Biomolecules – JEE Main ChemistryPractice Questions & PYQs

The ceric ammonium nitrate test is used to detect the presence of alcoholic hydroxyl ($-OH$) groups, whereas the carbylamine test serves as a specific diagnostic test for primary amine ($-NH_2$) groups. Examining the amino acid components, serine contains an $-OH$ group in its side chain, while lysine possesses a free primary amine group in its side chain. Consequently, the dipeptide Ser - Lys contains both necessary functional groups, allowing it to yield a positive result for both tests.

The classification of amino acids as basic depends on the presence of functional groups capable of accepting protons, particularly within the side chain. Histidine contains an imidazole ring that functions as an effective proton acceptor, which imparts significant basic character to the molecule. In the context of the compounds provided, this specific structural property identifies Histidine as the most basic amino acid among the choices.

Amylopectin is a branched-chain polysaccharide constructed entirely from repeating units of $\alpha$-D-glucose. The primary linear segments of the polymer are connected by glycosidic linkages between the $C_1$ and $C_4$ carbon atoms of adjacent glucose units. Furthermore, the characteristic branching of the molecule is established through distinct glycosidic linkages formed between the $C_1$ and $C_6$ carbon atoms of the sugar residues.

Glucose and galactose are stereoisomers known as epimers, a term describing sugars that possess identical configurations at every chiral center except for one. If you examine the structural arrangement of these molecules, you will see that they are structurally indistinguishable until you reach the C-4 carbon atom. The orientation of the hydroxyl group is inverted specifically at this C-4 position, which is the sole structural deviation that differentiates galactose from glucose.

In the open-chain structure of glucose, represented by the formula $\text{CHO}-(\text{CHOH})_4-\text{CH}_2\text{OH}$, the internal carbons located at positions 2, 3, 4, and 5 are each bonded to four distinct groups, identifying them as chiral centers. This arrangement establishes 4 stereocenters in the linear molecule. When the molecule undergoes cyclization to form a six-membered pyranose ring, the carbonyl carbon at position 1 participates in hemiacetal formation and becomes a new chiral center, known as the anomeric carbon. Because the existing stereocenters at positions 2 through 5 remain unchanged while this new center is created at position 1, the total count increases to 5 stereocenters in the cyclic form.

Acetic anhydride serves as a potent acetylating agent that selectively targets nucleophilic functional groups, particularly primary amino groups and hydroxyl groups, through nucleophilic acyl substitution. In the dipeptide Asn-Ser, the terminal amino group at the N-terminus and the side-chain hydroxyl group of the serine residue act as the primary nucleophiles, while the amide side chain of the asparagine residue remains largely unreactive under these conditions. Treatment with excess $(CH_3CO)_2O$ in the presence of the base $NEt_3$ promotes the acetylation of these specific sites, converting the N-terminal amine into an acetamide and the serine side-chain hydroxyl into an acetate ester. The resulting product 'P' effectively incorporates these two acetyl groups onto the respective nucleophilic centers while preserving the overall peptide backbone and the side chain of the asparagine residue.

Potassium ions are essential intracellular cations that perform several critical physiological functions within the body. These ions serve as important activators for many enzymes and are actively involved in the biochemical pathway responsible for the oxidation of glucose to produce adenosine triphosphate. Additionally, potassium ions work in conjunction with sodium ions to maintain the electrochemical gradients across cell membranes that are fundamental for the transmission of nerve signals.

RNA molecules utilize a specific set of nitrogenous bases to store genetic information, which helps distinguish them from DNA. While both nucleic acids share adenine, guanine, and cytosine, RNA specifically incorporates uracil rather than the thymine found in DNA. This structural difference is a fundamental characteristic of RNA that supports its role in protein synthesis and cellular regulation.

The ionization state of a molecule in a biological environment is determined by comparing the ambient pH to the substance's specific pKa value. With the pH of blood maintained at $7.35$ and the pKa of histamine identified as $6.0$, the surrounding environment is more basic than the molecule's acid dissociation constant. Consequently, the equilibrium conditions dictate that histamine exists predominantly in its monocationic form within human blood.

Adenosine is a nucleoside formed by the covalent attachment of an adenine base to a D-ribose sugar molecule through a $\beta$-N9-glycosidic linkage. The structure is characterized by the nitrogen atom at the 9th position of the adenine purine ring being bonded to the anomeric carbon ($C_1'$) of the ribofuranose moiety, distinguishing it from other nucleosides.

Disaccharides and polysaccharides are defined by the specific glycosidic linkages between their constituent monosaccharide units, which dictate their unique structural properties. Sucrose is formed by an $\alpha$-D-glucose and $\beta$-D-fructose unit joined by an $\alpha, \beta$-1,2-glycosidic bond, whereas amylose is a linear polymer of $\alpha$-D-glucose units connected solely by $\alpha$-1,4-glycosidic linkages. The claim that both sucrose and amylose share a 1,2-glycosidic linkage is inaccurate because amylose does not contain this specific bond type.

Glucose is composed of a straight chain of six carbon atoms, with oxygen atoms present in both hydroxyl and carbonyl groups. When subjected to prolonged heating with hydroiodic acid in the presence of red phosphorus, this potent reducing combination removes all oxygen atoms from the molecule. This process fully reduces the carbon backbone, resulting in the formation of the straight-chain alkane $n$-hexane.

The reagent $CH_3COCl$ acts as a potent electrophilic acylating agent that selectively targets the most nucleophilic site in the Gln-Gly dipeptide, which is the free $N$-terminal primary amino group. This nucleophilic substitution reaction results in the formation of an $N$-acetylated dipeptide, converting the terminal $-NH_2$ group into an $-NHCOCH_3$ group while leaving the side-chain amide and carboxylic acid functional groups chemically unaffected.

When an ester interacts with an aqueous KOH solution, it undergoes a nucleophilic acyl substitution, which is classified as an $S_N^{AE}$ reaction. This chemical transformation converts the ester moiety into an $\alpha$-hydroxy carbonyl compound, a structure that is inherently capable of reducing Tollen's reagent. Consequently, this structural change allows the compound to exhibit the characteristic properties of a reducing sugar within the alkaline medium.

Essential amino acids are those that cannot be synthesized by the human body and must be acquired through dietary sources, whereas non-essential amino acids can be produced endogenously. Among the given options, Valine is an essential amino acid, while Alanine, Aspartic acid, and Serine are classified as non-essential amino acids synthesized within the body.

$\alpha$-D-glucose and $\beta$-D-glucose are diastereomers known as anomers, defined by their differing configurations only at the anomeric carbon (C-1), rather than enantiomers which are non-superimposable mirror images requiring the inversion of all chiral centers. Since these sugars share identical stereochemistry at the C-2, C-3, C-4, and C-5 positions, they cannot be enantiomers. The remaining options accurately describe the structural composition of cellulose as a polymer of $\beta$-D-glucose units and the inability of the protected aldehyde group in glucose pentaacetate to undergo oxime formation with hydroxylamine.

The formation of Ruhemann's purple is a hallmark of the chemical reaction between ninhydrin and free amino groups, which are fundamental components found in $\alpha$-amino acids and proteins. Upon reacting with these substrates, ninhydrin undergoes a transformation that yields this characteristic blue or purple pigment. Consequently, this sensitive colorimetric reaction serves as a definitive confirmatory test to identify the presence of proteins within a sample.

The isoelectric point (pI) of an amino acid represents the pH at which the molecule carries no net electric charge, effectively existing as a neutral zwitterion. For an acidic amino acid like aspartic acid, which features an additional carboxyl group in its side chain, this point is determined by calculating the average of the $pK_1$ and $pK_R$ values associated with the two acidic groups. Using the provided constants $pK_1 = 1.88$ and $pK_R = 3.65$ in the formula $pI = \frac{pK_1 + pK_R}{2}$ leads to the calculation $\frac{1.88 + 3.65}{2}$, which results in 2.77.

A thiol group is defined by the presence of a sulfhydryl (-SH) functional group covalently bonded to a carbon atom. Cysteine is the amino acid among the choices that possesses this specific side chain, represented by the structural formula $HS-CH_2-CH(NH_2)-COOH$. Conversely, Cystine contains a disulfide bridge ($S-S$) resulting from the oxidation of two cysteine residues, Methionine features a thioether linkage ($R-S-CH_3$), and Cytosine is a heterocyclic nitrogenous base that does not contain sulfur.

Vitamins are categorized into two groups based on whether they dissolve in fat or water, which determines their physiological absorption and storage. In contrast to vitamins D, E, and K, which are fat-soluble and capable of being stored in the body's lipid tissues, vitamin C is water-soluble. This solubility characteristic means that vitamin C dissolves easily in aqueous fluids, ensuring it is readily processed and excreted rather than accumulated in fat stores.