Occurrence of bifurcated three‐center hydrogen bonds in protpins

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Abstract
Analysis of 13 high‐resolution protein X‐ray crystal structures shows that 1204 (24%) of all the 4974 hydrogen bonds are of the bifurcated three‐center type with the donor X‐H opposing two acceptors A1, A2. They occur systematically in α‐helices where 90% of the hydrogen bonds are of this type; the major component is (n+4)N‐H…O=C(n) as expected for a 3.613 α‐helix, and the minor component is (n+4)N‐H…O=C(n+1), as observed in 310 helices; distortions at the C‐temini of α‐helices are stabilized by three‐center bonds, In β‐sheets 40% of the hydrogen bonds are three‐centered. The frequent occurrence of three‐center hydrogen bonds suggests that they should not be neglected in protein structural studies.