Carbohydrate crystals

Plant materials are extracted to exhaustion by percolation with ethanol. Some very water soluble products may possibly be missed by this procedure but the convenience of the process outweighs this possibility. Many times highly water soluble substances such as carbohydrates crystallize from the extracts during concentration so clearly some quantity of even very water soluble substances are extracted by the ethanol percolation. 

A notable feature of carbohydrate crystals is the absence (or small proportion) of water of hydration. As monosaccharides are hydrophilic and water-soluble, considerable deposition of water (association) in a sugar crystal may be expected. The absence of water from the crystal lattice of a sugar must, therefore, be explained by supposing that the sugar is able to form sufficiently strong and numerous hydrogen bonds without the involvement of molecules of water this has been observed, although there are some exceptions. 

These three questions will be examined in relation to the conformational data which have been given by recent carbohydrate crystal structure determinations. Because of the added reliability and accuracy arising from the use of automatic diffractometers only those numerical data will be quoted where these instruments were used. 

Thble 6.1 shows some comparisons of values determined by X-ray and neutron diffraction studies for O-H and H- -O bond lengths for some carbohydrate crystal structures that have been analyzed by both methods. Application of the normalization reduces the discrepancy between the H - - O distances obtained by X-ray and neutron diffraction experiments from 0.102 A to 0.033 A. Similar results have been reported for NH -0=C bonds [387], where the random errors in the X-ray N-H and H 0 distances varied between 0.02 and 0.17 A with a mean of 0.065 A when compared with the same neutron diffraction values. Cor-... 

Three-center hydrogen bonds in the carbohydrate crystal structures. The first systematic study which suggested that three-center hydrogen bonds are more widespread than originally thought came from the examination of the results of the neutron diffraction crystal structures of the pyranose and pyranoside sugars referred to in Chapter 7 [58]. 

Table 9.2. Intramolecular hydrogen bonds between syndiaxially oriented O-H groups in carbohydrate crystal structures...

The proportion of three-center bonds in the pyrimidine, purine, and barbiturate crystal structures of about 30% is comparable to the number observed in the carbohydrate crystal structures (Thble 2.3). There is also a small proportion, about 1 %, of four-center bonds. A particular feature of the three-center bonds in these crystal structures is the occurrence of chelation, where both acceptor atoms are covalently bonded to the same atom(s), as in Fig. 15.7. 

The cooperative, infinite chains and cycles formed by O-H 0 hydrogen bonds in the a-cyclodextrin hydrates are a characteristic structural motif [109]. As with the simpler carbohydrate crystal structures described in Part II, Chapter 13, the hydrogen bonds can be traced from donor to acceptor in the cyclodextrin hydrate crystal structures. Networks of O-H 0-H 0-H interactions are observed in which the distribution of hydrogen bonds follows patterns with two characteristic motifs. One are the "infinite chains which run through the whole crystal lattice, and the others are the loops or cyclically closed patterns (a special case of the "infinite chains). As in the small molecule hydrates, such as a-maltose monohydrate, the chains and cycles are interconnected at the water molecules to form the complex three-dimensional networks illustrated schematically in Fig. 18.5, with some sections shown in more detail in Fig. 18.7 a, b, c. 

The carbohydrate crystal structures provide very limited data on hydrates, despite their ubiquitous solubility in water. Of the twenty simple pyranose and pyranoside crystal structures described in Part II, Chap. 13, only four are hydrates. Of these, three have four-coordinated water, and one, methyl-a-galactopyranoside monohydrate, has a three-coordinated water. 

Jeffrey GA, Ihkagi S (1978) Hydrogen-bond structure in carbohydrate crystals. Accts Chem Res 11 264-270... 

Jeffrey GA, Gress ME, Thkagi S (1977) Some experimental observations on H 0 hydrogen bond lengths in carbohydrate crystal structures. J Am Chem Soc 99 609-611... 

Gritsan VN, Panov VP, Kachur VG (1983) Theoretical analysis of hydrogen bonds in carbohydrate crystals. Carbohydr Res 112 11-21... 

Steiner, T., and Saenger, W., Geometry of C H—0 hydrogen bonds in carbohydrate crystal structures. Analysis of neutron diffraction data, J. Am. Chem. Soc. 114, 10146-10154 (1992). 

For the carbohydrates, there is the added complication of hydrogen bonding, concerning which there is very little quantitative understanding indeed. Nevertheless, it is worth while to examine the overall pattern of hydrogen bonding in carbohydrate crystals, to see if there are any apparent generalizations. 

French et al. have provided some insight into the ability of MM2 and MM3 to correctly predict the strengths of nonbonded interactions between sugar residues by computing the crystal lattice energies of microcrystalline ensembles of monosaccharides.. 3 Although simulations of carbohydrate crystal structures have been reported, the published investigations have focused primarily on the structural features.3 23.i04 worthwhile to note that these... 

Bugg CE (1973) Calcium binding to carbohydrates. Crystal structure of a hydrates ealcium bromide complex oflactose. JAm Chem Soc 95 908-913. doi 10.1021/ja00784a046... 

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