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 $pH =7.0$ ? $Image$

Peptide $R _1$ $R _2$

$I$ $H$ $H$

$II$ $H$ $CH _3$

$III$ $CH _2 COOH$ $H$

$IV$ $CH _2 CONH _2$ $( CH _2)_4 NH _2$

$V$ $CH _2 CONH _2$ $CH _2 CONH _2$

$VI$ $( CH _2)_4 NH _2$ $( CH _2)_4 NH _2$

$VII$ $CH _2 COOH$ $CH _2 CONH _2$

$VIII$ $CH _2 OH$ $( CH _2)_4 NH _2$

$IX$ $( CH _2)_4 NH _2$ $CH _3$

Q: 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 $pH =7.0$ ? $Image$ Peptide $R _1$ $R _2$ $I$ $H$ $H$ $II$ $H$ $CH _3$ $III$ $CH _2 COOH$ $H$ $IV$ $CH _2 CONH _2$ $( CH _2)_4 NH _2$ $V$ $CH _2 CONH _2$ $CH _2 CONH _2$ $VI$ $( CH _2)_4 NH _2$ $( CH _2)_4 NH _2$ $VII$ $CH _2 COOH$ $CH _2 CONH _2$ $VIII$ $CH _2 OH$ $( CH _2)_4 NH _2$ $IX$ $( CH _2)_4 NH _2$ $CH _3$

a For the polypeptide the isoelectric point will be more than $7$ . That means the given polypeptide is of basic nature so it must contain two or more amino groups. So $(iv), (vi), (viii)$ and $(ix)$ are the correct options.

Question

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 $pH =7.0$ ? $Image$

Peptide	$R _1$	$R _2$
$I$	$H$	$H$
$II$	$H$	$CH _3$
$III$	$CH _2 COOH$	$H$
$IV$	$CH _2 CONH _2$	$( CH _2)_4 NH _2$
$V$	$CH _2 CONH _2$	$CH _2 CONH _2$
$VI$	$( CH _2)_4 NH _2$	$( CH _2)_4 NH _2$
$VII$	$CH _2 COOH$	$CH _2 CONH _2$
$VIII$	$CH _2 OH$	$( CH _2)_4 NH _2$
$IX$	$( CH _2)_4 NH _2$	$CH _3$

A$4$
B$5$
C$6$
D$7$

IIT 2012, Advanced

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