Biomolecules and Chemistry in Everyday Life - Result Question 31

####32. 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 $pH=7.0$ ?

(2012)

Peptide $\boldsymbol{R} _1$
I $H$ $R-CO-NH-CH-COO$
II $H$ $R$
III $CH _2 COOH$ $H$
IV $CH _2 CONH$ $CH$
V $CH _2 CONH _2$ $\left(CH _2\right) _4 NH _2$
VI $\left(CH _2\right) _4 NH _2$ $\left(CH _2 CONH _2\right) _4 NH _2$
VII $CH _2 COOH _2$ $CH _2 CONH _2$
VIII $CH _2 OH _2$ $\left(CH _2\right) _4 NH _2$
IX $\left(CH _2\right) _4 NH _2$ $CH _3$
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Solution:

  1. The amino acid remain completely in Zwitter ionic form at its isoelectric point. Amino acids with additional acidic group have their isoelectric $pH$ less than 7.0 and increasing $pH$ above isoelectric point makes them anionic.

On the other hand, amino acids with additional basic group have their isoelectric $pH$ greater than 7.0 and decreasing $pH$ below isoelectric point (by adding acid solution) makes them cationic. The given peptide with followings $R _1$ and $R _2$ are basic, will remain protonated (cationic) at $pH=7.0$.

Peptide $\boldsymbol{R} _{\mathbf{1}}$ $\boldsymbol{R} _{\mathbf{2}}$
IV $CH _2 CONH _2$ $\left(CH _2\right) _4 NH _4$
VI $\left(CH _2\right) _4 NH _2$ $\left(CH _2\right) _4 NH _4$
VIII $CH _2 OH$ $\left(CH _2\right) _4 NH _4$
IX $\left(CH _2\right) _4 NH _2$ $CH _3$

Thus, 4 is the correct integer.



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