α螺旋Alpha helix

每圈 3.6 个残基，每个残基沿轴上升 0.15 nm，螺距 0.54 nm，主链二面角约 φ = −57°、ψ = −47°。所有主链 N–H 和 C=O 都在螺旋内部形成氢键（两端除外），因此结构稳定。所有肽键偶极同向排列，使螺旋具有宏观偶极（N 端带部分正电），常与带负电的配体或磷酸基作用。天然蛋白质中的 α 螺旋几乎都是右手螺旋，因为 L-氨基酸的侧链在左手螺旋中会与主链发生空间冲突。

There are 3.6 residues per turn, a rise of 0.15 nm per residue and a pitch of 0.54 nm, with backbone dihedral angles of about φ = −57° and ψ = −47°. Every backbone N–H and C=O inside the helix, except at the ends, is hydrogen-bonded, which makes it stable. All peptide dipoles point the same way, giving the helix a macroscopic dipole with a partly positive N-terminus that often binds negatively charged ligands or phosphates. Natural α-helices are almost always right-handed, because L-amino acid side chains would clash with the backbone in a left-handed helix.

1948 年鲍林在牛津讲学期间因感冒卧床，用纸条画出平面肽链并折叠，找到了满足氢键要求的螺旋。1951 年他与科里、布兰森正式发表 α 螺旋模型，鲍林因化学键研究获 1954 年诺贝尔化学奖。1952 年佩鲁茨在合成多肽中观测到 0.15 nm 的特征衍射，证实了该模型。

In 1948, laid up with a cold while lecturing in Oxford, Pauling drew a flat peptide chain on a strip of paper and folded it until he found a helix with satisfied hydrogen bonds. He, Corey and Branson published the α-helix model in 1951, and Pauling received the 1954 Nobel Prize in Chemistry for his work on the chemical bond. In 1952 Perutz saw the characteristic 0.15 nm reflection in synthetic polypeptides, confirming the model.

丙氨酸、亮氨酸、谷氨酸、甲硫氨酸倾向形成螺旋，脯氨酸和甘氨酸不利于螺旋。毛发和指甲中的 α-角蛋白由两条 α 螺旋缠绕成卷曲螺旋；膜蛋白的跨膜区通常是约 20 个疏水残基构成的 α 螺旋；DNA 结合蛋白常用“识别螺旋”嵌入 DNA 大沟。两亲性螺旋一侧亲水一侧疏水，常位于蛋白质表面。

Alanine, leucine, glutamate and methionine favour helices, while proline and glycine disfavour them. α-Keratin in hair and nails is two α-helices wound into a coiled coil; transmembrane segments of membrane proteins are usually α-helices of about 20 hydrophobic residues; DNA-binding proteins often insert a 'recognition helix' into the major groove. Amphipathic helices, hydrophilic on one face and hydrophobic on the other, often lie on protein surfaces.