zwit·ter·i·on 
(zwĭtər-ī′ən, swĭt-, tsvĭt-)
(zwĭtər-ī′ən, swĭt-, tsvĭt-)n.
A molecule, especially an amino acid, containing a positively charged ion at one end and a negatively charged ion at the other.
[German : Zwitter, hybrid (from Middle High German zwitarn, from Old High German, from zwi-, twice; see dwo- in the Appendix of Indo-European roots) + Ion, ion (from Greek; see ION).]
zwit′ter·i·onic (-ī-ŏnĭk) adj.