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?
$P e p t i d e$
$R_{1}$
$R_{2}$
I
$H$
$H$
II
$H$
$C H_{3}$
III
$C H_{2} C O O H$
$H$
IV
$C H_{3} C O N H_{2}$
${(C H_{2})}_{4} N H_{2}$
V
$C H_{2} C O N H_{2}$
$C H_{2} C O N H_{2}$
VI
${(C H_{2})}_{4} N H_{2}$
${(C H_{2})}_{4} N H_{2}$
VII
$C H_{2} C O O H$
$C H_{2} C O N H_{2}$
VIII
$C H_{2} O H$
${(C H_{2})}_{4} N H_{2}$
IX
${(C H_{2})}_{4} N H_{2}$
$C H_{3}$

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Solution

The correct option is B 4
Four peptides (IV, VI, VIII and IX) are positively charged at $p H 7.0$ .
These peptides have isoelectric point (pI) more than $7$ . In the neutral solution $(p H = 7)$ , they exist as cation. Hence, the given peptide is of basic nature. Hence, it must contain two or more amino groups.

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