Zwitterion
Definition and meaning of Zwitterion in chemistry.
A zwitterion is a neutral molecule that contains both a positive and a negative electrical charge at different locations within its structure. Amino acids exist as zwitterions in neutral aqueous solutions like the human bloodstream.
In more detail
The unusual term zwitterion originates from the German word zwitter, meaning hybrid or hermaphrodite, which perfectly describes a unique molecule harboring two opposing charges simultaneously. Unlike typical classical ions, which are entirely positive or entirely negative, a true zwitterion has a net electrical charge of exactly zero.
This fascinating duality occurs because the molecule possesses specific functional groups that can both donate and accept hydrogen protons. In an aqueous environment, these functional groups interact dynamically with the surrounding water, causing a rapid internal transfer of a proton from one side of the molecule to the other.
Amino acids, the fundamental biological building blocks of all proteins, are the most famous zwitterions. Every standard amino acid has a carboxylic acid group, which is naturally acidic, and an amine group, which is naturally basic. When dissolved in pure water at a neutral pH, the highly acidic carboxylic group readily donates a proton, instantly becoming a negatively charged carboxylate ion.
At the exact same time, the basic amine group eagerly accepts that exact proton, becoming a positively charged ammonium ion. The entire amino acid molecule now features two full electrical poles, yet remains completely neutral overall. The specific pH value at which a molecule exists perfectly as a neutral zwitterion is scientifically known as its isoelectric point.
If the surrounding solution becomes highly acidic, the negative pole will accept a proton and the zwitterion will become a regular positive cation. Conversely, in a highly basic solution, the positive pole will rapidly lose its extra proton, turning the molecule into a regular negative anion.
This unique ability to alter its charge based on environmental pH dictates exactly how proteins fold and function.
Key facts
| Field | Biochemistry |
|---|---|
| Net Charge | Zero |
| Contains | Positive and Negative Poles |
| Classic Example | Amino Acids |
| Key Condition | Isoelectric Point |
| Origin Word | German for hybrid |
| Environmental Factor | pH Level |
At a pH of 7, the amino acid glycine exists perfectly as a zwitterion, possessing a positively charged nitrogen end and a negatively charged oxygen end.
Frequently asked questions
Are zwitterions considered polar molecules?
Yes, despite having a perfectly neutral overall charge, the distinct positive and negative poles make them highly polar molecules that dissolve extremely well in water.
Can any molecule be a zwitterion?
No, a molecule must specifically contain both an acidic functional group to donate a proton and a basic functional group to successfully accept it.
What happens to an amino acid in stomach acid?
In a highly acidic environment, the negative carboxylate pole accepts an extra proton, neutralizing itself and turning the entire molecule into a positive cation.