Stereochemistry Carbohydrates Carbohydrates are used to provide energy to living cells in the body. They can also be used as storage in the body for example, in the form of glycogen. All carbohydrates contain the elements carbon, hydrogen and oxygen. The well known carbohydrates are monosaccharides, disaccharides and polysaccharides. Monosaccharides are single sugar units which are building blocks that are used to make up larger carbohydrates. There is a large variety of monosaccharides that are different due to the number of carbon atoms that they possess and how the atoms are arranged in the molecule.
An example of a monosaccharide is Glucose (C6H12O6) which is used to provide energy to heterotrophic cells. [1] Disaccharides are made up of two monosaccharides that contain covalent bonds that are held together by a glycosidic bond. Disaccharides are present in the food that people take in; these are then broken down into two single sugar molecules so that they are able to be easily absorbed in the small intestine. An example of a disaccharide is sucrose which is formed from glucose and fructose. [2]
Polysaccharides are large molecules which are made from single sugars which can be in different forms such as branched, unbranched and coiled. Cellulose and Starch are examples of a polysaccharide. Cellulose provides support and rigidity in the cell wall in plant cells. It is made of thousands of glucose molecules that are joined together by beta (1-4) glycosidic bonds. Each second glucose unit is inverted, this means that the sixth carbon is on the opposite side which causes hydrogen bonding to occur. This is what provides the rigidity in the cell wall as there is cross linking involved which provides also flexibility to the cell wall. 3] In monosaccharides, glucose can be structured as either a linear molecule or ring structure. It has the configurations of D-glucose and L-glucose. [pic] D-Glucose and L-Glucose are enantiomers. Enantiomers are when isomers that are mirror images but are not super imposable. This means that the L-Glucose and D-Glucose will always have the same melting point, boiling point and solubility. [4] The naming of L-Glucose is shown on the fischer projection shown above in Figure 1. This is due to the fact that the OH group is indicated on the left of the chiral carbon.
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If the OH was indicated to the right of the chiral carbon then it will be called D-Glucose. [5]D-Glucose is the most common isomer found of glucose, this is because it occurs naturally as it is essential as part of the building blocks that is used to make disaccharides. It is the only sugar present in cellulose and starch. Also since it is made naturally in animals and plants, this makes D-Glucose a main source of energy. [6]However, L-Glucose does not occur naturally and so is synthesised from D-Glucose which is the more common isomer.
This is because L-Glucose cannot be broken down by the body for the use of energy whereas D-Glucose can. [7] Since D-Glucose is biologically active, this means that L-Glucose can’t be used in the same way as D-Glucose. Another name for D-Glucose is Dextrose which means that it can be used in the food industry. This is because D-Glucose can take on a 6-ring configuration or a linear shape, which is the same as glucose which is also a 6-ring configuration. [8] Glyceraldehyde is also a three carbon monosaccharide which is essential for cellular respiration and photosynthesis. 9]Glyceraldehyde is a chiral molecule which means that there are four different substituent’s that are attached to the chiral carbon which consists of D-glyceraldehyde and L-glyceraldehyde. Also D-glyceraldehyde can be called R-glyceraldehyde and the same goes for L-glyceraldehyde as S-glyceraldehyde. [10] Also amino acids and sugars are synthesised from glyceraldehyde. The synthesised amino acid will be labelled after the D-isomer if it has been taken from D-glyceraldehyde when the order of the molecule is going in a clockwise otation and the same for L-glyceraldehyde but the order of the molecule is going in a counterclockwise rotation. [11] The D and L isomers of glyceraldehyde can be used to describe the stereochemistry of amino acids. This means that the D-amino acid and L-amino acid structure can be found by matching the structures to the D-glyceraldehyde and L-glyceraldehyde. By doing this, the stereochemistry of the amino acids can be defined as it is based on the comparisons of the chemical groups of the molecules. [12] Glucose and fructose are monosaccharide’s and are called structural isomers.
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This means that they both have the same general formula (C6H12O6) but have different structural formulas. Glucose is an aldose, which means that it is as monosaccharide that has an aldehyde group attached. [13]However, fructose is a ketose which means that it is a monosaccharide that has a ketone group attached. [14]This shows that they are also anomers of each other and will always have D/L isomers. Therefore in the synthesis of sucrose it is by a condensation reaction that uses ? -D-glucose and ? -D-fructose to produce sucrose. [15] Lactose is another common disaccharide which contains ? D-galactose which is attached to ? -D-glucose which then forms ? -(1-4) glycosidic bonds. D-galactose and D-glucose are isomers of each other with the same general formula. D-glucose is more common that D-galactose as it is a sugar that is needed to provide energy to the body. However D-galactose is also a sugar but is not as sweet in comparison to glucose. Both of these isomers are synthesised in the body to provide energy to the body. [16] Starch is the most common polysaccharide found that is used for storage in plants. It is made up of ? -amylose and amylopectin.
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Amylose is a linear structure that forms a coiled shape made up of thousands of ? -(1-4)-glucose units. Amylopectin is made up of both ? (1-4) and ? (1-6) linkages of D-glucose units. This makes amylopectin to form a branched and compact structure. Also, because of the stereochemistry of D-glucose in starch, this isomer can be digested in humans which means that it can be used as an energy source for humans. [17] [pic] [pic] Cellulose is made up of thousands of ? -D-glucose molecules that have ? -(1-4)glycosidic bonds. When it is fully hydrolysed, glucose is produced also cellulose can be synthesised from cellobiose. [18]
Stereochemistry is essential in carbohydrates as it can be used to produce many other molecules in different forms that can be used for many different functions. This is important as stereochemistry is important when producing drugs that are either harmless or dangerous to the human body. An example of this is that the body can produce and digest the sugars and carbohydrates of a specific stereochemistry. Also stereochemistry is important for certain proteins such as hair and skin, which are all made up of amino acids of a single stereochemistry. It is fascinating how starch and cellulose can come from one simple sugar which is glucose.
