What is Glucose?

Glucose is a sugar and contains hydroxyl groups substituted at five carbons and the 6^th carbon is an aldehydic group. Glucose is al aldohexose and is a reducing sugar. Glucose can exist in various different isomeric forms which are either linear or cyclic. In the cyclic forms, it exists as a five membered ring called ‘furan’ of as a six membered ring called ‘pyranose’. These 5 and 6 membered rings, each, can exist as two stereoisomers (the D and L forms). In spite of all possible conformations the cyclic structures can potentially exist as, in physiological pH, it exists as beta-glucopyranose in its chair conformation. This conformation is considered highly stable.

• Glucose and different conformational forms
• Stability of the chair conformation of glucose 
• Lesser known conformational forms
• Practice questions and applications

Glucose

In organic chemistry, we can represent certain compounds in different ways structurally. Compounds which are 6-membered rings or substituted 6-membered rings can be represented in two ways- chair conformation and boat conformation.

In the chair conformation, the 2^nd, 3^rd, 5^th and 6^th carbon atoms lie on one plane whereas the 1^st carbon is above the plane and 4^th carbon is below the plane. In the boat conformation, 1^st, 2^nd, 4^th and 5^th carbons are in one plane while the 3^rd and 6^th carbons are bent in the same plane outside of the plane.

In any case, the chair conformation is more stable than the boat conformation because it has zero ring strain unlike the boat form.

Taking cyclohexane as example, writing chair and boat conformation:

Most Stable Structure of Glucose

Glucose (C₆H₁₂O₆) is a six-carbon containing carbohydrate whose structure can be written in various different ways:

Fischer projection (open chain structure)

Haworth projection (ring form)

As indicated above, there are two isomeric forms of glucose- alpha and beta glucose (they are more specifically called ‘anomers’).

The Fischer structure cyclizes to give the Haworth structure and this cyclic structure yields the chair conformation of glucose.

The six-membered rings consisting of five carbons and one oxygen atom in the ring are collectively called pyranose and hence, the ring structure of glucose is referred to as glucopyranose.

Although the two anomers are in a state of equilibrium, the beta-glucose chair conformation because all the substituent groups are in the equatorial position (only the hydrogens are axial).

Here, we can see that the substituent groups (-OH) are all equatorial in position. This is the most stable conformation of glucose.  

Other conformations of glucose:         

There are other lesser known conformations possible for glucose and one of them is called the Reeves projection. We saw the Haworth projection where the cyclic ring is planar but in Reeves projection, the representation is non-planar with a stereo view of the ring. Here, the three bonds at the front have been emboldened: 

Apart from the main two conformations- chair (C) and boat (B), there are other lesser known conformational forms like- half-chair (H), envelope (E) and skew (S).

Although these other forms exist, the most predominant ones are just chair and boat.

How to Draw the Chair Form from the Haworth Projection

1. Draw the Haworth structure which includes a six membered heterocyclic ring wherein five carbons are carbon and the sixth atom is oxygen. The carbon to the right of oxygen is numbered 1 (C1) and the one to the left is numbered 5 (C5).
2. On C5, draw a vertical line with CH₂OHplaced on top and H at the bottom. On C1 and C3, draw a similar line and place the OH group on top and H at the bottom. On C2 and C4, the line has H above and OH group below.
3. The C1 carbon is called the anomeric carbon and the position of OH group on this carbon determines the alpha and beta forms.
4.  Next, to convert this into chair conformation, a cyclohexane structure like this down below,

5. Since this carbon chain is shaped like a chair, it is known as the chair conformation.

6. From this above structure, change one of the carbons in the ring to oxygen.

7. Write the hydroxyl groups in the equatorial position and the CH₂OH group is also placed in the equatorial position.

Applications of Glucose

• Glucose is an abundant sugar which is the most important biomolecule in the metabolism in all organisms.
• It is paired up with some amount of saline and intravenously given to patients as medicine.
• It is further metabolized into amino acids or other carbohydrates, etc.
• It is found in combination with hemoglobin in our blood and present as glycated hemoglobin.
• It is mostly a monomer unit which forms a variety of polysaccharides of significance like cellulose, glycogen, etc..,

Practice Questions

Q1. What makes beta-glucopyranose in the chair conformation most stable?

Answer: this question has to be answered in three parts:

• First part explaining why the linear structure of glucose cyclizes. Considering enthalpy conditions, we are aware that single bonds between carbon and oxygen are more favorable energetically than a carbon oxygen double bond. Further, this cyclisation reaction happens intramolecularly, so the entropic factor is low and all the hydroxyl groups remain intact. Hence, the equilibrium proceeds towards cyclisation.
• Second part explaining what makes pyranose more favorable than furanose.  In five membered rings (furanose), there is torsional strain which is owing to the fact that the bond angles are less than 109.5° (because the carbons are not exactly sp3 hybridized). However, in six membered rings (pyranose), the bond angles are found to be 109.5° and therefore, no torsional strain exist. This makes pyranose energetically more favorable.
• The third and the last part explaining why beta chair form is more preferred than the alpha form. In the alpha isomeric form, there exists a 1,3-diaxial strain which are caused due to the 1,3-diaxial interactions between large groups (hydroxyl groups) at the axial position and from these interactions arise a strain. Whereas, in beta isomer, the hydroxyl are placed in the equatorial positions and the strain is minimized. Therefore, beta form is more favored and stable.

Context and Applications

This topic is a prerequisite for the completion of certain undergraduate and graduate courses, especially for

• B.Sc Chemistry  
• M.Sc Chemistry
• B.Sc Life Sciences
• B.Sc Biochemistry
• M.Sc Biochemistry