Configurations of Monosaccharides Fischer Projections

Fischer projections and, 977-978 glycosides and, 989-990 l- 4 links in, 997-998 origin of name. 973 photosynthesis of, 973-974 see also Aldose, Monosaccharide vaccines from, 1004-1005 Carbon, ground-state electron configuration of, 6... 

Monosaccharides generally contain several chiral carbons and therefore exist in a variety of stereochemical forms, which may be represented on paper as Fischer projections. Epimers are sugars that differ in configuration at only one carbon atom. 

The designations D and L refer to the configuration of the stereocenter farthest from the aldehyde or ketone group on a monosaccharide chain. When the monosaccharide chain is drawn as a Fischer projection, a D-monosaccharide has the -OH on this carbon on the right an L monosaccharide has it on the left. 

Fischer projections are however, unsatisfactory when considering the physical properties and chemical reactivity of monosaccharides for which definitive spatial formulations are necessary. These are given below for D-glyceraldehyde, D-erythrose and D-threose, for which the (/ ,S configuration may be readily assigned at the appropriate chiral carbons. 

The Fischer projection is a convenient way of showing the configurations of the linear forms of monosaccharides. This convention depicts the concepts of stereochemistry established by Jacobus Henricus van t Hoff and Joseph Achille Le Bel in a simplified form. While these abbreviated structural formulas are simple to write and easy to visualize, there are some guidelines that should be taken into account when converting a three-dimensional structure into a Fischer projection and in its manipulation (Fig. 1.2) ... 

Figure 1.40 Structural summary of all main monosaccharides found in natural carbohydrates. ALL monosaccharides are displayed as their Fischer Projections to shown differences in absolute configuration.

You would do well to remember the configuration of groups on the Haworth projection of both a-D-glucopyranose and jS-D-glucopyranose as reference structures. Knowing how the Fischer projection of any other monosaccharide differs from that of D-glucose, you can then construct the Haworth projection of that other monosaccharide by reference to the Haworth projection of D-glucose. 

The designations refer to the configuration of the penultimate carbon. If the monosaccharide is drawn as a Fischer projection and the —OH group bonded to this carbon is on the right, the D designation is used likewise, a monosaccharide is l if the — OH group is on the left. 

Carbohydrates are polyhydroxy aldehydes and ketones that are commonly known as sugars. A monosaccharide is a carbohydrate composed of one sugar unit and a disaccharide is a carbohydrate composed of two monosaccharide units. An oligosaccharide is composed of 2-10 monosaccharide units a polysaccharide is composed of 10 or more monosaccharide units. An aldose is a carbohydrate that contains an aldehyde unit and a ketose is a carbohydrate that contains a ketone unit. Monosaccharides are categorized by the total number of carbons in the structure triose, tetrose, pentose, hexose, etc. The d and 1 configurations of a monosaccharide are based on the Fischer projection of d-glyceraldehyde. A Fischer projection is an older representation of sugars. 

A description of the pioneering work of Emil Fischer in the period around 1890, especially relating to his elucidation of the molecular configuration of glucose and aspects of his philosophical view of chemistry, has appeared. Lemieux and Spohr have looked in detail at how Fischer was led to the "lock and key" concept for enzyme specificity. An animated 386-based PC program with VGA graphics has been produced to assist visualization of the relationship between Fischer and Haworth projections of monosaccharides. ... 

Table 25.1 shows names and Fischer projections for all D-aldotetroses, pentoses, and hexoses. Each name consists of three parts. The letter D specifies the configuration of the penultimate carbon. Prefixes such as rib-, arabin-, and glue- specify the configuration of all other chiral centers in the monosaccharide. The suffix -ose shows that the compound is a carbohydrate. 

---