Abstract
The results of a survey of 439 hydrogen bonds in 95 recently determined crystal structures of amino acids, peptides and related molecules suggest that the following generalizations hold true for linear (angle X–H—Y > 150°) hydrogen bonds. (1) The charge on the acceptor group does not influence the length of a hydrogen bond. (2) For a given acceptor group, the hydrogen bond lengths increase in the order imidazolium N–H < ammonium N–H < guanidinium N–H; this order holds true for oxygen anion acceptor groups. Clions and the uncharged oxygen of water molecules. (3) The uncharged imidazole N–H group forms shorter hydrogen than the amide N–H group. (4) The carboxyl O–H groups form shorter hydrogen bonds than other hydroxyl groups. (5) The hydrogen bonds involving a halogen ion are longer than hydrogen bonds with other acceptors when corrected for their longer vander Waals radii. The observed differences between the lengths of hydrogen bonds formed by different donor and acceptor groups in amino acids and peptides, imply differences in the energetics of their formation.

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