Oxidation ofCarbohydrates

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 ofcarbohydrates it will be worthwhile to considertheir 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 cen

tury 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 toseven carbons...

with either a single aldehyde or ketone group and hydroxyl groups on nearly every other carbon.

They are therefore polyhydroxyaldehydes orpolyhydroxyketones.

Polyhydroxyaldehydes& Polyhydroxyketones

Are these compoundsdifferent?

NonsuperimposableMirror 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 calledenantiomers.

Enantiomers are one type of stereoisomers. Stereoisomers are molecules that have the sam

e bonds connecting the same atoms but different relative orientations of the bonds.

Enantiomers areDifferent 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 arenot Stereoisomers

StereoisomersHave 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 inLiving 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 gluc

ose 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 point

s down. Yet this one difference means that humans can dige

st 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 MonosaccharideEnantiomers

Since monosaccharide enantiomers are different compounds we need a way to indicate w

hich 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 enantiomericmonosaccharides:

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 chiralcarbon is the middle carbon forglyceraldehyde.

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 Fisch

er projections. The Fischer projection is named after the famou

s scientist, Emil Fischer, who established the molecular structures of many sugars.

Fischer Projections ofSome 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 e

ach 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 -> HaworthCorrespondence

Bonds pointing right in the Fischer structure correspond to bonds pointing down in the Haworth projection.

How do cyclicmonosaccharides 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 ringthe 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 α& β. αmeans the -OH points down, β

means the –OH points up. This carbon is called the anomeri

c carbon.

What are stereoisomers thatare 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 diastereomersnot 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 Propertiesof 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.

MonosaccharideStructure 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.