Hydrogen-Bond Acceptor Geometries

The lone-pair electrons of oxygen are conventionally assigned to sp3 orbitals, which suggests some tetrahedral directionality. However, these electron distributions are diffuse and easily polarized. So this directionality, if it exists, might be dependent on the configuration of the group to which the oxygen is bonded, as between and 0=C, for example 

Water molecules have the same characteristics as sp3 oxygen atoms, both in the organic and inorganic small molecule hydrates. 

Directional properties of acceptor groups are very soft or nonexistent. Just as the H A lengths and X-ft- -A angles for individual hydrogen bonds are perturbed by the other intermolecular forces in the crystal, so are the directional acceptor properties of the functional groups. These properties appear to be even softer than the hydrogen-bond geometries, and in consequence it is not possible to extract any characteristic trends from the surveys of the type described in this chapter. 

For the carbohydrates especially, the amount of available crystal structural data decreases sharply with molecular complexity [479]. With the exception of the cyclodextrins, discussed in Part III, Chapter 18, there are less than 40 crystal structure analyses of oligosaccharides, of which less than 10 are trisaccharides, one is a tetrasaccharide, and one a hexasaccharide (Part III, Chap. 18). The majority of the basic monosaccharides that are the subunits of the polysaccharides that occur naturally have been studied for example, the pyranose forms of /7-arabinose, a-xylose, a- and -glucose, / fructose, a-sorbose, a-mannose, a- and -galactose, a-fucose, a-rhamnose, N-acetyl glucosamine, and mannosamine (Box 13.2). How- 

Of the three major components of biological structure, the proteins, nucleic acids, and polysaccharides, least is known about the polysaccharides at the secondary and tertiary level of molecular structure. This is because the polysaccharides cannot be obtained in crystals which are large enough for single crystal X-ray or neutron structure analysis. What structural information there is comes from fiber X-ray diffraction patterns. However good these diffraction patterns are, the structures derived from them will always be model-dependent. This is because the number of variable atomic parameters which determine the diffraction intensities exceeds the number of observed intensities. 

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