Oxidation of Carbohydrates Most living organisms that live in air obtain energy by oxidation of carbohydrates. Glucose is the most common simple carbohydrate used as a fuel by living organisms. Before considering the chemical reactions of carbohydrates it will be worthwhile to consider their structure. Differences in the molecular structures of carbohydrates are often subtle, but critical to their function in living organisms.

What are carbohydrates? The word carbohydrates comes from the late 19th century idea that these compounds were “hydrates of carbon.” The molecular formulas of carbohydrates are often Cn(H2O)n Carbohydrates come in a variety of forms but there is usually at least one oxygen attached to each carbon and many of the oxygens are in hydroxyl groups. Carbohydrates can be found as monomers, dimers, oligomers and polymers.

Monosaccharides The units of carbohydrate structure are monosaccharides. Monosaccharides can be found with three to seven carbons... with either a single aldehyde or ketone group and hydroxyl groups on nearly every other carbon. They are therefore polyhydroxyaldehydes or polyhydroxyketones.

Polyhydroxyaldehydes & Polyhydroxyketones

Are these compounds different?

Nonsuperimposable Mirror Images

Different Compounds Since these two compounds are not superimposable they must be different compounds. They are not superimposable, even though they have all of the same atoms with all of the same bonds, because the middle carbon is chiral (not symmetric).

Chiral Compounds The mirror images of most compounds with a chiral carbon are not superimposable.

Enantiomers Nonsuperimposable mirror images are called enantiomers. Enantiomers are one type of stereoisomers. Stereoisomers are molecules that have the same bonds connecting the same atoms but different relative orientations of the bonds.

Enantiomers are Different Compounds The different relative orientations of bonds in a pair of enantiomers can’t be changed by rotation about the bonds. They can only be changed by breaking bonds and remaking them. Thus one enantiomer can only be changed into another enantiomer by chemical reactions.

Geometric Isomers are not Stereoisomers

Stereoisomers Have Different Properties Enantiomers are different compounds but they differ only in the direction in which they rotate polarized light and... in their interactions (attractions, repulsions and chemical reactions) with other chiral compounds. enantiomers behave identically when reacting or interacting with symmetric molecules

Differences in Living Organisms Since most of the components of living organisms are chiral this means that the effects of stereoisomers on living organisms usually differ. In fact the differences can be dramatic...

Dramatic Differences Starch and cellulose are both polymers made of glucose units linked together. In both of these polymers the C-4 of one glucose is bonded to the C-1 of the next glucose. The only difference is the direction of this bond. In cellulose the bond points up and in starch it points down. Yet this one difference means that humans can digest starch but not cellulose. Starch and cellulose are stereoisomers.

Stereoisomers There are many other examples of dramatic differences. Some drugs are chiral and one isomer may cure a fatal disease while the other isomer is lethal at the same dose. Distinguishing stereoisomers is therefore critical, even though the differences are subtle.

Naming Monosaccharide Enantiomers Since monosaccharide enantiomers are different compounds we need a way to indicate which compound we are talking about, i.e., a way to give them different names. Chemists have agreed that the orientation of the hydroxyl on the highest numbered chiral carbon be used to identify the isomer for monosaccharides.

How to name enantiomeric monosaccharides: D-glyceraldehyde L-glyceraldehyde  Orient the molecule so that the carbonyl is at the top. Vertical bonds should be pointing back and horizontal bonds forward.  In this orientation, if the hydroxyl on the highest numbered chiral carbon is on the right it is called the D-isomer and on the left, the L- isomer.  The highest numbered chiral carbon is the middle carbon for glyceraldehyde.

Fischer Projections  Drawing 2D images that accurately represent the 3D configuration of a molecule can be difficult.  Chemists have simplified this by agreeing on a set of rules for these drawings.  Acyclic compounds are drawn so that vertical bonds represent bonds pointing back and horizontal bonds are bonds that point forward.

Fischer Projections With these rules the distinguishing features of the 3D structure of stereoisomers can be easily and accurately represented with 2D drawings. Drawings made by these rules are called Fischer projections. The Fischer projection is named after the famous scientist, Emil Fischer, who established the molecular structures of many sugars.

Fischer Projections of Some Hexoses

L-Monosaccharides The L-version of each monosaccharide corresponds to the enantiomer of the D-version. Thus L-glucose has the opposite configuration at each of the chiral carbons compared to D- glucose. The only properties of L-glucose that differ from D- glucose are rotation of polarized light and interaction with other chiral substances. But does L-glucose taste sweet like D-glucose?

Cyclic Structures  Many monosaccharides have cyclic structures in aqueous solution.  Cyclic structures can be represented with Haworth projections.  The Haworth projection pictures the molecule in a flattened edge-on view.

Fischer -> Haworth Correspondence  Bonds pointing right in the Fischer structure correspond to bonds pointing down in the Haworth projection.

How do cyclic monosaccharides form?  The -OH of one carbon forms a hemiacetal or hemiketal bond to the carbonyl carbon.  One of the ring atoms is an oxygen.  The most common cyclic structures are five or six membered rings.

Furanoses & Pyranoses  The most common five membered ring is a furanose. furanose is derived from the furan ring the furan ring is a five membered ring containing one oxygen the ose ending of furanose indicates multiple hydroxyls  The most common six membered ring is a pyranose. pyranose is derived from the pyran ring the pyran ring is a six membered ring containing one oxygen

More Stereoisomers A new chiral carbon is formed when a monosaccharide cyclizes. Therefore two new stereoisomers are possible. These are designated  & .  means the -OH points down,  means the –OH points up. This carbon is called the anomeric carbon.

What are stereoisomers that are not mirror images? By now you may have noticed that not all stereoisomers are mirror images. Molecules that are stereoisomers but not enantiomers are diastereomers. Diastereomers have different configurations at one or more, but not at all corresponding pairs of chiral carbons. What are a pair of molecules called if they have opposite configurations at all corresponding pairs of chiral carbons?

Examples

When are diastereomers not possible? Diastereomers don’t exist for symmetric molecules, of course. But why don’t diastereomers exist for molecules that have only one chiral carbon? What properties of diastereomers differ?

Different Properties of Diastereomers...essentially all chemical and physical properties. melting & boiling points chemical reaction rates sweetness but these differences may be small or large depending on the compound and the property.

Monosaccharide Structure Summary Most monosaccharides in nature have the Dconfiguration. Most hexoses found in nature are diastereomers. Diastereomers differ in essentially all physical and chemical properties at least a little and sometimes a lot, particularly in their biological properties.