What Is a Zwitterion?
A zwitterion is a molecule that contains an equal number of positive and negative functional groups, resulting in an overall electrical charge of zero. Despite being neutral as a whole, it has localized positive and negative charges on different parts of its structure, making it a dipolar ion.
Section 2: Key Principles
Zwitterions are typically formed from compounds that have both an acidic group and a basic group. The acidic group, such as a carboxyl group (-COOH), can lose a proton (H+) to become negatively charged (-COO⁻). Simultaneously, the basic group, like an amino group (-NH₂), can gain a proton to become positively charged (-NH₃⁺).
Section 3: A Practical Example of a Zwitterion
The most common examples of zwitterions are amino acids. In a solution with a neutral pH, the carboxyl group of an amino acid donates a proton, and the amino group accepts it. For example, the amino acid alanine exists as H₃N⁺-CH(CH₃)-COO⁻, a zwitterion with a positive charge on the amino group and a negative charge on the carboxyl group, but a net charge of zero.
Section 4: Importance in Biology and Chemistry
The zwitterionic nature of amino acids is fundamental to protein structure and function. It explains their high melting points and solubility in water. It also allows amino acids to act as buffers, resisting changes in pH, which is a critical function in biological systems like blood.