peptide?
    c. In what order would the fragments be eluted from an anion-exchange column using a buffer of


47. Treatment of a polypeptide with 2-mercaptoethanol yields two polypeptides with the following primary sequences:

Val-Met-Tyr-Ala-Cys-Ser-Phe-Ala-Glu-Ser

Ser-Cys-Phe-Lys-Cys-Trp-Lys-Tyr-Cys-Phe-Arg-Cys-Ser

Treatment of the original intact polypeptide with chymotrypsin yields the following peptides:

a. Ala, Glu, Ser c. Tyr, Val, Met e. Ser, Phe, 2 Cys, Lys, Ala, Trp

b. 2 Phe, 2 Cys, Ser d. Arg, Ser, Cys f. Tyr, Lys

Determine the positions of the disulfide bridges in the original polypeptide.

48. Show how aspartame can be synthesized using DCC.


49. Reaction of a polypeptide with carboxypeptidase A releases Met. The polypeptide undergoes partial hydrolysis to give the
    following peptides. What is the sequence of the polypeptide?
    a. Ser, Lys, Trp e. Met, Ala, Gly i. Lys, Ser
    b. Gly, His, Ala f. Ser, Lys, Val j. Glu, His, Val
    c. Glu, Val, Ser g. Glu, His k. Trp, Leu, Glu
    d. Leu, Glu, Ser h. Leu, Lys, Trp l. Ala, Met


50. Glycine has values of 2.3 and 9.6. Would you expect the values of glycylglycine to be higher or lower than these values?


51. A mixture of 15 amino acids gave the fingerprint shown in the below (see also Problem 42). Identify the spots. (Hint 1:Pro reacts
    with ninhydrin to form a yellow color; Phe and Tyr form a yellow-green color. Hint 2:Count the number of spots before you start.)


52. Dithiothreitol reacts with disulfide bridges in the same way that 2-mercaptoethanol does. With dithiothreitol, however, the
    equilibrium lies much more to the right. Explain.

HO HO

SH RSSR

SH

S

HO HO S

+ + 2 RSH

dithiothreitol

Electrophoresis at pH

=^5

Chromatography

Origin

Starting mixture:

Ala

Arg

Asp

Glu

Gly

Ile

Leu

Met

Phe

Pro

Ser

Thr

Trp

Tyr

Val

−

+

pKa pKa

pH=5?

pH= 7 pH= 5 pH= 9

pKa

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