肽键Peptide bond

氮上的孤对电子与羰基共轭，使 C–N 键具有约 40% 的双键性质，键长 133 pm，介于单键（147 pm）和双键（127 pm）之间。因此肽键不能自由旋转，构成肽键的六个原子（Cα、C、O、N、H、Cα）共处一个平面，称为肽平面，几乎总是反式构型（脯氨酸前的肽键约 5–10% 为顺式）。主链的构象只能通过 φ（N–Cα）和 ψ（Cα–C）两个二面角变化，拉氏图描述了允许的组合。

The nitrogen lone pair is conjugated with the carbonyl, giving the C–N bond about 40% double-bond character and a length of 133 pm, between a single (147 pm) and a double bond (127 pm). The bond therefore does not rotate freely, and the six atoms Cα, C, O, N, H and Cα lie in one plane, the peptide plane, almost always trans (about 5–10% of bonds before proline are cis). Backbone conformation can change only through the dihedral angles φ (N–Cα) and ψ (Cα–C), whose allowed combinations are mapped by the Ramachandran plot.

1902 年埃米尔·费歇尔和霍夫迈斯特同时提出蛋白质中氨基酸以酰胺键连接，费歇尔创造了 peptide（肽）一词，并于 1901 年合成了第一个二肽甘氨酰甘氨酸。1930 年代鲍林和科里通过 X 射线研究确定了肽键的平面性。1963 年梅里菲尔德发明固相肽合成，获 1984 年诺贝尔化学奖。

In 1902 Emil Fischer and Franz Hofmeister independently proposed that amino acids in proteins are linked by amide bonds; Fischer coined the word peptide and had made the first dipeptide, glycylglycine, in 1901. In the 1930s and 1940s Pauling and Corey established the planarity of the peptide bond by X-ray studies. In 1963 Merrifield invented solid-phase peptide synthesis, earning the 1984 Nobel Prize in Chemistry.

肽键水解在热力学上有利但动力学上极慢，在中性水中半衰期可达数百年，所以蛋白质很稳定。体内由核糖体的肽酰转移酶中心（本身是 rRNA 核酶）催化形成肽键，由蛋白酶（如胰蛋白酶、胃蛋白酶）催化水解。实验室合成需要用 DCC、HATU 等缩合剂活化羧基并使用保护基。

Peptide-bond hydrolysis is thermodynamically favourable but kinetically very slow, with half-lives of centuries in neutral water, which is why proteins are stable. In cells the peptidyl transferase centre of the ribosome, itself an rRNA ribozyme, forms peptide bonds, and proteases such as trypsin and pepsin hydrolyse them. Laboratory synthesis activates the carboxyl with coupling agents such as DCC or HATU and uses protecting groups.