二级结构Protein secondary structure

α 螺旋中第 i 个残基的 C=O 与第 i+4 个残基的 N–H 形成氢键，每圈 3.6 个残基、螺距 0.54 nm。β 折叠由伸展的链并排排列，相邻链间形成氢键，可为平行或反平行。β 转角通常由 4 个残基组成，使肽链折回 180°，常含甘氨酸和脯氨酸。脯氨酸缺少 N–H 且环状结构限制了二面角，常是螺旋的“破坏者”。

In the α-helix the C=O of residue i hydrogen-bonds to the N–H of residue i+4, with 3.6 residues and 0.54 nm per turn. β-Sheets are extended strands lying side by side and hydrogen-bonded to their neighbours, parallel or antiparallel. β-Turns, usually of four residues, reverse the chain by 180° and often contain glycine and proline. Proline, lacking an N–H and restricted by its ring, often breaks helices.

1930 年代阿斯特伯里用 X 射线研究羊毛和丝，发现两种不同的纤维图谱（α 型与 β 型）。1951 年鲍林、科里和布兰森根据肽键平面性和最佳氢键几何，在《美国科学院院刊》上提出 α 螺旋和 β 折叠模型，次年被佩鲁茨的实验证实。1963 年拉马钱德兰绘制出允许的主链二面角图。

In the 1930s Astbury used X-rays on wool and silk and found two distinct fibre patterns, α and β. In 1951 Pauling, Corey and Branson, using the planarity of the peptide bond and ideal hydrogen-bond geometry, proposed the α-helix and β-sheet in the Proceedings of the National Academy of Sciences; Perutz confirmed the helix experimentally the next year. In 1963 Ramachandran mapped the allowed backbone dihedral angles.

圆二色光谱可快速估计蛋白质中 α 螺旋（208、222 nm 负峰）和 β 折叠（218 nm 负峰）的比例；红外光谱的酰胺 I 带（1600–1700 cm⁻¹）也反映二级结构。精确结构由 X 射线晶体学、核磁共振和冷冻电镜测定。1974 年周-法斯曼法开启了由序列预测二级结构的研究。

Circular dichroism quickly estimates α-helix (negative bands at 208 and 222 nm) and β-sheet (negative band at 218 nm) content, and the infrared amide I band (1600–1700 cm⁻¹) also reflects secondary structure. Exact structures come from X-ray crystallography, NMR and cryo-electron microscopy. The Chou–Fasman method of 1974 began the prediction of secondary structure from sequence.