α螺旋
蛋白质二级结构中的一种
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）。[1]
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。
资料来源
- 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.
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