Zwitterion An ion with both a positive and a negative charge. Amino acids and form zwitterions: the amino group has the form-NH3+ and the acid group is ionized as-COO-.

• 两性离子：既带有正电荷又带有负电荷的离子。氨基酸就是这样一种离子：氨基基团为-NH3+而羧基端为-COO-。