Monosaccharides conformation depiction

Similar to 2DR, ribose (C5H5O5) is one of the most important monosaccharides since it constitutes a subunit of the backbone of RNA. NMR studies have shown that ribose in solution is a mixture of a- and p-pyranose and a- and p-furanose forms, the p-pyranose form being predominant. The recently settled crystal structures have shown that the a- and P-pyranose forms are present in the solid phase [239-243]. The structure in the gas phase has been experimentally investigated using a laser ablation molecular beam Fourier transform microwave spectroscopy (LA-MBFTMW) technique [62]. The high resolution rotational spectrum has provided structural information on a total of six rotamers of ribose, three belonging to the a-pyranose forms and other three to the P-pyranose forms. Recently, D-ribose (m.p. 95°C) has been submitted to a laser ablation broadband (CP-FTMW) spectroscopic study and eight conformers (two new a-pyranose forms) have been identified. A broadband section of the spectra is shown in Fig. 35 and the detected conformers depicted in Fig. 36. 

William Mills described a similar convention to depict the structures of monosaccharides. While the ring atoms of the Haworth projections are oriented perpendicular to the paper, Mills chose to depict the carbon skeleton in the plane of the paper (Fig. 1.5). Although Fischer, Haworth, and Mills projections are useful tools for depicting the structures of carbohydrates, the planar nature of these representations does not provide an accurate picture of the actual geometry of the molecules. In order to understand carbohydrate function and reactivity, recognition of each distinct conformation and the properties associated with it is required [15]. 

Although Fischer projections are commonly used to depict monosaccharides with many stereogenic centers, care must be exercised in using them since they do not give a true picture of the three-dimensional structures they represent. Because each stereogenic center is drawn in the less stable eclipsed conformation, the Fischer projection of glucose really represents the molecule in a cylindrical conformation, as shown in Figure 27.2. 

The depictions of glucopyranose and fructofuranose shown in Figures 11.4 and 11.5 are Haworth projections. In such projections, the carbon atoms in the ring are not explicitly shown. The approximate plane of the ring is perpendicular to the plane of the paper, with the heavy line on the ring projecting toward the reader. Like Fischer projections, Haworth projections allow easy depiction of the stereochemistry of sugars. We will return to a more structurally realistic view of the conformations of cyclic monosaccharides shortly. 

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