Essential Organic Chemistry, Global Edition
3rd Edition
ISBN: 9781292089034
Author: Paula Yurkanis Bruice
Publisher: PEARSON
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Chapter 17, Problem 36P
A professor was preparing a manuscript for publication in which she reported that the pI of the tripeptide Lys-Lys-Lys was 10.6. One of her students pointed out that there must be an error in her calculations because the pKa of the side-chain amino group of lysine is 10.8 and the pI of the tripeptide has to be greater than any of its individual pKa values. Was the student correct?
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A professor was preparing a manuscript for publication in which she reported that the pI of the tripeptide Lys-Lys-Lys was 10.6. One of her students pointed out that there must be an error in her calculations because the pKa of the e-amino group of lysine is 10.8 and the pI of the tripeptide has to be greater than any of its individual pKa values. Was the student correct?
All amino acids have two ionizable functional groups: an α‑amino group (average pKa of 9.4) and an α‑carboxylic acid group (average pKa of 2.2). Aspartic acid has an ionizable side chain (R group) with a pKa of about 3.8. One of the possible ionization states of aspartic acid is shown in the image.
At what pH would the structure be the predominant ionization state? Consider the ionization state of all three of the functional groups.
Peptides can be separated using an ion-exchange column based on their isoelectric (pI) values. At which pH values would
two different peptides, one with a pl of 5.6 and the other with a pl of 8.6, bind to a cation- and anion-exchange column?
Each peptide may be capable of binding to each column at more than one pH value.
anion-exchange column at
pH = 4.0
pH = 6.5
pH
=
10.1
cation-exchange column at
= 4.0
pH
pH =
pH
= 6.5
= 10.1
Answer Bank
peptide B pl = 8.6
peptide A pl = 5.6
Chapter 17 Solutions
Essential Organic Chemistry, Global Edition
Ch. 17.1 - a. Explain why, when the imidazole ring of...Ch. 17.2 - Prob. 2PCh. 17.3 - Prob. 3PCh. 17.3 - Prob. 4PCh. 17.3 - Prob. 6PCh. 17.4 - Calculate the pI of each of the following amino...Ch. 17.4 - a. Which amino acid has the lowest pI value? b....Ch. 17.5 - What aldehyde is formed when valine is treated...Ch. 17.5 - Prob. 10PCh. 17.5 - Prob. 11P
Ch. 17.5 - Prob. 12PCh. 17.6 - Prob. 13PCh. 17.6 - What amino acid would be formed using the...Ch. 17.6 - What amino acid would be formed when the aldehyde...Ch. 17.7 - Pig liver esterase is an enzyme that catalyzes the...Ch. 17.8 - Prob. 17PCh. 17.8 - Prob. 18PCh. 17.8 - Prob. 19PCh. 17.8 - Prob. 20PCh. 17.10 - Prob. 21PCh. 17.10 - Prob. 22PCh. 17.10 - Why does cyanogen bromide not cleave on the C-side...Ch. 17.10 - Prob. 24PCh. 17.10 - Prob. 26PCh. 17.12 - Prob. 27PCh. 17.13 - a. Which would have the greatest percentage of...Ch. 17 - Draw the predominant form of the following amino...Ch. 17 - What is the pI of serine?Ch. 17 - Prob. 31PCh. 17 - Prob. 32PCh. 17 - Which would have a higher percentage of negative...Ch. 17 - Draw the form of aspartate that predominates at...Ch. 17 - Prob. 35PCh. 17 - A professor was preparing a manuscript for...Ch. 17 - a. Why is the pKa of the glutamate side chain...Ch. 17 - Prob. 38PCh. 17 - Determine the amino acid sequence of a polypeptide...Ch. 17 - Prob. 40PCh. 17 - Prob. 41PCh. 17 - Three peptides were obtained from a trypsin...Ch. 17 - Prob. 43PCh. 17 - After the polypeptide shown here was treated with...Ch. 17 - The disulfide bridges of a polypeptide were...Ch. 17 - -Amino acids can be prepared by treating an...Ch. 17 - Reaction of a polypeptide with carboxypeptidase A...Ch. 17 - Prob. 48PCh. 17 - Prob. 49PCh. 17 - Show how valine can be prepared by a. a Strecker...Ch. 17 - Prob. 51PCh. 17 - Why is proline never found in an -helix?Ch. 17 - Determine the amino acid sequence of a polypeptide...Ch. 17 - Prob. 55PCh. 17 - A chemist wanted to test his hypothesis that the...Ch. 17 - A normal polypeptide and a mutant of the...
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- 22-49 Based on your knowledge of the chemical properties of amino acid side chains, suggest a substitution for leucine in the primary structure of a protein that would probably not change the character of the protein very much.arrow_forward22-47 How many different tetrapeptides can be made (a) if the peptides contain the residues of asparagine, proline, serine, and metbionine and (b) if all 20 amino acids can be used?arrow_forwardOn complete hydrolysis, a polypeptide gives two alanine, one leucine, one methionine, one phenylalanine, and one valine residue. Partial hydrolysis gives the following fragments: Ala-Phe, Leu-Met, Val-Ala, Phe-Leu. It is known that the first amino acid in the sequence is valine and the last one is methionine. What is the complete sequence of amino acids?arrow_forward
- 22-31 A polypeptide chain is made of alternating valine and phenylalanine. Which part of the polypeptide is polar (hydrophilic)?arrow_forwardGlutathione (G-SH), one of the most common tripeptides in animals, plants, and bacteria, is a scavenger of oxidizing agents. In reacting with oxidizing agents, glutathione is converted to G-S-S-G. (a) Name the amino acids in this tripeptide. (b) What is unusual about the peptide bond formed by the N-terminal amino acid? (c) Write a balanced half-reaction for the reaction of two molecules of glutathione to form a disulfide bond. Is glutathione a biological oxidizing agent or a biological reducing agent? (d) Write a balanced equation for reaction of glutathione with molecular oxygen, O2 to form G-S-S-G and H2O. Is molecular oxygen oxidized or reduced in this process?arrow_forwardConsider a tripeptide (a peptide that's only 3 amino acids long) composed of one molecule each of lysine, leucine, and the hypothetical amino acid lupine (which doesn't actually exist, so don't try to look it up). Lupine has an ionizable side group with a pKa of 6.3. When this tripeptide is dissolved in an aqueous solution at pH 5.0, the net charge on the molecule is +2. Give the charge on the protonated form of the lupine side group, and the charge on the deprotonated form of the lupine side group.arrow_forward
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