等电点( p I)
K1K2=[H+]2[A-]/[A+], 等电点时,[A+]=[A-]
pI = (pK1+pK2) /2
pH = pI, 氨基酸净电荷为零 pH > pI, 氨基酸带负电荷 pH < pI, 氨基酸带正电荷