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chemistry
organic chemistry
Questions and Answers of
Organic Chemistry
D-Glucose most often exists as a pyranose, but it can also exist as a furanose. Draw the Haworth projection of -D-glucofuranose.
Which OH groups are in the axial position ina. β -D-mannopyranose? b. β-D-idopyranose?c. α-D-allopyranose?
Why is only a trace amount of acid used in the formation of an N-glycoside?
Name the following compounds, and indicate whether each is a reducing sugar or a nonreducing sugar:a.b. c. d.
What is the main structural difference between a. Amylose and cellulose? b. Amylose and amylopectin? c. Amylopectin and glycogen? d. Cellulose and chitin?
Explain why the C-3 OH group of vitamin C is more acidic than the C-2 OH group.
From the nature of the carbohydrate bound to red blood cells, answer the following questions:a. People with type O blood can donate blood to anyone, but they cannot receive blood from everyone. From
Indicate whether each of the following is D-glyceraldehyde or L-glyceraldehyde, assuming that the horizontal bonds point toward you and the vertical bonds point away from you (Section 5.6):a.b. c.
Give the product or products that are obtained when D-galactose reacts with the following: a. Nitric acid b. Tollens reagent c. H2/Pd/C d. Three equivalents of phenylhydrazine e. In water f. Ethanol
Identify the following sugars: a. An aldopentose that is not D-arabinose forms D-arabinitol when it is reduced with NaBH4. b. A sugar forms the same osazone as D-galactose with phenylhydrazine, but
Answer the following questions about the eight aldopentoses: a. Which are enantiomers? b. Which give identical osazones? c. Which form an optically active compound when oxidized with nitric acid?
The reaction of D-ribose with an equivalent of methanol plus HCl forms four products. Give the structures of the products.
Determine the structure of D-galactose, using arguments similar to those used by Fischer to prove the structure of D-glucose.
Dr. Isent T. Sweet isolated a monosaccharide and determined that it had a molecular weight of 150. Much to his surprise, he found that it was not optically active. What is the structure of the
The 1HNMR spectrum of D-glucose in D2O exhibits two high-frequency (low-field) doublets. What is responsible for these doublets?
Treatment with sodium borohydride converts aldose A into an optically inactive alditol. Ruff degradation of A forms B, whose alditol is optically inactive. Ruff degradation of B forms
D-Glucuronic acid is found widely in plants and animals. One of its functions is to detoxify poisonous HO-containing compounds by reacting with them in the liver to form glucuronides. Glucuronides
Hyaluronic acid, a component of connective tissue, is the fluid that lubricates the joints. It is an alternating polymer of N-acetyl- D-glucosamine and D-glucuronic acid joined by linkages. Draw a
In order to synthesize D-galactose, Professor Amy Losse went to the stockroom to get some D-lyxose to use as a starting material. She found that the labels had fallen off the bottles containing
When D-fructose is dissolved in and the solution is made basic, the D-fructose recovered from the solution has an average of 1.7 deuterium atoms attached to carbon per molecule. Show the mechanism
A D-aldopentose is oxidized by nitric acid to an optically active aldaric acid. A Ruff degradation of the aldopentose leads to a monosaccharide that is oxidized by nitric acid to an optically
How many aldaric acids are obtained from the 16 aldohexoses?
Calculate the percentages of -D-glucose and -D-glucose present at equilibrium from the specific rotations of -D-glucose, -D-glucose, and the equilibrium mixture. Compare your values with those given
Predict whether D-altrose exists preferentially as a pyranose or a furanose. (Hint: The most stable arrangement for a five-membered ring is for all the adjacent substituents to be trans.)
Propose a mechanism for the rearrangement that converts an into an (Section 22.6). a-hydroxyimine a-aminoketone
A disaccharide forms a silver mirror with Tollens reagent and is hydrolyzed by a When the disaccharide is treated with excess methyl iodide in the presence of (a reaction that converts all the OH
All the glucose units in dextran have six-membered rings. When a sample of dextran is treated with methyl iodide and silver oxide and the product is hydrolyzed under acidic conditions, the products
When a pyranose is in the chair conformation in which the group and the C-1 OH group are both in the axial position, the two groups can react to form an acetal. This is called the anhydro form of the
Devise a method to convert D-glucose into D-allose.
Give systematic names to the following compounds. Indicate the configuration (R or S) of each asymmetric carbon:a. D-glucose b. L-glucose
How many stereoisomers are possible for a. A 2-ketoheptose? b. An aldoheptose? c. A ketotriose?
What products are obtained from the reduction of a. D-idose? b. D-sorbose?
a. Explain why, when the imidazole ring of histidine is protonated, the double-bonded nitrogen is the nitrogen that accepts the proton.b. Explain why, when the guanidino group of arginine is
a. Which amino acid has the lowest pI value? b. Which amino acid has the highest pI value? c. Which amino acid has the greatest amount of negative charge at pH = 6.20? d. Which amino acid-glycine or
Explain why the pI values of tyrosine and cysteine cannot be determined by the method just described.
Why are buffer solutions of increasingly higher pH used to elute the column that generates the chromatogram shown in Figure 23.5?
Explain the order of elution (with a buffer of pH 4) of each of the following pairs of amino acids on a column packed with Dowex® 50 (Figure 23.3): a. Aspartate before serine b. Glycine before
In what order would the following amino acids be eluted with a buffer of pH 4 from a column containing an anion-exchange resin? histidine, serine, aspartate, valine
Why is excess ammonia used in the preceding reaction?
Pig liver esterase is an enzyme that catalyzes the hydrolysis of esters. It hydrolyzes esters of L-amino acids more rapidly than esters of D-amino acids. How can this enzyme be used to separate a
Amino acids can be synthesized by reductive amination α-keto of acids (Section 21.8).Biological organisms can also α-keto convert acids into amino acids, but because H2
Draw a peptide bond in a cis configuration.
a. Which isomer-(R)-alanine or (S)-alanine-is D-alanine? b. Which isomer-(R)-aspartate or (S)-aspartate-is D-aspartate? c. Can a general statement be made relating R and S to D and L?
Which bonds in the backbone of a peptide can rotate freely?
What is unusual about glutathione's structure? (If you can't answer this question, draw the structure you would expect for a tripeptide of glutamate, cysteine, and glycine, and compare your structure
What dipeptides would be formed by heating a mixture of valine and N-protected leucine?
Show the steps in the synthesis of the tetrapeptide Leu-Phe-Lys-Val.
a. Calculate the overall yield of bradykinin if the yield for the addition of each amino acid to the chain is 70%. b. What would be the overall yield of a peptide containing 15 amino acid residues if
Show the steps in the synthesis of the peptide in Problem 25, using Merrifield's method.
Write the mechanism for the reaction of a cysteine residue with iodoacetic acid.
A decapeptide undergoes partial hydrolysis to give peptides whose amino acid compositions are shown. Reaction of the intact decapeptide with Edman's reagent releases PTH-Gly. What is the sequence of
Why won't cyanogen bromide cleave at cysteine residues?
Determine the primary structure of an octapeptide from the following data: Acid hydrolysis gives 2 Arg, Leu, Lys, Met, Phe, Ser, Tyr. Carboxypeptidase A releases Ser. Edman's reagent releases
How long is an α-helix that contains 74 amino acids? Compare the length of this α-helix with the length of a fully extended peptide chain containing the same number of amino acids. (The distance
a. Which of the following water-soluble proteins would have the greatest percentage of polar amino acids-a spherical protein, a cigar-shaped protein, or a subunit of a hexamer? b. Which of these
Unlike most amines and carboxylic acids, amino acids are insoluble in diethyl ether. Explain.
Indicate the peptides that would result from cleavage by the indicated reagent: a. His-Lys-Leu-Val-Glu-Pro-Arg-Ala-Gly-Ala by trypsin b. Leu-Gly-Ser-Met-Phe-Pro-Tyr-Gly-Val by chymotrypsin c.
Aspartame has a pI of 5.9. Draw its most prevalent form at physiological pH.
Draw the form of aspartic acid that predominates at a. pH = 1.0 b. pH = 2.6 c. pH = 6.0 d. pH = 11.0
Dr. Kim S. Tree 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
A mixture of amino acids that do not separate sufficiently when a single technique is used can often be separated by two-dimensional chromatography. In this technique, the mixture of amino acids is
Explain the difference in the pKa values of the carboxyl groups of alanine, serine, and cysteine.
Which would be a more effective buffer at physiological pH, a solution of 0.1 M glycylglycylglycylglycine or a solution of 0.2 M glycine?
Identify the location and type of charge on the hexapeptide Lys-Ser-Asp-Cys-His-Tyr at a. pH = 7 b. pH = 5 c. pH = 9
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
Treatment of a polypeptide with 2-mercaptoethanol yields two polypeptides with the following primary
Show how aspartame can be synthesized using DCC.
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,
Draw the form in which each of the following amino acids predominantly exists at physiological pH (7.3): a. Histidine b. Glutamine c. Lysine d. Arginine e. Tyrosine
Glycine has pKa values of 2.3 and 9.6. Would you expect the pKa values of glycylglycine to be higher or lower than these values?
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
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.
α-amino acids can be prepared by treating an aldehyde with ammonia and hydrogen cyanide, followed by acid-catalyzed hydrolysis. a. Give the structures of the two intermediates formed in this
The UV spectra of tryptophan, tyrosine, and phenylalanine are shown here. Each spectrum is that of a 1 Ã 10-3 solution of the amino acid, buffered at Calculate the approximate molar
A normal polypeptide and a mutant of the polypeptide were hydrolyzed by an endopeptidase under the same conditions. The normal and mutant differ by one amino acid residue. The fingerprints of the
Determine the amino acid sequence of a polypeptide from the following results: a. Complete hydrolysis of the peptide yields the following amino acids: Ala, Arg, Gly, 2 Lys, Met, Phe, Pro, 2 Ser, Tyr,
The C-terminal end of a protein extends into the aqueous environment surrounding the protein. The C-terminal amino acids are Gln, Asp, 2 Ser, and three nonpolar amino acids. Assuming that ∆Go the
Professor Mary Gold wanted to test her hypothesis that the disulfide bridges that form in many proteins do so after the minimum energy conformation of the protein has been achieved. She treated a
Draw the form in which glutamic acid predominantly exists in a solution with the following pH: a. pH = 0 b. pH = 3 c. pH = 6 d. pH = 11
a. Why is the pKa of the glutamic acid side chain greater than the pKa of the aspartic acid side chain? b. Why is the pKa of the arginine side chain greater than the pKa of the lysine side chain?
Explain why the pI of lysine is the average of the pKa values of its two protonated amino groups.
Calculate the pI of each of the following amino acids: a. Asparagines b. Arginine c. Serine
The relative rate of reaction of the cis alkene (E) is given in Table 24.2. What would you expect the relative rate of reaction of the trans isomer to be?
Show all the products, and their configurations, that would be obtained from solvolysis of the trans-substituted compound illustrated in the preceding diagram.
Why do the nitro groups change the relative leaving tendencies of the carboxyl and phenyl groups in the tetrahedral intermediate in Problem 12?
Whether the ortho-carboxyl substituent acts as an intramolecular general-base catalyst or as an intramolecular nucleophilic catalyst can be determined by carrying out the hydrolysis of aspirin with
Which of the following C-terminal peptide bonds would be more readily cleaved by carboxypeptidase A? Ser-Ala-Phe or Ser-Ala-Asp Explain.
Carboxypeptidase A has esterase activity as well as peptidase activity. In other words, the compound can hydrolyze ester bonds as well as peptide bonds. When carboxypeptidase A hydrolyzes ester
Arginine and lysine side chains fit into trypsin's binding pocket. One of these side chains forms a direct hydrogen bond with serine and an indirect hydrogen bond (mediated through a water molecule)
If H2O18 were used to hydrolyze lysozyme, which ring would contain the label, NAM or NAG?
Compare each of the following mechanisms with the mechanism for each phase of the acid-catalyzed hydrolysis of an ester, indicating a. Similarities b. Differences 1. Acid-catalyzed formation of a
When apples that have been cut are exposed to oxygen, an enzyme-catalyzed reaction causes them to turn brown. They can be prevented from turning brown by coating them with lemon juice (pH = ~3.5.)
When D-glucose undergoes isomerization in the absence of the enzyme, three products result: D-glucose, D-fructose, and D-mannose (Section 22.5). Why is D-mannose not formed in the enzyme-catalyzed
The descending leg of the pH-rate profile for glucose-6-phosphate isomerase indicates that one of the amino acid side chains at the active site of the enzyme has a pKa value of 9.3. Identify the
Propose a mechanism for the aldolase-catalyzed cleavage of D-fructose-1,6-diphosphate if it did not form an imine with the substrate. What is the advantage gained by imine formation?
In glycolysis, why must D-glucose-6-phosphate isomerize to D-fructose-6-phosphate before the cleavage reaction with aldolase occurs?
Aldolase shows no activity if it is incubated with iodoacetic acid before D-fructose- 1,6-diphosphate is added to the reaction mixture. Suggest what could cause the loss of activity.
Which of the following two compounds would eliminate HBr more rapidly in basic solutions?
Which compound would form a lactone more rapidly?a.b.
Which compound would form an anhydride more rapidly?
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