(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

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Amino acid,cysteine easily undergoes dimerization due to the presence of a free $-SH$ group.

(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.

Class 12 Chemistry Biomolecules Amino Acids and Proteins

Medium

Identify the correct statement among the following $:$

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A
All naturally occurring amino acids except glycine contain one chiral centre.
B
All naturally occurring amino acids are optically active.
C
Glutamic acid is the only amino acid that contains a $-COOH$ group at the side chain.
D
Amino acid,cysteine easily undergoes dimerization due to the presence of a free $-SH$ group.

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Biomolecules

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Amino Acids and Proteins

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The correct structure of Rhumann's Purple,the compound formed in the reaction of ninhydrin with proteins is :

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The substituents $R_1$ and $R_2$ for nine peptides are listed in the table given below. How many of these peptides are positively charged at $pH = 7.0$?
Peptide $R_1$ $R_2$
$I$ $H$ $H$
$II$ $H$ $CH_3$
$III$ $CH_2COOH$ $H$
$IV$ $CH_2CONH_2$ $(CH_2)_4NH_2$
$V$ $CH_2CONH_2$ $CH_2CONH_2$
$VI$ $(CH_2)_4NH_2$ $(CH_2)_4NH_2$
$VII$ $CH_2COOH$ $CH_2CONH_2$
$VIII$ $CH_2OH$ $(CH_2)_4NH_2$
$IX$ $(CH_2)_4NH_2$ $CH_3$

Peptide	$R_1$	$R_2$
$I$	$H$	$H$
$II$	$H$	$CH_3$
$III$	$CH_2COOH$	$H$
$IV$	$CH_2CONH_2$	$(CH_2)_4NH_2$
$V$	$CH_2CONH_2$	$CH_2CONH_2$
$VI$	$(CH_2)_4NH_2$	$(CH_2)_4NH_2$
$VII$	$CH_2COOH$	$CH_2CONH_2$
$VIII$	$CH_2OH$	$(CH_2)_4NH_2$
$IX$	$(CH_2)_4NH_2$	$CH_3$