Occurrence of bifurcated three-center hydrogen bonds in proteins.

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 alpha-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 3.6(13) alpha-helix, and the minor component is (n + 4)N-H ... O = C(n + 1), as observed in 3(10) helices; distortions at the C-termini of alpha-helices are stabilized by three-center bonds. In beta-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.