Molecules consist of atoms chemically bonded together that act as a unit. They are electrically balanced, meaning they contain a total number of electrons equal to the total number of protons. If a molecule did not possess an equal number of electrons and protons, it would be termed an ion. Although molecules are neutral as a whole, there may be one or more local charges (molecular polarizations) resulting from molecular geometry or from atomic electronegativities. These charges rarely approach the unit charge of a proton or of an electron. There is one notable exception to this – the zwitterion. The zwitterion contains a full, formal positive and a full, formal negative charge at different locations within the same molecule.
Even as the human body consists of an assortment of cells (be they bone, muscle, or blood) and yet may be divided by function (into head, limbs, or digits), so molecules consist of a variety of atoms and yet may be divided according to functionality. In organic (carbon-based) chemistry there may be the “alkyl” backbone of carbon (C) and hydrogen (H) atoms. Other groups, called pendant groups, include hydroxyl, amino, and carboxylic groups (‐OH, ‐NH2, and ‐COOH), respectively. It is the chemistry of the individual pendant groups that leads so directly to the formation of the zwitterion.
Example Formation of a Zwitterion
One very simple example of how a near-neutral molecule can become a zwitterion is the amino acid glycine, NH2‐CH2‐COOH. Note the amino group shown to the far left, the carboxylic group to the far right, and the simple carbon and hydrogen, alkyl skeleton, consisting of a single ‐CH2‐ (methylene) group. This molecule does not exist stretched out in straight-line form. The bonds are at angles; in addition, the molecules are dynamic – able to twist and turn. The image associated with this article demonstrates how the molecule facilitates the transfer of a hydrogen atom resulting in the formation of a zwitterion.1 Written out in the style above, the zwitterion formed is +NH3‐CH2‐COO−. Thus a full positive charge, equivalent to that of a proton, is present at the left end of the molecule, while a full negative charge, equivalent to that of an electron, is present to the right. The total charge remains at zero. Thus the zwitterion possesses the distinction of being a double ion and a molecule.
Properties and Applications of the Zwitterion
Charge of the zwitterion encourages solubility in polar solvents, such as water. Since amino acids and certain other compounds found in nature form zwitterions, it is apparent that such solubility is important. In addition, the folding of long-chain or oligomeric molecules is considerably affected by electrical charge. An area of intense ongoing research is that of zwitterion transport. Outside the field of biochemistry, research into the zwitterion is even being used in conjunction with carbon nanotube and fullerene research.
1 Glycine forms the zwitterion when dissolved in water. Water lowers the energy required to form the double ion. For amino acids in general, the energy increase associated with carrying a negative charge is lessened by being “spread out” over the two oxygen atoms (see associated image). Such spreading out is called “resonance.”