zwitterion

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ion

 [i´on]
an atom or group of atoms having a positive (cation) or negative (anion) electric charge by virtue of having gained or lost an electron; substances forming ions are called electrolytes. adj., adj ion´ic.
dipolar ion an ion that has both positive and negative regions of charge.
hydrogen ion the positively charged hydrogen atom (H+), which is the positive ion of all acids. See also hydrogen ion concentration.
hydroxyl ion the negatively charged group, OH, present to excess in alkaline solutions.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.
Zwitterionclick for a larger image
Fig. 318 Zwitterion . Molecular structure of (a) undissociated amino acid, (b) Zwitterion.

zwitterion

the chemical form of an AMINO ACID with a dipolar ion state. The pH at which the amino acid exists as a zwitterion is called its ISOELECTRIC POINT and at this pH it will not migrate in an electric field, produced, for example, by ELECTROPHORESIS.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005
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