Zwitterion
A molecule carrying both a positive and a negative charge but no net overall charge.
Formula / equation
⁺H₃N–CHR–COO⁻
Unit
Dimensionless
In depth
Amino acids exist as zwitterions at physiological pH because the carboxyl group donates its proton to the amino group. This dual ionic character explains their high melting points, water solubility and negligible solubility in organic solvents, and lets them act as buffers.
Examples in the real world
Glycine melting above 230 °C rather than the low temperature its small size would suggest.