Zwitterions are ions that have no overall net charge, with the name coming from the German meaning 'hybrid ion'. Other common names to which it is sometimes referred to are 'dipolar salts' or 'inner salts'. 
The best example we have to show this are free amino acids living in cells, the primary amine group and carboxylate group that are both joined to the alpha carbon interact to obtain the property of no overall charge. This occurs when the amino acidis in a solution or environment that has a pH to match its isoelectric point, this point changes between molecules, depending on the prime function.  At this point a proton transfer occurs between the amine and carboxylate groups, that results in a change of –COOH to –COO − and the –NH 2 to–NH 3 +. The positive and negative charge on each of the groups therefore cancel out and create the electrostatic balance within the molecule.
- ↑ Matthew A. Fisher http://www.chemistryexplained.com/Va-Z/Zwitterions.html#ixzz3KLsP7CpK
- ↑ Weng Chung, https://www.physicsforums.com/threads/zwitterion-and-isoelectric-point.523370/
- ↑ Matthew A. Fisher http://www.chemistryexplained.com/Va-Z/Zwitterions.html
- ↑ Taylor, D. J.. Biological science. 3rd ed. Cambridge: Cambridge University Press, 1997 http://kimberlybiochemist.wordpress.com/tag/zwitterion/