Zwitterion

Zwitterion designates molecules containing both positive and negative charges at neutral pH while maintaining overall electrical neutrality, exhibiting unique physicochemical properties including high water solubility, minimal migration in electric fields at isoelectric points, and distinctive crystallisation behaviours. These amphoteric compounds include amino acids, certain drugs, and biological molecules, with charge state depending on pH through protonation equilibria.

The pharmaceutical industry encounters zwitterions frequently throughout drug development, formulation, and analysis. Many drugs exhibit zwitterionic character affecting formulation approaches, stability, and bioavailability. Zwitterionic drugs often demonstrate pH-dependent solubility with minimum solubility near isoelectric points, creating formulation challenges requiring appropriate pH selection or salt forms. Membrane permeability typically proves lower for zwitterions compared to neutral species, though transporters may facilitate uptake. Formulation scientists must understand zwitterionic behaviour optimising pH, selecting appropriate salts, or employing permeation enhancers. Analytical methods including electrophoresis exhibit characteristic zwitterion migration patterns useful for separations. Salt formation proves challenging with zwitterions potentially forming multiple salt species. As small molecule diversity increases and understanding improves regarding pH effects on drug properties, considering zwitterionic character remains important throughout development from early candidate selection through formulation ensuring optimal pharmaceutical properties supporting effective therapeutic delivery.