Disaccharides are sugars which contain two monosaccharide units joined together. Figure 1 illustrates the combination of a D-glucose and a D-fructose unit to form the disaccharide sucrose. While this animation indicates that creation of the disaccharide is accompanied by the formation of a water molecule, it is not intended to present an accurate picture of the mechanism of disaccharide formation.
Formation of a Disaccharide
Figure 2 highlights some of the important structural features of polysaccharides in general and of sucrose in particular.
Structural Features of Polysaccharides
Sucrose provides a good example of a non-reducing sugar; there are no aldehyde or hemiacetal groups in this molecule.
In sucrose, the oxygen atom attached to C1 of the D-glucopyranose ring is also bonded to C2' of the D-fructofuranose ring. This is called a 1,2-glycosidic linkage. Since sugars are polyhydroxyaldehydes and ketones, it should not be surprising that other linkages are possible. Maltose and lactose are disaccharides which contain alternative glycosidic linkages. Figure 3 presents the structures of these two sugars.
Maltose and Lactose: Alternative Glycosidic Linkages
Maltose contains two molecules of D-glucose. Lactose is comprised of one molecule of D-glucose and one of D-galactose. In both cases the monosaccharide units are connected by 1,4-glycosidic linkages, i.e. they both contain a C1-O-C4' link. In maltose the C1-O bond is in the a position, while in lactose it is oriented b to the D-galactose ring. Remember that the a and b designations refer to the disposition of the oxygen atom at an anomeric carbon.
Would you expect maltose and/or lactose to be reducing sugars?
By far the most common polysaccharide is cellulose. While the structure of cellulose is complex, the "backbone" of this polymer consists of D-glucopyranose rings joined together by C1-O-C4 links. Figure 4 illustrates this bonding mode for 4 D-glucopyranose units.
A Partial Structure of Cellulose
Note how the D-glucopyranose rings alternate their orientation along the "backbone". Apparently this arrangement allows for one cellulose chain to pack close to another, thus maximizing "inter-chain" hydrogen bonding interactions. Note also that all the glycosidic connections are b-1,4 linkages.
Starch is also a polymeric form of D-glucose. It consists of two components, amylose and amylopectin. Amylose is a linear poly-D-glucose in which the monosaccharides are connected by a-1,4 linkages. The basic repeat unit of amylose is illustrated in Figure 5.
The Repeat Unit of Amylose
Amylopectin is structurally more complex than amylose. It consists of multiple "strands" of amylose "cross-linked" by a-1,6 linkages. Approximately every 20th-25th D-glucose unit of one amylose chain is "cross-linked" to another amylose chain. Figure 6 gives a partial structure for amylopectin.
Partial Structure of Amylopectin
The structure of glycogen, the form in which D-glucose is stored in the liver, is similar to that of amylopectin. However, glycogen is even more highly "cross-linked". Approximately one out of every 10 glucose units in one amylopectin chain is connected to another amylopectin chain by an a-1,6 link. With a molecular weight of over 1,000,000, glycogen contains nearly 3,000 D-glucose units.
Analysis of Polysaccharides
One of the first steps in determining the structure of a polysacchride is acid catalysed hydrolysis of the glycosidic linkages to produce the monomeric components of the polymers. This reaction is the reverse of the nucleophilic addition of alcohols that is typical of aldehydes and ketones. Write a mechanism for the hydrolysis of sucrose.
Glycosylamines and Amino Sugars
Sugars in which the OH group at C1 is replaced by an amino group are called glycosylamines. A special category of glycosylamines is known as nucleosides. They are sugars in which C1 is connected to a nitrogen atom of a heterocyclic amine. The most common nucleosides are those found in RNA and DNA. Their structures are presented in Figures 7 and 8, respectively.
The Nucleosides of RNA
The glycosylamines in Figure 7 are also called ribonucleosides because the sugar component is D-ribose. These four molecules are the building blocks of ribonucleic acids, RNA.
The Nucleosides of DNA
The glycosylamines in Figure 8 are also called deoxyribonucleosides because there is no oxygen atom attached to C2 of the sugar. These four molecules are the building blocks of deoxyribonucleic acids, DNA.
The abbreviations RNA and DNA stand for ribonucelic acid and deoxyribonucleic acid, respectively. These molecules are polymeric sugars in which the ribose or 2'-deoxyribose units are linked together by phosphate bonds between hydroxyl group at C-3' of one sugar and the hydroxyl group at C-5' of the next. Figure 9 shows the bonding pattern for four ribose units of RNA .
The Glycosidic Bonds in Nucleic Acids
The polymeric chain is similar to that in cellulose. However, unlike cellulose, the glycosidic bonds do not involve the anomeric carbons. The designations B1, B2, etc. in Figure 9 represent heterocyclic bases that are depicted in Figure 7.
Amino sugars are carbohydrates in which a non-anomeric oxygen atom has been replaced by a nitrogen atom. The simplest example is b-D-glucosamine. The structure of this amino sugar and its N-acetyl derivative are given in Figure 10.