α螺旋
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α螺旋（alpha helix (α-helix)；Pauling–Corey–Branson α-helix；3.613-helix）是蛋白質中最常見最典型和含量最豐富的二級結構。它和β摺疊因肽段每個二面角Φ和Ψ值都處於同一或幾乎同一數值而被稱為規正二級結構（Cα-N夾角和Cα-C夾角）。[1]
對於L-胺基酸，α螺旋一般是右手螺旋，左手α螺旋的理論穩定性較小；對於D-胺基酸，最穩定的形式則是左手的。在α螺旋中，Φ=-57˚，Ψ=-47˚，平均每個螺旋周期包含3.6個胺基酸殘基，沿螺旋軸方向上升0.54nm，這些胺基酸殘基的側鏈伸向外側。同一肽鏈上的每個殘基的醯胺氫原子和位於它後面的第4個殘基上的羰基氧原子之間形成氫鍵。其中的H－O鍵長2.8Å。這種氫鍵大致與螺旋軸平行。一條多肽鏈呈α螺旋構象的原因是所有肽鍵上的醯胺氫和羰基氧之間形成的鏈內氫鍵。在水環境中，肽鍵上的醯胺氫和羰基氧既能形成分子內（α螺旋內）氫鍵，也能與水分子形成氫鍵。如果後者發生，多肽鏈呈現類似變性蛋白質那樣的伸展構象。非極性溶劑環境對於氫鍵形成沒有影響，因此更可能促進α螺旋的形成。
α螺旋中，胺基酸的R基團指向外側和下側，以避免與多肽鏈的主幹部分之間的位阻影響。α螺旋的核心部分緊密結合，原子之間以范德華力聯繫在一起。
因為其特殊的環狀R基團，脯氨酸一般不出現在α螺旋中部，而經常出現在α螺旋的開始處。
資料來源
- Donald Voet, Judith G. Voet, Charlotte Pratt. Fundamentals Of Biochemistry, upgrade edition
- ^ 朱長庚 徐長法. 生物化学. 高等教育出版社. 2017: 100–102. ISBN 9787040457988.
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