Monosaccharides configurational atom

All carbohydrates can exist in either of these two forms and the prefix of D or L only refers to the configuration around the highest numbered asymmetric carbon atom. Enantiomers have the same name (e.g. D-glucose and L-glucose) and are chemically similar compounds but have different optical properties. The majority of naturally occurring monosaccharides, whether they be aldoses or ketoses, are of the D configuration. 

The most important natural monosaccharide, D-glucose, is an aliphatic aldehyde with six C atoms, five of which carry a hydroxyl group (1). Since C atoms 2 to 5 represent chiral centers (see p. 8), there are 15 further isomeric aldohexoses in addition to D-glucose, although only a few of these are important in nature (see p.38). Most natural monosaccharides have the same configuration at C-5 as D-glyceraldehyde-they belong to the D series. 

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. 

Monosaccharides commonly form internal hemiacetals or hemiketals, in which the aldehyde or ketone group joins with a hydroxyl group of the same molecule, creating a cyclic structure this can be represented as a Haworth perspective formula. The carbon atom originally found in the aldehyde or ketone group (the anomeric carbon) can assume either of two configurations, a and /3, which are interconvertible by mutarotation. In the linear form, which is in equilibrium with the cyclized forms, the anomeric carbon is easily oxidized. 

The common nomenclature for di- or oligosaccharides specifies the order of monosaccharide units, the configuration at each anomeric carbon, and the carbon atoms involved in the glycosidic linkage (s). 

Compounds that have the same chemical formula but have different structures are called isomers. For example, fructose, glucose, mannose, and galactose are all isomers of each other, having the same chemical formula, C6H1206. If two monosaccharides differ in configuration around only one specific carbon atom (with the exception of the carbonyl carbon, see anomers below), they are defined as epimers of each other. (Of course, they are also isomers ) For example, glucose... 

Some monosaccharides may also be classified as being epimers. Epimers are compounds that have identical configurations except for one carbon atom. For example, a-D-glucose and a-D-fructose are epimers. Epimers sometimes react with the same reagent to form the same product. For example, both a-D-glucose and a-D-fructose react with phenylhydrazine to form the same osazone. 

A disaccharide is formed when two monosaccharides become joined by a glycosidic bond. The bond may be an a- or (3-bond depending on the configuration of the anomeric carbon atom involved in the bond. Usually the anomeric carbon atom of only one of the two monosaccharides is involved in the bond so that the disaccharide still has one free aldehyde or ketone group and is still reducing. However, in sucrose both anomeric carbon atoms are bonded together so that sucrose is a nonreducing disaccharide. 

Polyols. Linear polyols containing two carbon to seven carbon atoms all evoked neural responses. Two important results were observed (Figure 8)s (1) The effectiveness (CR q.) of the polyols increased as the chain length increased up to five carbon atoms (2) In contrast to monosaccharides, the configurations of linear polyol may not play a role in the taste response. This is indicated by the identical responses to the four pentitols D-arabinitol, L-arabinitol, D-ribitol, or D-xylitol. 

---