Biomolecules – JEE Main ChemistryPractice Questions & PYQs

Essential amino acids are the vital protein building blocks that the human body cannot synthesize on its own, necessitating their inclusion in one's diet. Among the amino acids listed, Arginine, Phenylalanine, Histidine, and Valine fall into this category, whereas the others are classified as non-essential. Therefore, counting these specific entries confirms that there are 4 essential amino acids in the provided group.

Arginine exists primarily as a zwitterion, which confers high polarity and ensures the molecule is soluble in polar solvents but insoluble in non-polar liquids like benzene. Because of this structure, the compound manifests as a crystalline solid with a high melting point and exhibits three distinct $pK_a$ values based on its ionizing side chains. Consequently, the assertion that arginine is highly soluble in benzene is inaccurate, as its ionic nature restricts it from dissolving in non-polar media.

The ammonolysis of alkyl halides typically yields a mixture of primary, secondary, and tertiary amines because the resulting alkyl amines are stronger nucleophiles than ammonia, leading to successive alkylation. In contrast, the reaction of $\alpha$-halocarboxylic acids with excess ammonia yields $\alpha$-amino carboxylic acids with high selectivity because the amino group formed is significantly less nucleophilic due to the strong electron-withdrawing effect of the adjacent carboxyl group, which prevents further alkylation. While the statement that amino acids exist as zwitterions in aqueous media is chemically accurate, this structural property is independent of the kinetic and mechanistic factors that prevent over-alkylation in this synthesis, meaning the reason does not explain the assertion.

Aldohexoses like mannose cyclize to form stable six-membered pyranose rings through the intramolecular reaction between the $C_1$ aldehyde group and the $C_5$ hydroxyl group. The specific configuration of D-mannose distinguishes it from other stereoisomers by the orientation of the hydroxyl groups at the chiral carbons, particularly the $C_2$ position where the hydroxyl group is directed upward in the Haworth projection. Option B accurately depicts this spatial arrangement, correctly representing the cyclic structure of D-mannose.

Proteins are composed of $\alpha$-amino acids, a structural class where both the amino group ($\mathrm{NH}_2$) and the carboxylic acid group ($\mathrm{COOH}$) are bonded to the same carbon atom known as the $\alpha$-carbon. Even for a protein 'X' with a molecular weight of 70,000 u, the hydrolysis products are always these specific monomeric building blocks. Among the choices provided, the structure $\mathrm{CH}_3-\mathrm{CH}(\mathrm{CH}_3)-\mathrm{CH}_2-\mathrm{CH}(\mathrm{NH}_2)-\mathrm{COOH}$ is the only one that satisfies this definition, as it is the only instance where both functional groups are attached to the $\alpha$-carbon.

A tripeptide is formed by linking three amino acids, and since each of these three positions can be filled by either valine or proline, there are two distinct choices available for every slot. Applying the multiplication principle of counting, the total number of unique sequences is found by raising the number of available amino acids to the power of the positions in the chain, which is $2^3$. This calculation yields a total of 8 possible tripeptides.

The mass percentage of an element within a compound is defined as the ratio of the total mass of that element in one molecule to the total molar mass of the compound, multiplied by 100. For uracil, which has the molecular formula $C_4H_4N_2O_2$, the first step is to calculate the total molar mass by summing the atomic weights of all constituent atoms: $4(12) + 4(1) + 2(14) + 2(16) = 112$.

Once the total molar mass is established, the nitrogen component is determined by calculating the mass contributed by the two nitrogen atoms, which equals 28. Dividing the mass of nitrogen by the total molar mass of uracil provides the fraction, and multiplying by 100 yields the percentage:

$\frac{28}{112} \times 100 = 25\%$

The one-letter code system assigns a unique character to each of the 20 proteinogenic amino acids to provide a standardized, concise representation for protein sequences. Arginine is denoted by R, Aspartic acid is represented by D, Asparagine is identified by N, and Alanine is designated by A. Mapping these established codes to the provided lists results in the correspondence (A)-IV, (B)-I, (C)-II, and (D)-III.

Amino acids are identified by specific one-letter codes that are often derived from phonetic associations or the chemical name itself. Glutamic acid is designated by E, which helps differentiate it from Glutamine, which is assigned the letter Q. Tyrosine is represented by Y due to the strong phonetic sound of the letter in its name, and Tryptophan is mapped to W because of the distinct 'w' sound prominent in its pronunciation.

Protein secondary ($2^{\circ}$) and tertiary ($3^{\circ}$) structures are maintained by a specific array of interactions that dictate the overall folding and spatial arrangement of the polypeptide chain. Hydrogen bonds, disulfide linkages ($-S-S-$), van der Waals forces, and electrostatic attractions are all established stabilizing forces that keep these complex protein geometries intact. In contrast, the $-O-O-$ linkage, which represents a peroxide bond, does not provide any inherent stabilization to these structures, making it the exception among the choices provided.

The Benedict test detects reducing sugars by utilizing their free aldehyde or ketone groups to reduce copper(II) ions into an insoluble orange-red $\text{Cu}_2\text{O}$ precipitate. Within the provided list, glucose, maltose, ribose, 2-deoxyribose, and lactose all contain these reactive functional groups and are classified as reducing sugars. Because sucrose and amylose are non-reducing carbohydrates that lack the necessary reactive center for this redox reaction, they are the only 2 compounds that will fail to produce the precipitate.

These diagnostic tests serve as qualitative indicators for identifying functional groups and classes of compounds based on their unique chemical reactivities. Molisch's test is the standard method for the detection of carbohydrates, whereas the biuret test is specifically employed to identify the presence of peptide linkages in protein or peptide structures. Furthermore, the carbylamine test provides a selective pathway to distinguish primary amines through the formation of distinctively foul-smelling isocyanides, while Schiff's test functions as a reliable procedure for confirming the presence of aldehydes by restoring the color of a previously decolorized reagent.

Determining the number of peptide linkages requires finding the total number of amino acid residues by adding the 3 moles of glycine, 2 moles of leucine, and 2 moles of valine, which results in 7 total amino acids. In any linear peptide chain, each peptide bond connects a pair of adjacent amino acids, following the relationship that the number of linkages is equal to the total number of amino acids minus one. Applying this principle to the peptide results in $7 - 1 = 6$ peptide linkages.

The Kiliani-Fischer synthesis increases the carbon chain length of an aldose by adding a cyanide group, creating a new chiral center that results in two diastereomeric aldonic acids which are subsequently oxidized by nitric acid to dicarboxylic acids. Oxidation of these intermediates yields tartaric acid derivatives, where the stereochemistry of the two resulting diastereomers leads to one being the achiral mesotartaric acid and the other being an optically active tartaric acid isomer. Consequently, the final mixture consists of one optically active product and one meso product, as the molecular symmetry of the meso form prevents optical activity through internal compensation.

Every naturally occurring $\alpha$-amino acid possesses an $\alpha$-amino group which functions as a basic site. Lysine, histidine, and arginine each contain an additional basic moiety in their side chains-a primary amine, an imidazole ring, and a guanidino group, respectively-making them amino acids with two basic groups. Asparagine, however, contains a neutral amide side chain ($-CH_2CONH_2$), leaving the $\alpha$-amino group as the only basic functional group present in its structure.

The distinction between $\alpha$ and $\beta$ anomers in Haworth projections relies entirely on the configuration of the hydroxyl group attached to the anomeric carbon relative to the orientation of the terminal primary alcohol group. In the glucopyranose structure, the $\alpha$-anomer is identified by the hydroxyl group at the first carbon pointing downward, whereas the $\beta$-anomer features this group pointing upward. Similarly, for fructofuranose, the $\alpha$-anomer is characterized by the hydroxyl group at the second carbon directed downward, and the $\beta$-anomer is defined by that same hydroxyl group directed upward, which confirms the structural identity of each species in the provided set.

Seliwanoff's test serves as a diagnostic tool to distinguish between ketoses and aldoses, relying on the observation that ketohexoses dehydrate significantly faster than aldoses when heated in an acidic medium. This rapid dehydration leads to the production of 5-hydroxymethylfurfural, which immediately condenses with resorcinol to form a characteristic cherry-red or brownish-red complex. Because this rate difference is the underlying principle, the assertion that ketoses yield a positive test more rapidly is entirely correct. The reasoning statement is false, however, as the chemical pathway driving this reaction is dehydration rather than $\beta$-elimination.

The relative stability of various vitamin C structures is primarily dictated by the presence of intramolecular hydrogen bonding. The most stable configuration allows for an optimal geometric arrangement where these internal hydrogen bonds significantly decrease the molecule's potential energy. Conversely, structures that do not facilitate such interactions remain at a higher energy state, making them comparatively less stable.

In an oligopeptide, carbon atoms exhibit $\text{sp}^2$ hybridization within carbonyl groups and aromatic systems. Alanyl-Glycyl-Phenylalanyl-Isoleucine is a tetrapeptide composed of three peptide linkages, which contribute three carbonyl carbons, and one terminal carboxylic acid group, which contributes one additional carbonyl carbon, all of which are $\text{sp}^2$ hybridized. Furthermore, the phenylalanine residue incorporates a phenyl side chain containing a benzene ring, where all six carbons are $\text{sp}^2$ hybridized due to the ring's conjugated double bond structure. Summing these contributions yields $3 + 1 + 6 = 10$ $\text{sp}^2$ hybridized carbons.

Constructing a cyclic tripeptide from two amino acids, A and B, requires arranging three amino acid units in a closed loop where rotationally equivalent sequences represent the same molecule. First, consider the cases where all three amino acids are the same, resulting in the sequences AAA and BBB, which yield two distinct cyclic structures. Next, consider cases where the amino acids are not identical, requiring either two A's and one B, or one A and two B's. The combination of two A's and one B creates one unique cyclic arrangement, as AAB is rotationally equivalent to ABA and BAA, and the combination of one A and two B's creates another unique cyclic arrangement, as ABB is rotationally equivalent to BBA and BAB, resulting in a total of 4 cyclic tripeptides.