Amino Acids And Peptides PS
CH 600 Lc
PROBLEM SET
AMINO ACIDS AND PEPTIDES
1. How do D-amino acids differ from L-amino acids? What biological roles are played by peptides that contain D-amino acids?
2. For each of the following, name an amino acid in which the R group contains it: a hydroxyl group, a sulfur atom, a second chiral carbon atom, an amino group, an amide group, an acid group, an aromatic ring, and a branched side chain.
3. Predict the predominant ionized forms of the following amino acids at pH 7: glutamic acid, leucine, threonine, histidine, and arginine.
4. Draw structures of the following amino acids, indicating the charged form that exists at pH 4: histidine, asparagine, tryptophan, proline, and tyrosine.
5. Calculate the isoelectric point of each of the following amino acids: glutamic acid, serine, histidine, lysine, tyrosine, and arginine.
6. Suggest a reason why amino acids are usually more soluble at pH extremes than they are at neutral pH.
7. Based on the pKa values of the amino acids, is there any amino acid that could serve as a buffer at pH 8? If so, which one?
8. If you were to have a mythical amino acid based on glutamic acid, but one in which the hydrogen that is attached to the g-carbon were replaced by another amino group, what would be the predominant form of this amino acid at pH 4, 7, and 10, if the pKa value were 10 for the unique amino group?
9. Consider the peptides Ser—Glu—Gly—His—Ala and Gly—His—Ala—Glu—Ser. How do these two peptides differ?
10. 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?
PROBLEM SET
AMINO ACIDS AND PEPTIDES
1. How do D-amino acids differ from L-amino acids? What biological roles are played by peptides that contain D-amino acids?
2. For each of the following, name an amino acid in which the R group contains it: a hydroxyl group, a sulfur atom, a second chiral carbon atom, an amino group, an amide group, an acid group, an aromatic ring, and a branched side chain.
3. Predict the predominant ionized forms of the following amino acids at pH 7: glutamic acid, leucine, threonine, histidine, and arginine.
4. Draw structures of the following amino acids, indicating the charged form that exists at pH 4: histidine, asparagine, tryptophan, proline, and tyrosine.
5. Calculate the isoelectric point of each of the following amino acids: glutamic acid, serine, histidine, lysine, tyrosine, and arginine.
6. Suggest a reason why amino acids are usually more soluble at pH extremes than they are at neutral pH.
7. Based on the pKa values of the amino acids, is there any amino acid that could serve as a buffer at pH 8? If so, which one?
8. If you were to have a mythical amino acid based on glutamic acid, but one in which the hydrogen that is attached to the g-carbon were replaced by another amino group, what would be the predominant form of this amino acid at pH 4, 7, and 10, if the pKa value were 10 for the unique amino group?
9. Consider the peptides Ser—Glu—Gly—His—Ala and Gly—His—Ala—Glu—Ser. How do these two peptides differ?
10. 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?