α螺旋（alpha helix (α-helix)；Pauling–Corey–Branson α-helix；3.613-helix）是蛋白質的二級結構。它和β摺疊一起被稱為「規則二級結構」，因為他們都具有重複的Φ和Ψ值（Cα-N夾角和Cα-C夾角）。
α螺旋一般是右手螺旋。在α螺旋中，平均每個螺旋周期包含3.6個胺基酸殘基，殘基側鏈伸向外側，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，H－O距離是2.8Å。這種氫鍵大致與螺旋軸平行。一條多肽鏈呈α螺旋構象的推動力就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內部（α螺旋內）的氫鍵，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。疏水環境對於氫鍵的形成沒有影響，因此，更可能促進α螺旋結構的形成。
α螺旋中，胺基酸的R基團指向外和向下。這樣以來就避免了與多肽鏈的主幹部分之間的的位阻影響。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，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
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