ROLE OF C-H...O HYDROGEN-BONDS IN THE COORDINATION OF WATER-MOLECULES - ANALYSIS OF NEUTRON-DIFFRACTION DATA

C-H...O hydrogen bonds with water acceptors are analyzed from 101 water molecules in 46 neutron crystal structures. The shortest observed hydrogen bond distances are H...O(W) is similar to 2.3 angstrom and C...O(W) is similar to 3.1 angstrom. About 8% of the water molecules accept C-H...O(W) interactions with H...O(W) separations < 2.5 angstrom and 39% with H...O(W) < 2.8 angstrom. CH donors may coordinate to water molecules in concert with OH and NH donors and with metal ions in many different combinations. The most frequent function of C-H...O(W) hydrogen bonds is to complete a tetrahedral coordination geometry around the water. Also, there are many examples where a distorted tetrahedral O-H...O(W) and N-H...O(W) hydrogen bond coordination leaves ''free'' acceptor potential that is satisfield by additional C-H...O(W) interactions. In essence, a water molecule will prefer to accept the strong hydrogen bonds from OH and NH donors (or to interact with metal ions), but if these are not available in suitable configuration, it will resort to the weaker C-H...O(W) hydrogen bonds rather than leaving its multiple acceptor potential unsatisfied. This is also evident in structural biology; as shown for the enzyme actinidin, tetrahedral coordination geometries of internal water molecules may involve C-H...O(W) interactions.