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Diễn đàn » Hóa học đại học và sau đại học » Tiếng Anh chuyên ngành hóa học (English for Special Purposes) » Lesson forty two (Glucose)

Lesson forty two
ProfVietanhNgày: Thứ tư, 2010-06-02, 10:28 AM | Tin nhắn # 1
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Glucose

Models of glucose

Chemical name: 6-(hydroxymethyl)oxane-2,3,4,5-tetrol

Glucose (Glc), a monosaccharide (or simple sugar), is an important carbohydrate in biology. The cell uses it as a source of energy and metabolic intermediate. Glucose is one of the main products of photosynthesis and starts cellular respiration in both prokaryotes and eukaryotes. The name comes from the Greek word glykys, which means "sweet", plus the suffix "-ose" which denotes a carbohydrate.

Two stereoisomers of the aldohexose sugars are known as glucose, only one of which (D-glucose) is biologically active. This form (D-glucose) is often referred to as dextrose (dextrose monohydrate), especially in the food industry. This article deals with the D-form of glucose. The mirror-image of the molecule, L-glucose, cannot be used by cells.

Glucose is commonly available in the form of a white substance or as a solid crystal. It can also be commonly found as an aqueous solution.


Phạm Bá Việt Anh

Department of Analytical Chemistry
Faculty of Chemistry
Hanoi National University of Education
Mobile - Tel: (84) 943 919 789
 
ProfVietanhNgày: Thứ tư, 2010-06-02, 10:29 AM | Tin nhắn # 2
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Structure

Glucose (C6H12O6) contains six carbon atoms one of which is part of an aldehyde group and is therefore referred to as an aldohexose. The glucose molecule can exist in an open-chain (acyclic) and ring (cyclic) form (in equilibrium), the latter being the result of an intramolecular reaction between the aldehyde C atom and the C-5 hydroxyl group to form an six-membered cyclic hemiacetal. In water solution both forms are in equilibrium, and at pH=7 the cyclic form is predominant. As the ring contains five carbon atoms and one oxygen atom, which resembles the structure of pyran, the cyclic form of glucose is also referred to as glucopyranose. In this ring, each carbon is linked to a hydroxyl side group with the exception of the fifth atom, which links to a sixth carbon atom outside the ring, forming a CH2OH group.


Phạm Bá Việt Anh

Department of Analytical Chemistry
Faculty of Chemistry
Hanoi National University of Education
Mobile - Tel: (84) 943 919 789
 
ProfVietanhNgày: Thứ tư, 2010-06-02, 11:31 AM | Tin nhắn # 3
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Isomers

Aldohexose sugars have 4 chiral centers giving 24 = 16 stereoisomers. These are split into two groups, L and D, with 8 sugars in each. Glucose is one of these sugars, and L and D-glucose are two of the stereoisomers. Only 7 of these are found in living organisms, of which D-glucose (Glu), D-galactose (Gal) and D-mannose (Man) are the most important. These eight isomers (including glucose itself) are all diastereoisomers in relation to each other and all belong to the D-series.

An additional asymmetric center at C-1 (called the anomeric carbon atom) is created when glucose cyclizes and two ring structures, called anomers are formed — α-glucose and β-glucose. These anomers differ structurally with respect to the relative positioning of their hydroxyl group linked to C-1 and the group at C-6, which is termed the reference carbon. When D-glucose is drawn as a Haworth projection or in the standard chair conformation, the designation α means that the hydroxyl group attached to C-1 is positioned trans to the -CH2OH group at C-5, while β means it is cis. Another popular method of distinguishing α from β is by observing whether the C-1 hydroxyl is below or above the plane of the ring, respectively, but this method is an inaccurate definition and may fail if the glucose ring is drawn upside down or in an alternative chair conformation. The α and β forms interconvert over a timescale of hours in aqueous solution, to a final stable ratio of α:β 36:64, in a process called mutarotation.



Phạm Bá Việt Anh

Department of Analytical Chemistry
Faculty of Chemistry
Hanoi National University of Education
Mobile - Tel: (84) 943 919 789
 
ProfVietanhNgày: Thứ tư, 2010-06-02, 11:32 AM | Tin nhắn # 4
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Production

Natural
Glucose is one of the products of photosynthesis in plants and some prokaryotes.

In animals and fungi, glucose is the result of the breakdown of glycogen, a process known as glycogenolysis. In plants - the breakdown substrate is starch.

In animals, glucose is synthesized in the liver and kidneys from non-carbohydrate intermediates, such as pyruvate and glycerol, by a process known as gluconeogenesis.

Commercial
Glucose is produced commercially via the enzymatic hydrolysis of starch. Many crops can be used as the source of starch. Maize, rice, wheat, potato, cassava, arrowroot, and sago are all used in various parts of the world. In the United States, cornstarch (from maize) is used almost exclusively.
This enzymatic process has two stages. Over the course of 1-2 hours near 100 °C, these enzymes hydrolyze starch into smaller carbohydrates containing on average 5-10 glucose units each. Some variations on this process briefly heat the starch mixture to 130 °C or hotter one or more times. This heat treatment improves the solubility of starch in water, but deactivates the enzyme, and fresh enzyme must be added to the mixture after each heating.

In the second step, known as "saccharification", the partially hydrolyzed starch is completely hydrolyzed to glucose using the glucoamylase enzyme from the fungus Aspergillus niger. Typical reaction conditions are pH 4.0–4.5, 60 °C, and a carbohydrate concentration of 30–35% by weight. Under these conditions, starch can be converted to glucose at 96% yield after 1–4 days. Still higher yields can be obtained using more dilute solutions, but this approach requires larger reactors and processing a greater volume of water, and is not generally economical. The resulting glucose solution is then purified by filtration and concentrated in a multiple-effect evaporator. Solid D-glucose is then produced by repeated crystallizations.


Phạm Bá Việt Anh

Department of Analytical Chemistry
Faculty of Chemistry
Hanoi National University of Education
Mobile - Tel: (84) 943 919 789
 
Diễn đàn » Hóa học đại học và sau đại học » Tiếng Anh chuyên ngành hóa học (English for Special Purposes) » Lesson forty two (Glucose)
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