两性离子与等电点(Zwitterions and pI)1.pdf
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两性离⼦与等电点(Zwitterions andpI)
1. 两性离⼦就是具有双极性的离⼦(A zwitterion is a dipolar ion).
2. 氨基酸同时带有碱性的氨基和酸性的羧基,所以就具有两性解离的性 质,即可以以两性离⼦的形式存在(Amino acids are zwitterions, since they contain an amino group and a carboxylic group which can undergo an internal acid-base reaction).
3. 此外,肽、蛋白质、核苷酸和核酸也能够以两性离⼦的形式存在 (Besides, peptides, proteins, nucleotides and nucleic acids also can exist in zwitterions)
4. 具有两性解离性质的分⼦就具有等电点。等电点是⼀个特殊的pH ,在 这个pH 下相应的两性解离分⼦所带有的净电荷刚好是0 (The molecule with acid-base disassociation has a special pH, or isoelectric point- pI, at which its net charge is exactly zero). 在酸性溶液 In acid solution zwitterion protonated 两性离子 质子化 在碱性溶液 In base solution
zwitterion deprotonated 两性离子 去质子化 如何确定⼀种氨基酸的等电点 (Determination ofpI of an amino acid)
1. 实验测定- 滴定(By experiment-titration)
2. 计算(By calculatio