等电点Isoelectric point

氨基酸在水中以两性离子 H₃N⁺–CHR–COO⁻ 形式存在。pH 低于 pI 时带正电，高于 pI 时带负电。对只有一个氨基和一个羧基的氨基酸，pI = (pKa₁ + pKa₂)/2，如甘氨酸 pI = (2.34 + 9.60)/2 = 5.97。酸性氨基酸（天冬氨酸 pI ≈ 2.8）pI 较低，碱性氨基酸（赖氨酸 pI ≈ 9.7、精氨酸 ≈ 10.8）pI 较高。蛋白质的 pI 由所有可电离侧链共同决定。

In water an amino acid exists as the zwitterion H₃N⁺–CHR–COO⁻. Below its pI it is positively charged and above it negatively charged. For an amino acid with one amino and one carboxyl group, pI = (pKa₁ + pKa₂)/2; for glycine pI = (2.34 + 9.60)/2 = 5.97. Acidic amino acids (aspartic acid pI ≈ 2.8) have low pI values and basic ones (lysine ≈ 9.7, arginine ≈ 10.8) high values. A protein's pI is set by all its ionisable side chains together.

20 世纪初哈迪研究胶体粒子在电场中的迁移，发现存在不迁移的 pH。1910 年代米凯利斯系统研究了蛋白质和酶的等电点，并发展了缓冲溶液方法。1960 年代斯文松和维斯特贝里发展了载体两性电解质等电聚焦技术，可按 pI 差 0.01 的精度分离蛋白质。

Around 1900 Hardy, studying how colloidal particles move in an electric field, found a pH at which they do not move. In the 1910s Michaelis studied the isoelectric points of proteins and enzymes systematically and developed buffer methods. In the 1960s Svensson and Vesterberg developed isoelectric focusing with carrier ampholytes, which separates proteins differing in pI by as little as 0.01.

等电聚焦和双向电泳（第一向按 pI、第二向按分子量）是蛋白质组学的基础技术。调节 pH 至等电点可沉淀蛋白质，如酪蛋白在 pH 4.6 沉淀，这是制作奶酪和酸奶凝固的原理；工业上用等电点沉淀法提取谷氨酸。胶体与表面化学中，氧化物的零电荷点决定其吸附行为。

Isoelectric focusing and two-dimensional electrophoresis (pI in the first dimension, mass in the second) are basic tools of proteomics. Adjusting the pH to the pI precipitates proteins: casein precipitates at pH 4.6, the basis of cheese and yoghurt curdling, and glutamic acid is recovered industrially by isoelectric precipitation. In colloid and surface chemistry, an oxide's point of zero charge governs its adsorption behaviour.