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Question
How do you explain the amphoteric behaviour of amino acids?
Solution
In aqueous solution, the carboxyl group of an amino acid can lose a proton and the amino group can accept a proton to give a dipolar ion known as zwitter ion.
\[\begin{array}{cc}
\phantom{.........}\ce{O}\phantom{........................}\ce{O}\phantom{....}\\
\phantom{......}||\phantom{.........................}||\phantom{.}\\
\ce{R - CH - C - O - H <=> R - CH - C - O-}\\
|\phantom{.........................}|\phantom{........}\\
\ce{:NH2}\phantom{..................}\ce{\underset{(Zwitter ion)}{^+NH3}}\phantom{.....}
\end{array}\]
Therefore, in zwitter ionic form, the amino acid can act both as an acid and as a base.
\[\begin{array}{cc}
\phantom{.....}\ce{O}\phantom{.....................}\ce{O}\phantom{......................}\ce{O}\\
\phantom{.....}||\phantom{......................}||\phantom{......................}||\\
\ce{R - CH - C - O- <=>[H+][OH-] R - CH - C - O- <=>[H+][OH-] R - CH - C - OH}\\
|\phantom{......................}|\phantom{.......................}|\phantom{........}\\
\ce{NH2}\phantom{.................}\ce{^+NH3}\phantom{..................}\ce{^+NH3}\phantom{.....}
\end{array}\]
Thus, amino acids show amphoteric behaviour.
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Define the following term as related to proteins:
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(i)
\[\begin{array}{cc}
\ce{(CH3)2CH - CH - COOH}\\
\phantom{..}|\\
\phantom{.....}\ce{NH2}
\end{array}\]
(ii)
\[\begin{array}{cc}
\ce{HOOC - CH2 - CH2 - CH - COOH}\\
\phantom{............}|\\
\phantom{...............}\ce{NH2}
\end{array}\]
(iii)
\[\ce{H2N - CH2 - CH2 - CH2 - COOH}\]
(iv)
\[\begin{array}{cc}
\ce{HOOC - CH2 - CH - COOH}\\
\phantom{.....}|\\
\phantom{........}\ce{NH2}
\end{array}\]
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