Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

29. Amino Acids

L and D Amino Acids

Organic Chemistry

29. Amino Acids

L and D Amino Acids

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6:27 minutes

Problem 1a

Textbook Question

Draw three-dimensional representations of the following amino acids.
(a) L-phenylalanine

Verified step by step guidance

Understand the structure of l-phenylalanine: It is an amino acid with a central alpha carbon (Cα) bonded to four groups: an amino group (-NH₂), a carboxylic acid group (-COOH), a hydrogen atom (H), and a phenyl group (-CH₂-C₆H₅).

Determine the stereochemistry: The 'l-' designation indicates that the amino acid is in the L-configuration, which corresponds to the S-configuration for most amino acids (except cysteine). This means the groups around the alpha carbon should be arranged in a specific 3D orientation.

Draw the backbone in 3D: Place the alpha carbon (Cα) at the center. Draw the carboxylic acid group (-COOH) pointing downward, the amino group (-NH₂) pointing upward, and the hydrogen atom (H) pointing backward (into the plane, represented by a dashed wedge).

Add the side chain: Attach the phenyl group (-CH₂-C₆H₅) to the alpha carbon. The -CH₂ group should point forward (out of the plane, represented by a solid wedge), and the phenyl ring (C₆H₅) extends from the -CH₂ group.

Verify the stereochemistry: Ensure that the groups are arranged in the correct L-configuration by assigning priorities to the substituents around the alpha carbon and applying the Cahn-Ingold-Prelog (CIP) rules to confirm the S-configuration.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Amino Acids Structure

Amino acids are organic compounds that serve as the building blocks of proteins. Each amino acid consists of a central carbon atom (the alpha carbon) bonded to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable side chain (R group) that determines the specific properties of the amino acid. Understanding this structure is essential for visualizing and drawing amino acids in three dimensions.

Chirality in Amino Acids

Most amino acids, except glycine, are chiral, meaning they exist in two enantiomeric forms that are mirror images of each other. The 'L' and 'D' designations refer to the orientation of the amino group in relation to the carboxyl group. L-phenylalanine, specifically, is the naturally occurring form used in proteins, and recognizing chirality is crucial for accurately representing its three-dimensional structure.

Three-Dimensional Molecular Representation

Three-dimensional representations of molecules, such as amino acids, can be depicted using various models, including ball-and-stick and space-filling models. These representations help illustrate the spatial arrangement of atoms and the overall geometry of the molecule. For l-phenylalanine, understanding how to depict its side chain and the orientation of functional groups is vital for accurate visualization.

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Related Practice

Textbook Question

An opioid pentapeptide has the following structure: Tyr-Cys-Gly-Phe-Cys

a. Draw the structure of the pentapeptide including all the side chains.

b. Write its structure following mild oxidation.

Textbook Question

Metabolism of arginine produces urea and the rare amino acid ornithine. Ornithine has an isoelectric point close to 10. Propose a structure for ornithine.

Textbook Question

Suggest a method for the synthesis of the unnatural D enantiomer of alanine from the readily available L enantiomer of lactic acid.

Textbook Question

Draw the structure of the phenylthiohydantoin derivatives of

(a) alanine.

Textbook Question

Most naturally occurring amino acids have chiral centers (the asymmetric α carbon atoms) that are named (S) by the Cahn–Ingold–Prelog convention (Section 5-3). The common naturally occurring form of cysteine has a chiral center that is named (R), however.

(a) What is the relationship between (R)-cysteine and (S)-alanine? Do they have the opposite three-dimensional configuration (as the names might suggest) or the same configuration?

Textbook Question

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 racemic mixture of amino acids?

Textbook Question

Draw the structure of the phenylthiohydantoin derivatives of

Textbook Question

Sometimes chemists need the unnatural D enantiomer of an amino acid, often as part of a drug or an insecticide. Most L-amino acids are isolated from proteins, but the D-amino acids are rarely found in natural proteins. D-amino acids can be synthesized from the corresponding L-amino acids. The following synthetic scheme is one of the possible methods.

(a) Draw the structures of intermediates 1 and 2 in this scheme.

(b) How do we know that the product is entirely the unnatural D configuration?

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Draw three-dimensional representations of the following amino acids. (a) L-phenylalanine

Key Concepts

Amino Acids Structure

Chirality in Amino Acids

Three-Dimensional Molecular Representation

Watch next

Draw three-dimensional representations of the following amino acids.
(a) L-phenylalanine