蛋白质的二级结构

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In the early 1930s, William Astbury showed that there were drastic changes in the X-ray fiber diffraction of moist wool or hair fibers upon significant stretching. The data suggested that the unstretched fibers had a coiled molecular structure with a characteristic repeat of ~5.1 Å (= 0.51 nm).
②非键合原子间的最小接触距离
在相邻的两个肽单位的构象中，非键合原 子间的接近有无障碍，是否符合标准接触距 离，即能量是否达到最低，也是肽链构象能 否稳定存在的重要立体化学原则。
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Kaj Ulrik Linderstrø m-Lang (November 29, 1896 - May 25, 1959) was a Danish protein scientist, who was the director of the Carlsberg Laboratory （嘉士伯实验室） from 1939 until his death.
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history 如图所示。 1、肽单位中碳基碳原子和氮原子之间所成的键(肽键)的键长为 0.32nm。这个键长介于单键C—N(0.149nm)和双键C=N（0.127间， 具有部分双键的性质，是刚性的。 2、相反，α -碳原子与羰基碳原子之间是单键，因此。α -碳原子 与氮原于之间也是一个纯粹的单键， 因此，在刚性的肽单位两侧的这些键具有充分转动自由。围绕这 两个键进行的转动用角φ和ψ来代表。Φ代表绕Cα一C单链的转动； ψ代表绕Cα-N单链的转动。如果每一个氨基酸残基的φ和ψ已知， 多肽主链的构象就完全确定。
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Linus Pauling, Robert Corey and Herman Branson in 1951 developed the α-helix and the β-strand (Astbury's nomenclature was kept)
Linus Pauling and Robert Corey (A) and Herman Branson (B). Pauling's deep understanding of chemical structure and bonding, his retentive memory for details, and his creative flair were all factors in in the discovery of the α-helix. Robert Corey was a dignified and shy x-ray crystallographer with the knowhow and patience to work out difficult structures, providing Pauling with the fundamental information he needed. Herman Branson was a physicist on leave at the California Institute of Technology, who was directed by Pauling to find all helices consistent with the rules of structural chemistry that he and Corey had determined.
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history
history
二面角（dihedral angle）
肽平面1 围绕 Cα 2—N1 单键旋 转，其旋转的角 度用Φ 表示； 肽平面2 也可 以围绕 Cα 2—C2 单键旋转，其旋 转的角度用Ψ 表 示。
history
多肽链主链骨架的构象是由每个Cα的成 对二面角（Φ ，Ψ ）所决定的。
蛋白质的二级结构
The Secondary Structure of Protein
history 蛋白质构象研究的开创人鲍林(Pauling)和科里(Corey)在30年代后 期的研究中提出了一个重要的结论： 即蛋白质肽单位的刚性和共面性。
R1 O C H N C α H H
R2 O N C OH C α H H
Definition of secondary structure 蛋白质的二级结构是蛋白质中肽链骨架中局部肽段的稳定构象。 它们是完整肽链构象（三级结构）的结构单元，是蛋白质复 杂结构的空间构象的基础——构象单元
Classification of secondary structure 1、regular规正的二级结构 包括α-螺旋（α-helix）和其他形式的螺旋、β-折叠链（β-pleated strand） 2、partially regular部分规正的二级结构 转角reverse turn，Ω环型(Ω-loops) 3、nonregular-“无规”卷曲
310 helix
310螺旋每圈含三个残基，在氢键的供体和受体之间有十个原子， 由此而得名。
ቤተ መጻሕፍቲ ባይዱ
The amino acids in a 310-helix are arranged in a right-handed helical structure. Each amino acid corresponds to a 120° turn in the helix (i.e., the helix has three residues per turn), and a translation of 2.0 Å (= 0.2 nm) along the helical axis. Most importantly, the N-H group of an amino acid forms a hydrogen bond with the C = O group of the amino acid three residues earlier; this repeated i + 3 → i hydrogen bonding defines a 310helix.
α-螺旋（α-helix）和其他形式的螺旋 α-螺旋
α-螺旋（α-helix）和其他形式的螺旋
有 極 性
α-螺旋（α-helix）和其他形式的螺旋
α螺旋可随其卷曲的松紧而有所改变。这种改变是由于第 n个残基 与第n+5个或第n+3个残基的氢键，取代了第n个残基与第n+4个残 基的氢键。
α-螺旋（α-helix）和其他形式的螺旋
His most notable scientific contributions were： the development of sundry physical techniques to study protein structure and function ，especially （氢-氘置换） hydrogen-deuterium exchange， his definitions of protein primary, secondary, tertiary and quaternary structure.
Classification of secondary structure 1、规正的二级结构 形成的原因： (1) Peptide bond 不能转动→ Peptide bond 平面 (2) 一个氨基酸R 基团与前后R 基团的限制→ Peptide bond 平面不能任意转动 (3) R 基团的大小、电荷限制→ 只做规律折迭 → α Helix, β Sheet Ramachandron plot (4) 稳定二级构造的力量： 氢键
α-螺旋（α-helix）和其他形式的螺旋 α-螺旋
● α-Helix(螺旋)结构要点：
(1) Right handed (右手旋)
(2) 每3.6氨基酸绕一圈，每圈5.4 Å 高 (3) Carbonyl (C=O) 与下游H-N- 生成氢键
(4) 每个氢键以13 个原子夹着(α13)
(5) 氢键与螺旋长轴基本平行。 (6) 整个α-helix 呈圆筒状，且有偶极性 (7)肽链中氨基酸侧链R，分布在螺旋外侧，其形状、大 小及电荷影响α-螺旋的形成。
Classification of secondary structure
α-helix
β-sheet
α-螺旋（α-helix）和其他形式的螺旋 α-螺旋 Pauling 和Corey于1951 年提出蛋白质的α-螺旋（α-helix）结构 模型。
α-螺旋（α-helix）和其他形式的螺旋 α-螺旋
history
Linus Pauling, Robert Corey and Herman Branson in 1951 developed the α-helix and the β-strand (Astbury's nomenclature was kept)
The wooden helix between Pauling and Corey has a scale of 1 inch per Å, an enlargement of 254,000,000 times. (A) Courtesy of the Archives, California Institute of Technology. (B) Courtesy of the Lincoln University of Pennsylvania Archives.
Astbury initially proposed a kinked-chain structure for the fibers. He later joined other researchers (notably the American chemist Maurice Huggins) in proposing that: the unstretched protein molecules formed a helix (which he called the α-form); and the stretching caused the helix to uncoil, forming an extended state (which he called the β-form).
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Reverse Turns回折:
Ω环型
Science 14 November 1986: Vol. 234. no. 4778, pp. 849 - 855 Jacquelyn Leszczynski and George D. Rose Loops in globular proteins: a novel category of secondary structure The protein loop, a novel category of nonregular secondary structure, is a segment of contiguous polypeptide chain that traces a "loop-shaped" path in three-dimensional space; the main chain of an idealized loop resembles a Greek omega (omega). A systematic study was made of 67 proteins of known structure revealing 270 omega loops. Although such loops are typically regarded as "random coil," they are, in fact, highly compact substructures and may also be independent folding units. Loops are almost invariably situated at the protein surface where they are poised to assume important roles in molecular function and biological recognition. They are often observed to be modules of evolutionary exchange and are also natural candidates for bioengineering studies.