Introduction:

Two linked monosaccharides are termed as a disaccharide and these are the simplest polysaccharides. Illustrations comprise sucrose and lactose. They are comprised of two monosaccharide units bound altogether via a covalent bond termed as a glycosidic linkage formed by a dehydration reaction, resultant in the loss of a hydrogen atom from one monosaccharide and a hydroxyl group from the other. The formula of unchanged disaccharides is C12H22O11. However there are many types of disaccharides, a handful of disaccharides are specifically notable.

Composition of Disaccharides:

Disaccharides are manufactured of two monosaccharides. An example of this is illustrated in the table below.

Disaccharide    Description           Component monosaccharides

Sucrose         common table         sugar glucose 1α→2 fructose

Maltose         product of starch     hydrolysis glucose 1α→4 glucose

Trehalose      found in fungi          glucose 1α→1 glucose

Lactose         main sugar in milk    galactose 1β→4 glucose

Melibiose       found in legumes     galactose 1α→6 glucose

Sucrose:

Sucrose, as well known as saccharose, is common table sugar refined from the sugar cane or sugar beets. This is the major ingredient in turbinado sugar, evaporated or dried cane juice, brown sugar and confectioner's sugar. Sucrose is the richest disaccharide, and the most important form in which carbohydrates are transported in plants. It is composed of one D-glucose molecule and one D-fructose molecule. The systematic name for sucrose, O-α-D-glucopyranosyl-(1→2)-D-fructofuranoside, points out four things:

a) Its monosaccharides: glucose and fructose

b) Their ring kinds: glucose is a pyranose and fructose is a furanose

c) How they are linked altogether: the oxygen on carbon number 1 (C1) of α-D-glucose is linked to C2 of D-fructose.

d) The -oside suffix points out that the anomeric carbon of both monosaccharides partakes in the glycosidic bond.

It consists of a glycosidic bond linking the anomeric hydroxyls of glucose and fructose. As the configuration at the anomeric carbon of glucose is α (that is, O points down from the ring), the linkage is designated α (1→2). 

Fig: Sucrose structure

Lactose:

Lactose consists of a molecular structure comprising of galactose and glucose having β (1→4) linkage from the anomeric hydroxyl of galactose. Its full name is β-D-galactopyranosyl-(1→4)-β-D-glucopyranose. It takes place naturally in mammalian milk. This is of interest as it is related by the lactose intolerance which the intestinal distress is caused due to a deficiency of lactase, an intestinal enzyme required to absorb and digest lactose in milk. Undigested lactose ferments in the colon and causes abdominal pain, gas, bloating and diarrhea. Yogurt doesn't cause such problems as lactose is used by the bacteria which transform milk to yogurt.

Fig: Lactose structure

Maltose:

Maltose comprises of two α-D-glucose molecules having the alpha bond at carbon 1 of one molecule linked to the oxygen at carbon 4 of the second molecule. This is termed as a 1α→4 glycosidic linkage. This is a disaccharide having an α(1 →4) glycosidic linkage between the C1 hydroxyl of one glucose and the C4 hydroxyl of the second glucose. Maltose is the β-anomer, as the O at C1 points down from the ring.

Fig: Maltose structure

Trehalose:

Trehalose consists of two α-D-glucose molecules linked via carbon number one in a 1α→1 linkage. It is as well termed as mycose or tremalose, is a natural alpha-linked disaccharide made by an α,α-1,1-glucoside bond between the two α-glucose units. Trehalose is a non-reducing sugar made from two glucose units linked through a 1-1 alpha bond, providing it the name of α-D-glucopyranosyl-(1→1)-α-D-glucopyranoside. The bonding forms trehalose very resistant to acid hydrolysis, and thus is stable in the solution at high temperatures, even in acidic conditions.

Fig: Trehalose structure

Cellobiose:

Cellobiose is the disaccharide comprising of two β-D-glucose molecules which encompass a 1β→4 linkage as in cellulose. This is a product of cellulose breakdown. The configuration at the anomeric C1 is β (that is, O points up from the ring). The β (1→4) glycosidic linkage is symbolized as a 'zig-zag' line, however one glucose residue is in reality flipped over relative to the other. Cellobiose consists of no taste, while maltose and trehalose are around one-third as sweet as sucrose.

Fig: Cellobiose structure

Gentiobiose:

Gentiobiose is the disaccharide comprised of two units of D-glucose linked by a β(1->6) linkage. This is a white crystalline solid which is soluble in water or hot methanol. Gentiobiose is incorporated to the chemical structure of crocin, the chemical compound which provides saffron its color. This is a product of caramelization of glucose and its systematic name is 6-O-β-D-Glucopyranosyl-D-glucose

Fig: Gentiobiose structure

Reducing and Non-reducing Disaccharides:

Whenever the alcohol component of a glycoside is given by means of a hydroxyl function on the other monosaccharide, the compound is termed as a disaccharide. Acid-catalyzed hydrolysis of such disaccharides results glucose as the only product. Enzyme-catalyzed hydrolysis is choosy for a particular glycoside bond; therefore an alpha-glycosidase cleaves maltose and trehalose to glucose, however doesn't cleave cellobiose or Gentiobiose. A beta-glycosidase consists of the opposite activity. In order to sketch a representative structure for cellobiose, one of the glucopyranose rings should be rotated through 180º, however this characteristic is frequently omitted in favor of retaining the usual viewpoint for the individual rings. The bonding between the glucopyranose rings in cellobiose and maltose is from the anomeric carbon in ring A to the C-4 hydroxyl group on ring B. This leaves the anomeric carbon in ring B free, therefore cellobiose and maltose both might suppose alpha and beta anomers at that site. Gentiobiose consists of a beta-glycoside link, originating at C-1 in ring A and terminating at C-6 in ring B. Its alpha-anomer is sketch in the diagram. As cellobiose, maltose and Gentiobiose are hemiacetal then they are all reducing sugars (that is, oxidized via Tollen's reagent). Trehalose, a disaccharide found in some mushrooms, is a bis-acetal, and is thus a non-reducing sugar.

A methodical nomenclature for disaccharides exists, however as the given illustrations explain, these are often lengthy.

Comparison of Disaccharides:

However, all the disaccharides are prepared of two glucopyranose rings; their properties are different in interesting manners. Maltose, at times termed as malt sugar, comes from the hydrolysis of starch. It is around one third as sweet as cane sugar (that is, sucrose), is simply digested via humans, and is fermented via yeast. Cellobiose is acquired by the hydrolysis of cellulose. It virtually has no taste, is indigestible via humans, and is not fermented via yeast. Some of the bacteria encompass beta-glucosidase enzymes which hydrolyze the glycosidic bonds in cellobiose and cellulose. The presence of such bacteria in the digestive tracts of cows and termites permits such animals to make use of cellulose as a food. Lastly, it might be noted that trehalose consists of a noticeably sweet taste, however Gentiobiose is bitter. 

The Disaccharides made up of other sugars are acknowledged, however glucose is often one of the components. Lactose, as well termed as milk sugar, is a galactose-glucose compound linked as a beta-glycoside. This is a reducing sugar due to the hemiacetal function remaining in the glucose moiety. Most of the adults, specifically those from areas where milk is not a dietary staple, encompass a metabolic intolerance for lactose. Infants encompass a digestive enzyme that cleaves the beta-glycoside bond in lactose; however production of this enzyme stops with weaning. Cheese is less subject to the lactose intolerance problem, as most of the lactose is eliminated by the whey. Sucrose, or cane sugar, is our most generally employed sweetening agent. This is a non-reducing disaccharide comprised of glucose and fructose linked at the anomeric carbon of each through glycoside bonds (that is, one alpha and one beta).

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