Question

An amino acid under certain conditions have both positive and negative charges simultaneously in the same molecule. Such a form of’ amino acid is called

• A. acidic form
• B. basic form
• C. aromatic form
• D. zwitterionic form

Answer 1

Answer: (D)

In neutral solution, an amino acid molecule exists as a dipolar ion (zwitter ion), i.e., molecule containing positive and negative ionic groups. The charge on the ion changes with $pH$. In an acidic solution (low $pH$) the amino acid picks up $H^{+}$ions and becomes positively charged. In alkaline solution (high $pH$) the amino acid donates $H^{+}$ions to the medium and becomes negatively charged. The $pH$ at which the amino acid is electrically neutral or the molecule exists as a zwitterion is called the isoelectric $pH$.