Amino Acids and Proteins Questions in English

(B) By analyzing the structure of the tetrapeptide from the $N$-terminal to the $C$-terminal:
$1$. The first amino acid residue has a side chain $-CH_2Ph$,which corresponds to Phenylalanine $(F)$.
$2$. The second amino acid residue has a side chain $-CH_2CH(CH_3)_2$,which corresponds to Leucine $(L)$.
$3$. The third amino acid residue has a side chain $-CH_2COOH$,which corresponds to Aspartic acid $(D)$.
$4$. The fourth amino acid residue has a side chain $-CH_2(C_6H_4)OH$,which corresponds to Tyrosine $(Y)$.
Thus,the sequence is $F-L-D-Y$.

(D) The total number of amino acid residues in the peptide is the sum of the moles of each amino acid produced upon hydrolysis.
Total amino acid residues $= 3 (G) + 2 (L) + 2 (V) = 7$.
For a peptide chain containing $n$ amino acid residues,the number of peptide linkages is given by $(n - 1)$.
Therefore,the number of peptide linkages $= 7 - 1 = 6$.

(B) Proteins are polymers of $\alpha$-amino acids. An $\alpha$-amino acid is one in which the amino group $(-NH_2)$ is attached to the carbon atom adjacent to the carboxylic acid group $(-COOH)$,i.e.,the $\alpha$-carbon.
In option $(B)$,the structure is $CH_3-CH(CH_3)-CH_2-CH(NH_2)-COOH$. Here,the $-NH_2$ group is attached to the carbon atom directly adjacent to the $-COOH$ group. This is the structure of the amino acid Leucine.
In the other options,the $-NH_2$ group is attached to a carbon atom further away from the $-COOH$ group,meaning they are not $\alpha$-amino acids.

(C) Denaturation is a process that causes the loss of the secondary and tertiary structures of proteins,leading to the loss of their biological activity.
$1$. It is caused by changes in physical conditions such as temperature or chemical conditions like $pH$.
$2$. During this process,the helical structure of the protein uncoils.
$3$. Option $C$ describes the primary structure of a protein (formation from amino acids linked by peptide bonds),which remains intact during denaturation.
Therefore,the statement in option $C$ is incorrect regarding the process of denaturation.

(D) The one-letter codes for the given amino acids are as follows:
$A$. Arginine is represented by $R$.
$B$. Aspartic acid is represented by $D$.
$C$. Asparagine is represented by $N$.
$D$. Alanine is represented by $A$.
Therefore,the correct matching is $A-IV, B-I, C-II, D-III$.

(B) The amino acids that contain sulphur in their side chains are cysteine and methionine.
$1$. Cysteine: Contains a thiol $(-SH)$ group.
$2$. Methionine: Contains a thioether $(-S-CH_3)$ group.
Therefore,the correct options are $(b)$ and $(d)$.

(C) The single-letter codes for amino acids are as follows:
$A$. Glutamic acid is represented by $E$ $(III)$.
$B$. Glutamine is represented by $Q$ $(I)$.
$C$. Tyrosine is represented by $Y$ $(IV)$.
$D$. Tryptophan is represented by $W$ $(II)$.
Therefore,the correct matching is $A-III, B-I, C-IV, D-II$.

(C) The $2^{\circ}$ and $3^{\circ}$ structures of proteins are stabilized by various interactions including hydrogen bonding,disulphide linkages $(-S-S-)$,van der Waals forces,and electrostatic forces of attraction.
An $-O-O-$ linkage (peroxide linkage) is not a standard interaction involved in the stabilization of protein structures.

(B) Assertion $A$: The reaction of glycine $(H_2N-CH_2-COOH)$ with $Cl_2$ in the presence of red phosphorus is the Hell-Volhard-Zelinsky $(HVZ)$ reaction. This reaction replaces an $\alpha$-hydrogen with a chlorine atom,resulting in $H_2N-CH(Cl)-COOH$ ($2$-chloro$-2-$aminoacetic acid). The central carbon atom is bonded to four different groups $(-H, -NH_2, -Cl, -COOH)$,making it a chiral carbon atom. Thus,Assertion $A$ is true.
Reason $R$: $A$ molecule with $2$ chiral carbons is not always optically active. For example,meso compounds contain chiral centers but are optically inactive due to an internal plane of symmetry. Thus,Reason $R$ is false.
Therefore,$A$ is true but $R$ is false.

(B) The oligopeptide structure contains $4$ carbonyl groups $(C=O)$,each contributing $1$ $sp^2$ hybridized carbon. Total carbons from carbonyl groups = $4 \times 1 = 4$.
Additionally,the phenyl ring (side chain of phenylalanine) contains $6$ $sp^2$ hybridized carbons.
Total number of $sp^2$ hybridized carbons = $4 + 6 = 10$.

(B) The starting material is $4$-ethoxyphenylacetaldehyde.
$1$. Strecker synthesis: Reaction with $NH_4Cl/KCN$ followed by hydrolysis gives $4$-ethoxyphenylalanine ('$A$').
$2$. Nitration: Treatment with $Conc. HNO_3-H_2SO_4$ $(2 \ eq)$ introduces two nitro groups at the ortho positions relative to the ethoxy group,yielding $3,5$-dinitro-$4$-ethoxyphenylalanine ('$B$').
$3$. Acetylation: Reaction with $(CH_3CO)_2O$ protects the amino group as an acetamide.
$4$. Esterification: Reaction with $EtOH/\Delta$ converts the carboxylic acid to an ethyl ester.
$5$. Reduction: $H_2, Pd/C$ reduces the two nitro groups to amino groups.
$6$. Diazotization: $HNO_2$ converts the amino groups to diazonium salts.
$7$. Sandmeyer-like reaction: $NaI$ replaces the diazonium groups with iodine atoms.
The final product '$D$' is $3,5$-diiodo-$4$-ethoxyphenylalanine ethyl ester derivative.
The molecular formula is $C_{15}H_{19}NO_4I_2$.
Comparing this with $C_xH_{19}NO_4I_2$,we find $x = 15$.

(C) The general structure of an amino acid contains one $-NH_2$ group and one $-COOH$ group.
$Arginine$,$Lysine$,and $Histidine$ are basic amino acids because they contain an additional basic functional group in their side chains.
$Asparagine$ $(Asn)$ has the side chain $-CH_2CONH_2$. The amide group $(-CONH_2)$ is neutral,not basic. Thus,$Asparagine$ contains only one basic amino group $(-NH_2)$ attached to the $\alpha$-carbon,making it the correct choice among the given options.

(D) Arginine is an amino acid that exists as a zwitterion in its solid state.
Due to the presence of ionic charges,it exhibits a high melting point and is a crystalline solid.
It is highly soluble in polar solvents like water but has low solubility in non-polar solvents like benzene.
Therefore,the statement that it has high solubility in benzene is incorrect.

(A) Boiling an egg causes denaturation of its protein.
During denaturation,the $Secondary$,$Tertiary$,and $Quaternary$ structures of the protein are disrupted due to the breaking of hydrogen bonds and other non-covalent interactions.
However,the $Primary$ structure,which consists of the specific sequence of amino acids held together by peptide bonds,remains intact.

(B) Proteins are natural polymers composed of $\alpha$-amino acids which are connected by peptide linkages.
Hence,proteins upon acidic hydrolysis produce $\alpha$-amino acids.

(C) Let the $3$ different amino acids be $A, B,$ and $C$.
Since each amino acid is used exactly once,the number of possible tripeptides is equivalent to the number of permutations of $3$ distinct items taken $3$ at a time.
This is calculated as $3! = 3 \times 2 \times 1 = 6$.
The possible combinations are: $ABC, ACB, BAC, BCA, CAB, CBA$.

(D) The ninhydrin test is a chemical test used to detect the presence of amino acids or proteins.
When ninhydrin reacts with amino acids or proteins,it produces a characteristic purple or blue color (Ruhemann's purple).
Among the given options,$A$,$B$,and $C$ are carbohydrates or synthetic polymers that do not contain amino groups.
$Egg \text{ } albumin$ is a protein,which is a polymer of amino acids,and therefore it will give a positive ninhydrin test.

(A) Coagulation of egg,on heating,leads to the loss of biological activity and the unfolding of the protein,which is known as denaturation of protein.

(D) The chemical structure of tyrosine is $HO-C_6H_4-CH_2-CH(NH_2)-COOH$.
Counting the carbon atoms:
$1$ carbon atom in the carboxylic acid group $(-COOH)$,
$1$ carbon atom in the alpha-carbon position $(-CH(NH_2)-)$,
$1$ carbon atom in the methylene bridge $(-CH_2-)$,
$6$ carbon atoms in the benzene ring.
Total number of carbon atoms $= 1 + 1 + 1 + 6 = 9$.

(A) Essential amino acids are those that cannot be synthesized by the human body and must be obtained from the diet.
From the given list:
$1$. Arginine (Essential for children/semi-essential)
$2$. Phenylalanine (Essential)
$3$. Histidine (Essential)
$4$. Valine (Essential)
Aspartic acid,Cysteine,and Proline are non-essential amino acids.
Note: Arginine and Histidine are often classified as essential in the context of human nutrition.
Counting the essential amino acids from the list: Arginine,Phenylalanine,Histidine,and Valine.
Total count = $4$.

(B) Step $1$: Oxidation of $1,2-$diethylbenzene $(O)$ with $KMnO_4 / H^+$ yields phthalic acid $(P)$.
Step $2$: Reaction of phthalic acid $(P)$ with ammonia followed by heating yields phthalamide $(Q)$.
Step $3$: Hofmann bromamide degradation of phthalamide $(Q)$ with $Br_2 / NaOH$ yields $o-$phenylenediamine $(R)$.
Step $4$: Heating phthalamide $(Q)$ yields phthalimide $(S)$.
Step $5$: Gabriel phthalimide synthesis using phthalimide $(S)$ and ethyl $2-$bromopropanoate followed by hydrolysis yields alanine $(T)$.
Thus,$R$ is $o-$phenylenediamine (not explicitly labeled as $A$-$D$ in the provided image options,but based on standard chemistry,$R$ is $o-$phenylenediamine) and $T$ is alanine $(B)$. Since the question asks to identify $R$ and $T$ from the options provided,and $T$ is clearly alanine $(B)$,the correct choice is $B$.

(A) In the given structure of lysine,the basic groups are the amino group $(-NH_2)$ and the carboxylate ion $(-COO^-)$,which can act as a base by accepting a proton.
Therefore,there are $2$ basic groups present in this form of lysine.

(A) The given structure is a tetrapeptide with a terminal $-NH_3^+$ group and a terminal $-COO^-$ group. At $pH = 7.0$,the net charge depends on the side chains $R_1$ and $R_2$.
For a peptide to be positively charged at $pH = 7.0$,it must contain basic side chains (like $-(CH_2)_4NH_2$) that remain protonated,while acidic side chains (like $-CH_2COOH$) must be absent or balanced.
Let us analyze the side chains:
- $I$: $R_1=H, R_2=H$ (Neutral)
- $II$: $R_1=H, R_2=CH_3$ (Neutral)
- $III$: $R_1=CH_2COOH$ (Acidic),$R_2=H$ (Net negative charge)
- $IV$: $R_1=CH_2CONH_2$ (Neutral),$R_2=(CH_2)_4NH_2$ (Basic) $\rightarrow$ Positively charged
- $V$: $R_1=CH_2CONH_2, R_2=CH_2CONH_2$ (Neutral)
- $VI$: $R_1=(CH_2)_4NH_2, R_2=(CH_2)_4NH_2$ (Basic) $\rightarrow$ Positively charged
- $VII$: $R_1=CH_2COOH$ (Acidic),$R_2=CH_2CONH_2$ (Neutral) $\rightarrow$ Negatively charged
- $VIII$: $R_1=CH_2OH$ (Neutral),$R_2=(CH_2)_4NH_2$ (Basic) $\rightarrow$ Positively charged
- $IX$: $R_1=(CH_2)_4NH_2$ (Basic),$R_2=CH_3$ (Neutral) $\rightarrow$ Positively charged
The peptides $IV, VI, VIII,$ and $IX$ are positively charged at $pH = 7.0$. Thus,there are $4$ such peptides.

(D) The peptide is $Tyr-Asp-Lys$. The ionizable groups are the $N$-terminal $-NH_2$ $(pK_a \approx 9)$,the $C$-terminal $-COOH$ $(pK_a \approx 2)$,the side chain $-COOH$ of $Asp$ $(pK_a \approx 3.9)$,the side chain $-OH$ of $Tyr$ $(pK_a \approx 10)$,and the side chain $-NH_2$ of $Lys$ $(pK_a \approx 10.5)$.
At $pH = 2$: The $N$-terminal $-NH_2$ is protonated ($-NH_3^+$,$+1$),the $C$-terminal $-COOH$ is neutral $(0)$,the $Asp$ side chain $-COOH$ is neutral $(0)$,the $Tyr$ side chain $-OH$ is neutral $(0)$,and the $Lys$ side chain $-NH_2$ is protonated ($-NH_3^+$,$+1$). Net charge $z_1 = +2$,so $|z_1| = 2$.
At $pH = 6$: The $N$-terminal $-NH_2$ is protonated ($-NH_3^+$,$+1$),the $C$-terminal $-COOH$ is deprotonated ($-COO^-$,$-1$),the $Asp$ side chain $-COOH$ is deprotonated ($-COO^-$,$-1$),the $Tyr$ side chain $-OH$ is neutral $(0)$,and the $Lys$ side chain $-NH_2$ is protonated ($-NH_3^+$,$+1$). Net charge $z_2 = (+1) + (-1) + (-1) + (+1) = 0$,so $|z_2| = 0$.
At $pH = 11$: The $N$-terminal $-NH_2$ is neutral $(0)$,the $C$-terminal $-COOH$ is deprotonated ($-COO^-$,$-1$),the $Asp$ side chain $-COOH$ is deprotonated ($-COO^-$,$-1$),the $Tyr$ side chain $-OH$ is deprotonated ($-O^-$,$-1$),and the $Lys$ side chain $-NH_2$ is neutral $(0)$. Net charge $z_3 = (-1) + (-1) + (-1) = -3$,so $|z_3| = 3$.
Therefore,$|z_1| + |z_2| + |z_3| = 2 + 0 + 3 = 5$.

(A) The tetrapeptide has alanine $(Ala)$ at the $C$-terminal position because the $-COOH$ group is on alanine.
The remaining three amino acids are glycine $(Gly)$,valine $(Val)$,and phenylalanine $(Phe)$.
For the $-NH_2$ group to be attached to a chiral center,the $N$-terminal amino acid must be chiral.
Glycine is achiral (optically inactive),so it cannot be at the $N$-terminal position.
Therefore,the $N$-terminal position can be occupied by either valine $(Val)$ or phenylalanine $(Phe)$.
If $Val$ is at the $N$-terminal,the possible sequences are:
$Val-Phe-Gly-Ala$
$Val-Gly-Phe-Ala$
If $Phe$ is at the $N$-terminal,the possible sequences are:
$Phe-Val-Gly-Ala$
$Phe-Gly-Val-Ala$
Total number of possible sequences = $2 + 2 = 4$.

(A) Complete acidic hydrolysis of the given peptide breaks all amide bonds.
The peptide structure contains the following amino acid residues:
$1$. Glycine $(NH_2-CH_2-COOH)$
$2$. Cyclopropylalanine (an unnatural amino acid)
$3$. tert-Leucine (an unnatural amino acid)
Among these,only glycine is a naturally occurring amino acid.
Therefore,the total number of distinct naturally occurring amino acids obtained is $1$.

(C) Aspartame is a dipeptide formed from aspartic acid and phenylalanine. Specifically,it is the methyl ester of the dipeptide $L$-aspartyl-$L$-phenylalanine. The structure consists of the amino group of phenylalanine forming an amide bond with the carboxyl group of aspartic acid,and the carboxyl group of phenylalanine is esterified with methanol. The correct structure is shown in option $C$.

(C) The $\alpha-$helix and $\beta-$pleated sheet are common types of secondary structures of proteins.
These structures are stabilized primarily by hydrogen bonds between the carbonyl oxygen and the amide hydrogen of the polypeptide backbone.

(A) Essential amino acids are those that cannot be synthesized by the human body and must be obtained through the diet.
Among the given options,$(A)$ Valine,$(C)$ Lysine,and $(D)$ Threonine are essential amino acids.
Proline and Tyrosine are non-essential amino acids.

(D) $1$. Isoleucine and threonine have $2$ chiral centers,so option $A$ is incorrect.
$2$. Glycine is achiral and optically inactive,so option $B$ is incorrect.
$3$. Aspartic acid also contains a $-COOH$ group in its side chain,so option $C$ is incorrect.
$4$. Cysteine contains a free $-SH$ group,which can undergo oxidation to form a disulfide bond (cystine),leading to dimerization. Thus,option $D$ is correct.

(D) tetrapeptide is formed by the condensation of $4$ amino acids.
Since the tetrapeptide contains $4$ different amino acids $(Gly, Ala, Val, Leu)$ in equimolar proportions,each amino acid is present exactly once.
The number of different sequences (arrangements) possible for $4$ distinct items is given by the permutation formula $n!$.
Here,$n = 4$.
Number of sequences $= 4! = 4 \times 3 \times 2 \times 1 = 24$.

(A) To form the dipeptide $Gly-Ala$ (Glycyl-Alanine),the amino group $(-NH_2)$ of the second amino acid $(Alanine)$ must attack the activated carboxyl group $(-COCl)$ of the first amino acid $(Glycine)$.
$Glycine$ is $NH_2-CH_2-COOH$. Its activated form is $NH_2-CH_2-COCl$.
$Alanine$ is $NH_2-CH(CH_3)-COOH$.
The reaction proceeds as follows:
$NH_2-CH_2-COCl + NH_2-CH(CH_3)-COOH \rightarrow NH_2-CH_2-CONH-CH(CH_3)-COOH + HCl$.
Thus,the correct pair of reactants is $NH_2-CH_2-COCl$ and $NH_2-CH(CH_3)-COOH$.

(B) $1$. Hydrolysis of dipeptide $X$ yields two amino acids,$Y$ and $Z$.
$2$. $Y$ reacts with aqueous $HNO_2$ to form lactic acid $(CH_3CH(OH)COOH)$. This indicates that $Y$ is alanine $(CH_3CH(NH_2)COOH)$.
$3$. $Z$ on heating forms a cyclic molecule. The structure provided is $2,5-$diketopiperazine,which is formed by the dimerization of glycine $(NH_2CH_2COOH)$. Thus,$Z$ is glycine.
$4$. Therefore,the dipeptide $X$ is $alanine-glycine$.

(C) $(i)$ Proteins on hydrolysis yield a large number of $\alpha$-amino acids,not $\beta$-amino acids. Thus,Statement $(I)$ is incorrect.
$(ii)$ Fibrous proteins are generally insoluble in water. Denaturation involves the loss of biological activity and changes in the secondary and tertiary structures,but it does not involve the conversion of water-soluble fibrous proteins to water-insoluble forms in the manner described. Thus,Statement $(II)$ is also incorrect.

(A) Amino acids are amphoteric in nature. In an acidic medium (low $pH$),the amino group $(-NH_2)$ gets protonated to form $-NH_3^+$.
Thus,at $pH = 2$,the structure $A$ is $H_3N^+-CH(CH(CH_3)_2)-COOH$.
In a basic medium (high $pH$),the carboxylic acid group $(-COOH)$ gets deprotonated to form $-COO^-$.
Thus,at $pH = 10$,the structure $B$ is $H_2N-CH(CH(CH_3)_2)-COO^-$.
Therefore,the correct option is $A$.

(C) During the denaturation of proteins,the $2^{\circ}$ and $3^{\circ}$ structures are destroyed due to the disruption of hydrogen bonds and other interactions,while the $1^{\circ}$ structure (the sequence of amino acids) remains intact. Therefore,the statement in option $C$ is false because it incorrectly claims that the $3^{\circ}$ structure remains the same while the $1^{\circ}$ structure is destroyed.

(A) The secondary structure of proteins,such as the $\alpha$-helix,is primarily stabilized by hydrogen bonds formed between the carbonyl oxygen $(-C=O)$ of one amino acid residue and the amide hydrogen $(-NH)$ of another amino acid residue located further along the polypeptide chain. While peptide bonds hold the primary structure together,hydrogen bonds are responsible for maintaining the specific helical conformation.

(D) The ninhydrin test is a chemical test used to detect ammonia or primary and secondary amines. In biochemistry,it is specifically used to detect proteins and amino acids,which produce a characteristic blue or purple color.
Benedict's solution is used to detect reducing sugars (monosaccharides and some disaccharides).
Since proteins are not reducing sugars,they give a negative Benedict's test.
Therefore,a protein is the compound that gives a positive ninhydrin test and a negative Benedict's solution test.

(D) The general structure of an $\alpha$-amino acid is $R-CH(NH_2)-COOH$.
For serine,the side chain $(R)$ is a hydroxymethyl group,which is represented as $HO-CH_2-$.
Therefore,the correct option is $D$.

(C) Essential amino acids are those that cannot be synthesized by the human body and must be obtained through the diet.
Among the given options,$Histidine$ is an essential amino acid.
$Tyrosine$,$Serine$,and $Glycine$ are non-essential amino acids as they can be synthesized by the body.

(A) Amino acids are classified as acidic,basic,or neutral based on the number of amino $(-NH_2)$ and carboxyl $(-COOH)$ groups present in their structure.
$1$. $Glu$ $(Glutamic \ acid)$ contains two $-COOH$ groups and one $-NH_2$ group,making it an acidic amino acid.
$2$. $Arg$ $(Arginine)$ and $Lys$ $(Lysine)$ contain more amino groups than carboxyl groups,making them basic amino acids.
$3$. $Trp$ $(Tryptophan)$ contains one $-NH_2$ group and one $-COOH$ group in its structure,making it a neutral amino acid.
Therefore,the correct option is $A$.

(B) Amino acids that cannot be synthesized by the human body and must be obtained through the diet are called essential amino acids.
Among the given options,$Tryptophan$ is an essential amino acid.
Tyrosine,Glutamine,and Glycine are non-essential amino acids,meaning they can be synthesized by the human body.

(A) The amino acids that contain sulfur in their side chain $(R)$ are Methionine and Cysteine.
Methionine has the side chain structure: $-CH_2-CH_2-S-CH_3$.
Cysteine has the side chain structure: $-CH_2-SH$.
Among the given options,Methionine is the correct answer.

(C) The amino acid $Histidine$ contains an imidazole ring,which is a five-membered heterocyclic ring containing two nitrogen atoms in its side chain $(R)$ group.
Threonine contains a hydroxyl group $(-OH)$.
Cysteine contains a thiol group $(-SH)$.
Valine contains an isopropyl group $(-CH(CH_3)_2)$.
Therefore,the correct answer is $Histidine$.

(B) The general structure of an amino acid is $R-CH(NH_2)-COOH$.
For Alanine,the side chain $R$ is a methyl group $(-CH_3)$.
Therefore,the structure of Alanine is $CH_3-CH(NH_2)-COOH$.

(D) Amino acids are classified as acidic,basic,or neutral based on the number of amino and carboxyl groups present in their structure.
An acidic amino acid contains more carboxyl $(-COOH)$ groups than amino $(-NH_2)$ groups.
Among the given options:
$Thr$ (Threonine) is a neutral amino acid.
$Trp$ (Tryptophan) is a neutral amino acid.
$His$ (Histidine) is a basic amino acid.
$Glu$ (Glutamic acid) contains two carboxyl groups and one amino group,making it an acidic amino acid.
Therefore,the correct option is $D$.

(C) Amino acids are classified as acidic,basic,or neutral based on the number of amino $(-NH_2)$ and carboxyl $(-COOH)$ groups present in their structure.
$1$. $Arg$ $(Arginine)$ is a basic amino acid due to the presence of an extra amino group in its side chain.
$2$. $Asp$ $(Aspartic \ acid)$ is an acidic amino acid due to the presence of an extra carboxyl group in its side chain.
$3$. $Leu$ $(Leucine)$ is a neutral amino acid because it contains one amino group and one carboxyl group,and its side chain is non-polar and non-ionizable.
$4$. $His$ $(Histidine)$ is a basic amino acid.
Therefore,$Leu$ is the correct neutral amino acid.

(D) Amino acids are classified as acidic,basic,or neutral based on the number of amino and carboxyl groups present in their structure.
An acidic amino acid contains more carboxyl groups $(-COOH)$ than amino groups $(-NH_2)$.
Among the given options:
$Arg$ $(Arginine)$ is a basic amino acid.
$Lys$ $(Lysine)$ is a basic amino acid.
$Met$ $(Methionine)$ is a neutral amino acid.
$Asp$ $(Aspartic \ acid)$ contains two carboxyl groups and one amino group,making it an acidic amino acid.
Therefore,the correct option is $D$.

(D) The general structure of an amino acid is $R-CH(NH_2)-COOH$.
For Leucine,the side chain $(R)$ is an isobutyl group.
The structure of the isobutyl group is $(CH_3)_2CH-CH_2-$.
Therefore,the correct side chain for Leucine is $Me_2CH-CH_2-$.

(D) Essential amino acids are those that cannot be synthesized by the human body and must be obtained through the diet.
There are nine essential amino acids: $Histidine$,$Isoleucine$,$Leucine$,$Lysine$,$Methionine$,$Phenylalanine$,$Threonine$,$Tryptophan$,and $Valine$.
Among the given options,$Histidine$ is an essential amino acid.