Amino Acids and Proteins Questions in English

(C) In an aqueous solution,the carboxyl group $(-COOH)$ of an amino acid can lose a proton to form a carboxylate ion $(-COO^-)$,and the amino group $(-NH_2)$ can accept a proton to form an ammonium ion $(-NH_3^+)$.
This internal proton transfer results in the formation of a dipolar ion known as a zwitterion,represented as $H_3N^+-CHR-COO^-$.
Therefore,amino acids exist as zwitterions in aqueous solutions.

(A) An $\alpha-$amino acid has a general structure $R-CH(NH_2)-COOH$.
$A$ chiral carbon is a carbon atom bonded to four different groups.
In Glycine,the $R$ group is a hydrogen atom $(-H)$.
Thus,the structure of Glycine is $H_2N-CH_2-COOH$.
Since the central carbon is bonded to two identical hydrogen atoms,it is achiral.
All other amino acids listed have different $R$ groups,making their $\alpha-$carbon chiral.

(C) zwitterion is a dipolar ion that contains both a positive and a negative charge.
It is formed when an acidic group (like $-COOH$ or $-SO_3H$) donates a proton to a basic group (like $-NH_2$) within the same molecule.
Therefore,both $-NH_2, -COOH$ and $-NH_2, -SO_3H$ combinations can form zwitterions.

(B) Sanger's reagent is $1$-fluoro-$2,4$-dinitrobenzene $(FDNB)$.
It reacts with the free amino group of the $N$-terminal residue of a peptide chain to form a yellow-colored dinitrophenyl derivative,which helps in identifying the $N$-terminal amino acid.

(A) Hemoglobin is a respiratory pigment present in red blood cells. It is a conjugated protein consisting of a protein part called globin and a prosthetic group called heme. The heme group contains an $Fe^{2+}$ ion at its center,which is responsible for binding with oxygen. Therefore,hemoglobin primarily contains iron.

(D) $\alpha$-Amino acetic acid (Glycine) exists as a zwitterion in aqueous solution.
It contains both positive and negative charges within the same molecule,behaving like an internal salt.
$NH_2-CH_2-COOH \rightleftharpoons NH_3^+-CH_2-COO^-$.
This dipolar ion structure is characteristic of amino acids.

(B) The position $(X)$ corresponds to the carboxylic acid group $(-COOH)$,which is the most acidic site in the molecule.
Between the two amino groups at positions $(Y)$ and $(Z)$,the position $(Y)$ is closer to the electron-withdrawing $-COOH$ group.
The electron-withdrawing effect of the $-COOH$ group decreases the electron density on the nitrogen atom at position $(Y)$ more than at position $(Z)$,making the conjugate acid of $(Y)$ more stable (or the amino group at $(Y)$ less basic,which implies its conjugate acid is more acidic).
Therefore,the correct order of acidity is $X > Y > Z$.

(A) . Pepsin is a digestive enzyme that is composed of protein. Adrenaline is a hormone,$ATP$ is a nucleotide,and Glutamine is an amino acid.

(D) is the correct answer.
Benzidine is not an amino acid; it is an aromatic amine with the formula $(NH_2C_6H_4)_2$.

(C) zwitter ion is a dipolar ion that contains both a positive and a negative charge within the same molecule,resulting in a net charge of zero.
Amino acids containing both an amino group $(-NH_2)$ and a carboxylic acid group $(-COOH)$ form zwitter ions by internal proton transfer.
Similarly,compounds containing an amino group $(-NH_2)$ and a sulfonic acid group $(-SO_3H)$ can also form zwitter ions through internal proton transfer.
Therefore,both pairs of functional groups can lead to the formation of a zwitter ion.

(B) The thiol $(-SH)$ functional group present in the amino acid cysteine participates in the formation of disulphide bonds $(-S-S-)$ in proteins through an oxidation reaction.

(B) The correct statement is that the third base of the codon is less specific,which is known as the $Wobble$ $Hypothesis$.
$m-RNA$ does not directly recognize amino acids; instead,$t-RNA$ acts as an adaptor molecule.
Most amino acids are coded by more than one codon (degeneracy of genetic code).
Every $t-RNA$ molecule has only one specific amino acid attachment site at its $3'$-end.

(C) The amino acids are organic compounds containing an amino group $(-NH_2)$ and a carboxylic acid group $(-COOH)$.
Among the given options:
$A$. Lysine is a basic amino acid.
$B$. Serine is a hydroxy-containing amino acid.
$C$. Cysteine has the structure $HS-CH_2-CH(NH_2)-COOH$,which contains a thiol $(-SH)$ group,making it a sulphur-containing amino acid.
$D$. Tyrosine is an aromatic amino acid.
Therefore,the correct option is $C$.

(C) Denaturation is a process in which the secondary and tertiary structures of proteins are destroyed due to physical or chemical changes,such as change in $pH$ or temperature. During this process,the globules unfold and the helix becomes uncoiled,causing the protein to lose its biological activity. Therefore,the statement that denaturation makes proteins more active is incorrect.

(C) The central dogma of molecular biology describes the flow of genetic information within a biological system.
It states that genetic information flows from $DNA$ to $RNA$ and then to proteins.
The process is represented as: $DNA$ $\xrightarrow{\text{Transcription}} RNA$ $\xrightarrow{\text{Translation}} \text{Protein}$.

(A) The peptide bond is an amide bond $(-CONH-)$ formed by the condensation reaction between the carboxyl group $(-COOH)$ of one amino acid and the amino group $(-NH_2)$ of another amino acid,with the elimination of a water molecule $(H_2O)$.
This linkage is responsible for the primary structure of proteins.

(C) Statement $1$ is correct: Denaturation of proteins involves the unfolding of the polypeptide chain,leading to the loss of secondary and tertiary structures,while the primary structure remains intact.
Statement $2$ is correct: Denaturation of $DNA$ involves the separation of the two strands of the double helix into single strands due to the breaking of hydrogen bonds.
Statement $3$ is incorrect: Denaturation does not affect the primary structure of proteins,as the peptide bonds remain intact.
Therefore,statements $1$ and $2$ are correct.

(A) The thiol group $(-SH)$ is a functional group consisting of a sulfur atom bonded to a hydrogen atom.
Among the given options,$Cysteine$ is an amino acid that contains a thiol group in its side chain.
The structure of $Cysteine$ is $HS-CH_2-CH(NH_2)-COOH$.

(A) The primary structure of a protein refers to the specific sequence of $\alpha -$ amino acids in the polypeptide chain.
Secondary structure refers to the regular folding patterns of the polypeptide backbone,such as the $\alpha -$ helix and $\beta -$ pleated sheet structures.
These structures are stabilized by hydrogen bonding between the carbonyl oxygen $(C=O)$ and the amide hydrogen $(N-H)$ of the peptide backbone.

(A) The Biuret test is a chemical test used to detect the presence of peptide bonds ($CONH$ linkage).
Since carbohydrates do not contain peptide bonds,they do not give a positive Biuret test.
Polypeptides and proteins contain multiple peptide bonds and thus give a positive test.
Urea $(NH_2CONH_2)$ also gives a positive Biuret test because it contains a structure similar to the biuret molecule $(H_2N-CO-NH-CO-NH_2)$.

(B) Amino acids are organic molecules consisting of a basic amino group $(-NH_2)$,an acidic carboxyl group $(-COOH)$,and an $R$ group (side chain) unique to each amino acid.
Amino acids are classified based on the chemical properties of their $R$ group.
Serine is an amino acid where the $R$ group is $-CH_2OH$,which contains a hydroxyl $(-OH)$ group,characteristic of an alcohol functional group.
Therefore,Serine is the correct answer.

(C) Amino acids are classified based on the relative number of amino and carboxyl groups in their structure.
Basic amino acids contain more amino groups than carboxyl groups.
Arginine contains a guanidino group in its side chain,which makes it strongly basic.
The structure of Arginine is $H_2N-C(=NH)-NH-CH_2-CH_2-CH_2-CH(NH_2)-COOH$.

(B) Aspartic acid is an acidic amino acid with three ionizable groups. The isoelectric point $(pI)$ is the $pH$ at which the molecule exists as a zwitterion with no net charge,resulting in no net migration during electrophoresis.
For an acidic amino acid,the $pI$ is calculated using the two lowest $pK_a$ values:
$pI = \frac{pK_1 + pK_2}{2} = \frac{2.0 + 3.90}{2} = 2.95$.
At $pH = 1.0$ $(pH < pI)$,the molecule is positively charged and migrates towards the $(-)$ electrode.
At $pH = 2.95$ $(pH = pI)$,the molecule has no net charge and shows no net migration.
At $pH = 7.0$ and $pH = 9.0$ $(pH > pI)$,the molecule is negatively charged and migrates towards the $(+)$ electrode.

(C) Ninhydrin is a chemical reagent used to detect the presence of proteins and $\alpha$-amino acids.
When $\alpha$-amino acids react with ninhydrin,they undergo oxidative deamination followed by decarboxylation to form a purple-colored compound known as Ruhemann's purple.
This color change allows for the detection of $\alpha$-amino acids.

(D) Globular proteins are proteins where the polypeptide chain is folded into a spherical shape.
They are found in various biological systems:
$1$. In blood,albumins and globulins are globular proteins.
$2$. In milk,caseinogen is a globular protein.
$3$. In egg white,albumin is a globular protein.
Therefore,all of the given options contain globular proteins.

(D) peptide linkage is a chemical bond formed between two molecules when the carboxyl group $(-COOH)$ of one molecule reacts with the amino group $(-NH_2)$ of the other molecule,releasing a molecule of water $(H_2O)$.
This is a condensation reaction and usually occurs between amino acids.
The resulting $-CO-NH-$ bond is called a peptide bond.

(C) Isoleucine is an essential amino acid with the $IUPAC$ name $2-\text{amino}-3-\text{methylpentanoic acid}$.
The structure consists of a pentanoic acid chain ($5$ carbons) with an amino group $(-NH_2)$ at the $C-2$ position and a methyl group $(-CH_3)$ at the $C-3$ position.
Breaking down the chain: The backbone is $CH_3-CH_2-CH(CH_3)-CH(NH_2)-COOH$.
Comparing this with the given options,the correct structural formula matches Option $C$.

(A) An amino acid is optically active if it contains a chiral carbon atom.
Glycine,with the structure $NH_2-CH_2-COOH$,has two hydrogen atoms attached to the $\alpha$-carbon,making it achiral.
Therefore,Glycine is the only optically inactive amino acid among the standard amino acids.

(B) The isoelectric point $(pI)$ of an amino acid is the $pH$ at which it exists as a zwitterion and has no net charge.
$1$. $(Amino \ acid)_{III}$ has $pI = 3.9$,which is acidic. This corresponds to an acidic amino acid like Aspartic acid.
$2$. $(Amino \ acid)_{I}$ has $pI = 6$,which is near neutral. This corresponds to a neutral amino acid like Glycine or Alanine.
$3$. $(Amino \ acid)_{II}$ has $pI = 9.8$,which is basic. This corresponds to a basic amino acid like Lysine,which contains an extra amino group.

(A) Alanine is $CH_3CH(NH_2)COOH$. When heated $(\Delta)$,it undergoes intermolecular dehydration to form a cyclic dipeptide known as a diketopiperazine. Specifically,two molecules of alanine condense to form $3,6$-dimethyl$-2,5-$diketopiperazine. Thus,the total number of products formed is $1$ (the cyclic dipeptide) plus water as a byproduct,but in the context of organic synthesis questions regarding the main organic product,the formation of the cyclic dimer is the primary result. However,strictly speaking,the reaction $2 \text{ Alanine} \xrightarrow{\Delta} \text{Diketopiperazine} 2H_2O$ yields the cyclic product and water. Given the options,the question asks for the number of organic products,which is $1$.

(D) An amino acid is achiral if it does not possess a chiral carbon atom.
In $Glycine$ $(NH_2-CH_2-COOH)$,the central carbon atom is bonded to two identical hydrogen atoms.
Since it lacks four different groups attached to the $\alpha$-carbon,it is achiral.

(A) In this reaction,the chlorine atom at the $\alpha$-position of the carboxylic acid is substituted by an amino group $(-NH_2)$ via nucleophilic substitution using ammonia $(NH_3)$.
$CH_3-CHCl-COOH + NH_3 \rightarrow CH_3-CH(NH_2)-COOH + HCl$
The resulting product $[X]$ is $2$-aminopropanoic acid (Alanine),which is an $\alpha$-amino acid.

(C) Aspartame ($L$-aspartyl-$L$-phenylalanine methyl ester) is unstable in aqueous solution.
On storage for extended periods,the amino group of the aspartic acid residue attacks the methyl ester of the phenylalanine residue.
This intramolecular reaction leads to the formation of a cyclic diketopiperazine ($3$-benzyl$-6-$carboxymethyl$-2,5-$diketopiperazine) and the release of methanol $(CH_3OH)$.

(C) Aspartame is the methyl ester of the dipeptide formed from aspartic acid and phenylalanine.
In an aqueous solution,the methyl ester group $(-COOCH_3)$ is susceptible to hydrolysis.
Upon standing in an aqueous medium,the ester group undergoes hydrolysis to form a carboxylic acid group $(-COOH)$,while the peptide bond remains intact under these conditions.
Therefore,the structure changes to the corresponding dipeptide,which is $L-aspartyl-L-phenylalanine$.

(C) Ninhydrin reacts with $\alpha$-amino acids to form a purple-colored complex known as Ruhemann's purple.
This reaction requires the presence of a free primary amino group ($-NH_2$ group).
Proline is an imino acid where the nitrogen atom is part of a five-membered pyrrolidine ring.
Because the nitrogen in proline is a secondary amine $(-NH-)$ rather than a primary amine,it reacts with ninhydrin to produce a yellow-colored product instead of the characteristic purple color.

(A) Alanine is an amino acid with the structure $CH_3-CH(NH_2)-COOH$.
$1$. At low $pH$ ($pH = 2$,acidic medium),the basic amino group $(-NH_2)$ accepts a proton to form a cation: $CH_3-CH(N^{+}H_3)-COOH$.
$2$. At high $pH$ ($pH = 10$,basic medium),the acidic carboxylic group $(-COOH)$ loses a proton to form an anion: $CH_3-CH(NH_2)-COO^{-}$.

(C) The amino acid $Arginine$ contains a guanidino group in its side chain.
This group is represented by the structure $H_2N-C(=NH)-NH-R$,which contains the $HN=C(NH_2)-$ linkage.
Therefore,the correct option is $(c)$.

(C) When concentrated $HNO_3$ comes in contact with the skin (which contains proteins),it causes the nitration of the aromatic amino acids present in the protein,such as tyrosine and tryptophan.
This reaction forms yellow-colored nitro compounds,a process known as the xanthoproteic reaction.
Therefore,the yellow stain on the skin is due to the nitration of proteins.

(B) An asymmetric carbon (or chiral center) is a carbon atom bonded to four different groups.
Glycine has the structure $H_2N-CH_2-COOH$.
In glycine,the central carbon atom is bonded to two identical hydrogen atoms,so it is not asymmetric (achiral).
All other amino acids listed,such as histidine,$\alpha-$ alanine,and threonine,contain at least one chiral carbon atom.
Therefore,the correct option is $B$.

(A) Amino acids consist of an amine group $(-NH_2)$ and a carboxyl group $(-COOH)$ attached to the same alpha-carbon atom.
An amino acid is optically active if the alpha-carbon is chiral,meaning it is bonded to four different groups.
Glycine,with the structure $H_2N-CH_2-COOH$,has two hydrogen atoms attached to the alpha-carbon.
Since the alpha-carbon is not bonded to four different groups,it is achiral.
Therefore,$Glycine$ is the only optically inactive amino acid among the $20$ common amino acids.

(B) Glycine is the simplest amino acid. Its structure is $H_2N-CH_2-COOH$. Since the $\alpha$-carbon is bonded to two identical hydrogen atoms,it is achiral and thus optically inactive.

(B) In the Fischer projection of $L$-amino acids,the $COOH$ group is placed at the top and the side chain ($R$ group) at the bottom. For $L$-alanine,the amino group $(-NH_2)$ must be on the left side of the vertical carbon chain.
$1$. The general structure of an amino acid is $R-CH(NH_2)-COOH$.
$2$. For $L$-alanine,$R = CH_3$ (methyl group,denoted as $Me$).
$3$. In the Fischer projection,placing $COOH$ at the top and $CH_3$ at the bottom,the $-NH_2$ group is placed on the left to represent the $L$-configuration.
$4$. Comparing this with the given options,option $A$ shows the $-NH_2$ group on the left,$COOH$ at the top,$H$ at the top (wait,let's re-examine the images).
Re-evaluating the images:
- Option $A$: $H$ is top,$Me$ is bottom,$H_2N$ is left,$COOH$ is right. This is not the standard orientation.
- Option $B$: $Me$ is top,$COOH$ is bottom,$H_2N$ is left,$H$ is right. This is the correct $L$-configuration where the amino group is on the left when the carbon chain is vertical.
Therefore,the correct structure is $B$.

(D) Amino acids are the building blocks of proteins,composed of $C, H, O, N$ and sometimes $S$.
They contain a carboxyl group and an amine group attached to the $\alpha$-carbon.
Amino acids can exist as zwitterions.
Aromatic amino acids are those that contain an aromatic ring (benzene ring) in their side chain.
Examples of aromatic amino acids include phenylalanine,tyrosine,and tryptophan.
Therefore,all the given options are correct.

(C) chiral center is a carbon atom bonded to four different groups.
In $Glycine$ $(NH_2-CH_2-COOH)$,the central carbon atom is bonded to two hydrogen atoms,one amino group $(-NH_2)$,and one carboxyl group $(-COOH)$.
Since two of the groups attached to the central carbon are identical (both are hydrogen atoms),it is not a chiral center.
Therefore,$Glycine$ is the only optically inactive amino acid among the options.

(C) The given compound is a dipeptide,Alanyl-glycine.
Upon hydrolysis of the peptide bond $(-CO-NH-)$,the molecule breaks into its constituent amino acids.
The two amino acids obtained are Alanine $(NH_2-CH(CH_3)-COOH)$ and Glycine $(NH_2-CH_2-COOH)$.

(D) Essential amino acids are those that cannot be synthesized by the human body and must be obtained through the diet.
Non-essential amino acids are those that can be synthesized by the body.
$Valine$,$Leucine$,and $Arginine$ are essential amino acids.
$Aspartic$ $acid$ is a non-essential amino acid because it can be synthesized by the body from oxaloacetate via transamination.

(D) peptide linkage is an amide bond formed between the carboxyl group $(-COOH)$ of one amino acid and the amino group $(-NH_2)$ of another amino acid.
$1$. Proteins are polymers of amino acids linked by peptide bonds.
$2$. Nylon-$66$ is a polyamide formed by the condensation polymerization of hexamethylenediamine and adipic acid,containing amide linkages $(-CONH-)$,which are chemically identical to peptide linkages.
$3$. Biurides (or biurets) are compounds containing the $-NH-CO-NH-CO-NH_2$ group,which also contain amide linkages.
Therefore,all the given options contain amide/peptide linkages.

(B) Aspartame is a methyl ester of a dipeptide formed from two amino acids: $L$-aspartic acid and $L$-phenylalanine.
Upon hydrolysis,the peptide bond and the ester bond are broken,yielding $L$-aspartic acid,$L$-phenylalanine,and methanol.
Therefore,the correct pair is $Aspartic \ acid + Phenylalanine$.

(C) peptide bond is formed by the condensation reaction between the carboxyl group $(-COOH)$ of one amino acid and the amino group $(-NH_2)$ of another amino acid.
During this reaction,a molecule of water $(H_2O)$ is eliminated.
The reaction can be represented as: $R-CH(NH_2)-COOH + R'-CH(NH_2)-COOH \rightarrow R-CH(NH_2)-CONH-CH(R')-COOH + H_2O$.

(D) Zwitter ion is a molecule that contains an equal number of positively and negatively charged functional groups,making it electrically neutral overall.
In the given options,the structure in option $D$ represents the dipolar form of sulfanilic acid,where the $-NH_3^+$ group and the $-SO_3^-$ group are present on the same molecule,which is a classic example of a Zwitter ion.
Options $A$,$B$,and $C$ are simple ionic salts,not Zwitter ions.