A survey of hydrogen bond geometries in the crystal structures of amino acids

An analysis of the geometries of the hydrogen bonds observed by neutron diffraction in thirt-two crystal structures of amino acids shows the following results. Of the 168 hydrogen bonds in the data set, 64 involve the zwitterion groups  and CO2. Another 18 are from to sulphate or carbonyl oxygens. The majority, 46, of these H … O bonds are three-centered (bifurcated). Nine are four-centered (trifurcated). The geometry in which the three-centered hydrogen bond involves both oxygens of the same carboxylate group is not especially favoured. When it does occur, one hydrogen bond is generally shorter and the other longer, than when the bonding involves oxygens on different carboxylate groups. The shortest hydrogen bonds are the OH … O C, from a carboxylic acid hydroxyl to a carboxylate oxygen, and NH … OC when the nitrogen is the ring atom in histidine or proline. Carboxylate groups, on average, accept six hydrogen bonds, with no examples of less than four bonds. The reason for the large number of three-centered H … OC bonds is therefore a proton deficiency arising from the disparity between the tripled donor property of the groups and the sextuple, on average, acceptor property of the carboxylate groups. There is good geometrical evidence for the existence of H … O and H … Cl− hydrogen bonds, especially involving the hydrogen atoms on α-atoms.