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Indicate whether each statement is true or false. (a) Tryptophan is an aromatic amino acid. (b) Lysine is positively charged at pH 7. (c) Asparagine has two amide bonds. (d) Isoleucine and leucine are enantiomers. (e) Valine is probably more water-soluble than arginine.

Short Answer

Expert verified
(a) True, (b) True, (c) False, (d) False, (e) False

Step by step solution

01

Statement (a)

Tryptophan is an aromatic amino acid. An aromatic amino acid is an amino acid that contains an aromatic ring in its side chain. Tryptophan has an indole ring, which is an aromatic ring, in its side chain. Hence, **(a) is true**.
02

Statement (b)

Lysine is positively charged at pH 7. Lysine is a basic amino acid that has a positively charged side chain at a neutral pH of around 7. Its side chain contains an ammonium ion (NH3+). Therefore, **(b) is true**.
03

Statement (c)

Asparagine has two amide bonds. Asparagine is an amino acid with an amide group in its side chain. Its side chain structure is \( \mathrm{H}_{2}\mathrm{N-CO-R} \), where R is the amino acid backbone. There is only one amide bond in its side chain. Therefore, **(c) is false**.
04

Statement (d)

Isoleucine and leucine are enantiomers. Enantiomers are non-superposable mirror images of each other. Isoleucine and leucine are both aliphatic amino acids with similar molecular structures, but they are not mirror images of each other. They simply have different side-chain arrangements; they are isomers, but not enantiomers. Thus, **(d) is false**.
05

Statement (e)

Valine is probably more water-soluble than arginine. Water solubility of amino acids depends on their hydrophilicity or hydrophobicity. In general, amino acids with polar or charged side chains are more water-soluble, whereas those with nonpolar side chains are less water-soluble. Valine is a nonpolar, hydrophobic amino acid, while arginine is a basic amino acid with a positively charged side chain, making it more hydrophilic and more water-soluble. Therefore, **(e) is false**.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Aromatic Amino Acids
Aromatic amino acids play a vital role in protein structure due to their unique aromatic rings. These rings allow them to participate in interactions like hydrophobic stacking and absorption of UV light. This property is significant for understanding protein behavior and function. Tryptophan, phenylalanine, and tyrosine are classic examples of aromatic amino acids. Tryptophan, for instance, includes an indole ring—a type of aromatic ring—in its side chain, which contributes to its characteristics. The presence of such rings makes aromatic amino acids crucial for various biochemical activities.

Aromatic rings contribute to the protein's folding and stability by embedding within the protein structure. These rings are planar and cyclic, with a cloud of electrons shared across the ring, making them very stable. This electron cloud can interact with other electron clouds, which aids stacking interactions in the protein's tertiary (3D) structure. These factors underscore their importance in maintaining protein integrity and function.
Water Solubility of Amino Acids
Water solubility is an essential property of amino acids as it influences their biological roles and interactions. Amino acids can be categorized based on the nature of their side chains which can be either hydrophilic or hydrophobic. Hydrophilic amino acids have side chains that can form hydrogen bonds with water, making them soluble in water. These often include polar or charged side chains. For example, arginine is a basic amino acid with a positively charged side chain, increasing its solubility.

On the other hand, amino acids like valine have nonpolar, hydrophobic side chains, decreasing their water solubility. The ability of an amino acid to dissolve in water impacts its role in metabolic pathways and how it interacts in the body. Soluble amino acids tend to be found on the exterior of proteins, interacting with the aqueous environment, while hydrophobic amino acids are usually buried in the core of proteins.
Enantiomers in Chemistry
Enantiomers are fascinating subjects in chemistry due to their unique mirror-image properties. These molecules are non-superposable, meaning no matter how you rotate them, you can't align one with another. This concept is crucial in many biological systems, where enzymes and receptors can distinguish between enantiomers—binding to one but not the other.

In amino acids, the concept of chirality is derived from four different groups attached to the central carbon, leading to asymmetric carbon structures. Isoleucine and leucine, however, are not enantiomers as they do not form such non-superposable mirror images; instead, they are structural isomers with different atomic arrangements. Understanding how enantiomerism works is essential since only one enantiomer in drugs, often the so-called "active enantiomer," may provide the desired therapeutic effect, while the other may be inactive or even harmful.
Charged Amino Acids
Charged amino acids are pivotal in biochemical processes, owing to their ability to participate in ionic interactions. At physiological pH levels, some amino acids carry a charge, which influences how they interact with other molecules and proteins. For example, lysine is positively charged at a neutral pH. This occurs because its side chain contains an ammonium group that remains protonated, giving it a positive charge.

These charged properties enable amino acids to engage in electrostatic interactions. Such interactions are crucial in the formation and stability of protein structures, signaling pathways, and enzyme functions, as they can attract or repel each other. Understanding these charged states aids in deciphering protein structure and function, contributing to advancements in fields like drug design and molecular biology.

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Most popular questions from this chapter

Draw the two possible heterodimeric dipeptides formed by condensation reactions between glycine and alanine.

Which of the following peptides have a net positive charge at \(\mathrm{pH} 7 ?\) (a) Asn-His-Arg, (b) Glu-Lys-Phe, (c) Thr-Leu-Ala.

The protein ribonuclease A in its native, or most stable, form is folded into a compact globular shape: (a) Does the native form have a lower or higher free energy than the denatured form, in which the protein is an extended chain? (b) What is the sign of the system's entropy change in going from the denatured to the folded form? (c) In the native form, the molecule hasfour \(-\mathrm{S}-\mathrm{S}-\) bonds that bridge parts of the chain. What effect do you predict these four linkages to have on the free energy and entropy of the native form relative to the free energy and entropy of a hypothetical folded structure that does not have any \(-\mathrm{S}-\mathrm{S}-\) linkages? Explain. (d) A gentle reducing agent converts the four \(-\mathrm{S}-\mathrm{S}-\) linkages in ribonuclease \(\mathrm{A}\) to eight \(-\mathrm{S}-\mathrm{H}\) bonds. What effect do you predict this conversion to have on the tertiary structure and entropy of the protein? (e) Which amino acid must be present for \(-\mathrm{SH}\) bonds to exist in ribonuclease \(\mathrm{A}\) ?

Indicate whether each statement is true or false. (a) All carbon atoms in propene are \(s p^{2}\) hybridized. (b) Acetone is another name for propanone. (c) The phenyl group contains six \(s p^{2}\) -hybridized carbons. (d) A hydrocarbon containing only \(s p^{3}\) carbons must be an alkane.

Indicate whether each statement is true or false. (a) Alkanes do not contain any carbon-carbon multiple bonds. (b) Cyclobutane contains a four-membered ring. (c) Alkenes contain carbon-carbon triple bonds. (d) Alkynes contain carbon-carbon double bonds. (e) Pentane is a saturated hydrocarbon but 1 -pentene is an unsaturated hydrocarbon. (f) Cyclohexane is an aromatic hydrocarbon. (g) The methyl group contains one less hydrogen atom than methane.

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