Hexoses ring structures

Pentose Rings (Figure 9.10, Figure 9.11, Table 9.2, Structure) Hexose Rings (Structure, Figure 9.12, Figure 9,13)... 

Hexose monosaccharides can form both five- and six-membered rings. In most cases, the six-membered ring structure is more stable, but fructose is an important example of a hexose that is more stable as a five-membered ring. The structure ofy6-fhictose is shown in Figure 13-17. Notice that there are — CH2 OH fragments bonded to two positions of this five-membered ring. Examples and explore the structures of monosaccharides in more detail. 

The cyclic forms adopted by the hexoses and pentoses can be depicted as symmetrical ring structures called Haworth projection formulae, which give a better representation of the spatial arrangement of the functional groups with respect to one another. The nomenclature is based on the simplest organic compounds exhibiting a similar five- or six-membered ring... 

Figure 9.7 Six- and five-membered cyclic ethers. The stable ring structures which are adopted by hexoses and pentoses are five- or six-membered and contain an oxygen atom. They are named as derivatives of furan or pyran, which are the simplest organic compounds with similar ring structures, e.g. glucofuranose or glucopyranose for five-or six-membered ring structures of glucose respectively.

General acid catalysis by glutamic acid-35 represents at present the mechanism for lysozyme best able to explain the kinetic and structural data. For it to occur, however, distortion of the hexose ring in subsite D to a half-chair must take place so that relief of strain in the transition state will make bond breaking sufficiently easy. A question that must be answered for this picture to be tenable is whether relief of strain can so greatly facilitate bond breaking when the carbonium ion is a glycosyl ion [see also the discussion in Fife (1972) and Atkinson and Bruice (1974)]. 

Monomers and polymers of carbohydrates, (a) The most common carbohydrates are the simple six-carbon (hexose) and five-carbon (pentose) sugars. In aqueous solution, these sugar monomers form ring structures, (b) Polysaccharides are usually composed of hexose monosaccharides covalently linked together by glycosidic bonds to form long straight-chain or branched-chain structures. 

The carbohydrate most likely to appear on the MCAT is fructose or glucose. Both are six carbon carbohydrates called hexoses. These may appear as Fischer projections or ring structures. The Fischer projections are shown below ... 

Because poiysaccharides are natural macromolecules occurring in all living organisms, the structure of some polysaccharides can be much more complex, as they are not made only from simple monosaccharides. In the composition of natural polymeric carbohydrates, a wide variety of sugars are found. Among these, the most common are pentoses and hexoses. The structural formulas of three common pentoses are shown below. They frequently form cyclic structures with five-member rings (furanoses). 

Another variation of the classical primary polynucleotide structure (10.90a) was established by the synthesis of derivatives based on hexose rings (10.92b). The use of the latter in place of ribose rings produces a more linear chain which forms a non-helical double-stranded arrangement. However, some of these polyhexose chains appear to form double-stranded arrangements more stable than duplex DNA built from (10.92a). Similarly for RNA analogues [62]. 

The hexoses have the ring structure, and are linked together by an oxygen atom, as shown in the following diagrams ... 

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