Α螺旋

α螺旋（alpha helix (α-helix)；Pauling–Corey–Branson α-helix；3.613-helix）是蛋白质中最常见最典型和含量最丰富的二级结构。它和β折叠因肽段每个二面角Φ和Ψ值都处于同一或几乎同一数值而被称为规正二级结构（Cα-N夹角和Cα-C夹角）。 对于L-氨基酸，α螺旋一般是右手螺旋，左手α螺旋的理论稳定性较小；对于D-氨基酸，最稳定的形式则是左手的。在α螺旋中，Φ=-57˚，Ψ=-47˚，平均每个螺旋周期包含3.6个氨基酸残基，沿螺旋轴方向上升0.54nm，这些氨基酸残基的侧链伸向外侧。同一肽链上的每个残基的酰胺氢原子和位于它后面的第4个残基上的羰基氧原子之间形成氢键。其中的H－O键长2.8Å。这种氢键大致与螺旋轴平行。一条多肽链呈α螺旋构象的原因是所有肽键上的酰胺氢和羰基氧之间形成的链内氢键。在水环境中，肽键上的酰胺氢和羰基氧既能形成分子内（α螺旋内）氢键，也能与水分子形成氢键。如果后者发生，多肽链呈现类似变性蛋白质那样的伸展构象。非极性溶剂环境对于氢键形成没有影响，因此更可能促进α螺旋的形成。 α螺旋中，氨基酸的R基团指向外侧和下侧，以避免与多肽链的主干部分之间的位阻影响。α螺旋的核心部分紧密结合，原子之间以范德华力联系在一起。 因为其特殊的环状R基团，脯氨酸一般不出现在α螺旋中部，而经常出现在α螺旋的开始处。 Donald Voet, Judith G. Voet, Charlotte Pratt. Fundamentals Of Biochemistry, upgrade edition 朱长庚 徐长法. 生物化学. 高等教育出版社. 2017: 100–102. ISBN 9787040457988.  Tooze J, Brändén CI. Introduction to protein structure. New York: Garland Pub. 1999. ISBN 0-8153-2304-2. . Eisenberg D. The discovery of the alpha-helix and beta-sheet, the principal structural features of proteins. Proceedings of the National Academy of Sciences of the United States of America. September 2003, 100 (20): 11207–10. Bibcode:2003PNAS..10011207E. PMC 208735 . PMID 12966187. doi:10.1073/pnas.2034522100.  Astbury WT, Woods HJ. The Molecular Weights of Proteins. Nature. 1931, 127 (3209): 663–665. Bibcode:1931Natur.127..663A. doi:10.1038/127663b0.  Astbury WT, Street A. X-ray studies of the structures of hair, wool and related fibres. I. General. Trans. R. Soc. Lond. 1931, A230: 75–101. Bibcode:1932RSPTA.230...75A. doi:10.1098/rsta.1932.0003.  Astbury WT. Some Problems in the X-ray Analysis of the Structure of Animal Hairs and Other Protein Fibers. Trans. Faraday Soc. 1933, 29 (140): 193–211. doi:10.1039/tf9332900193.  Astbury WT, Woods HJ. X-ray studies of the structures of hair, wool and related fibres. II. The molecular structure and elastic properties of hair keratin. Philosophical Transactions of the Royal Society of London Series A. 1934, 232 (707–720): 333–394. Bibcode:1934RSPTA.232..333A. doi:10.1098/rsta.1934.0010.  Astbury WT, Sisson WA. X-ray studies of the structures of hair, wool and related fibres. III. The configuration of the keratin molecule and its orientation in the biological cell. Proceedings of the Royal Society. 1935, A150 (871): 533–551. Bibcode:1935RSPSA.150..533A. doi:10.1098/rspa.1935.0121.  Sugeta H, Miyazawa T. General Method for Calculating Helical Parameters of Polymer Chains from Bond Lengths, Bond Angles, and Internal-Rotation Angles. Biopolymers. 1967, 5 (7): 673–679. doi:10.1002/bip.1967.360050708.  Wada A. The alpha-helix as an electric macro-dipole. Advances in Biophysics. 1976: 1–63. PMID 797240.  Chothia C, Levitt M, Richardson D. Structure of proteins: packing of alpha-helices and pleated sheets. Proceedings of the National Academy of Sciences of the United States of America. October 1977, 74 (10): 4130–4. Bibcode:1977PNAS...74.4130C. PMC 431889 . PMID 270659. doi:10.1073/pnas.74.10.4130.  Chothia C, Levitt M, Richardson D. Helix to helix packing in proteins. Journal of Molecular Biology. January 1981, 145 (1): 215–50. PMID 7265198. doi:10.1016/0022-2836(81)90341-7.  Hol WG. The role of the alpha-helix dipole in protein function and structure. Progress in Biophysics and Molecular Biology. 1985, 45 (3): 149–95. PMID 3892583. doi:10.1016/0079-6107(85)90001-X.  Barlow DJ, Thornton JM. Helix geometry in proteins. Journal of Molecular Biology. June 1988, 201 (3): 601–19. PMID 3418712. doi:10.1016/0022-2836(88)90641-9.  Murzin AG, Finkelstein AV. General architecture of the alpha-helical globule. Journal of Molecular Biology. December 1988, 204 (3): 749–69. PMID 3225849. doi:10.1016/0022-2836(88)90366-X.  NetSurfP ver. 1.1 - Protein Surface Accessibility and Secondary Structure Predictions （页面存档备份，存于互联网档案馆） α-helix rotational angle calculator （页面存档备份，存于互联网档案馆） Artist Julie Newdoll's website （页面存档备份，存于互联网档案馆） Artist Julian Voss-Andreae's website （页面存档备份，存于互联网档案馆） template:Spirals（英语：template:Spirals）