BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 11. STRUCTURE AND PROPERTIES OF CARBOHYDRATES

11.2. Oligosaccharides

Oligosaccharides are polymeric CARBOHYDRATES whose molecules yield from two to ten monosaccharide molecules upon Hydrolysis. Oligosaccharides are widely distributed in microorganisms, plant, and animal Tissues, where they perform essential biological Functions, serving in particular as an energy source, providers of building blocks for the Synthesis of Other macromolecules, and playing a crucial role in The Development of Immunity, among other functions.

Some oligosaccharides are of paramount importance in Human Nutrition, which is why they are produced in massive quantities from natural raw Materials. First and foremost is The production of sucrose from sugar beet and sugar cane (current global sucrose production is ~ 5·10⁷ tons/year).

Based on the number of monose molecules formed during the hydrolysis of an oligosaccharide molecule, they are classified into: di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, and decasaccharides.

Depending on whether the oligosaccharide molecule contains residues of a single type of monosaccharide or different monoses, oligosaccharides are referred to as homooligosaccharides or heterooligosaccharides, respectively. Oligosaccharides may have a linear or branched polyglycosidic chain Structure, for example:

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Disaccharides are crystalline or amorphous, highly Water-soluble, optically active, sweet-tasting substances. Monosaccharide residues in disaccharide molecules can be linked either through their hemiacetal hydroxyls—via a glycosido-glycosidic bond—or through a glycosido-glucoside bond between the hemiacetal hydroxyl of one monosaccharide and one of the alcoholic hydroxyls of the second. Based on the type of bond, disaccharides are divided into glycosido-Glycosides and glycosido-glucosides.

Disaccharides of the first type (trehalose, sucrose), which lack free glycosidic hydroxyls, exist only in cyclic form

and do not participate in reactions characteristic of aldehyde or keto groups. They do not undergo mutarotation and do not reduce metals from their oxides; therefore, such disaccharides are called non-reducing. However, as polyhydroxy compounds, they can be alkylated and acylated.

Disaccharides of the second type (maltose, lactose, cellobiose) are similar in chemical properties to Monosaccharides. They exist in two tautomeric forms—cyclic and carbonyl—and are characterized by Reactions Involving the carbonyl and alcohol groups: they are oxidized to aldonic acids, reduced to polyhydric alcohols, form osazones, and undergo alkylation and acylation. Due to their reducing properties, they are called reducing disaccharides.

Sucrose (saccharose, beet or cane sugar) is found in the tissues of many plants. It accumulates in particularly large amounts in sugar beet ROOT crops (up to 28 %) and sugar cane stalks (about 20 %), making these plants the primary raw material for its production. Sucrose has a sweet taste, is highly soluble in water, and insoluble in ethanol, carbon tetrachloride, chloroform, and other Hydrocarbons. Sucrose solutions do not undergo mutarotation. It crystallizes without water, with a melting point of 185 °C. For a sucrose solution, [α]_D²⁰ = 66.5°. Upon hydrolysis, sucrose breaks down into monoses: glucose and fructose. The sucrose hydrolyzate, which is an equimolar mixture of these monoses, has a specific rotation angle of -20°, because these hexoses have opposite specific rotation angles ([α]_D²⁰ of glucose = +52.5°, [α]_D²⁰ of fructose = -92.5°). Due to the change from right-handed to left-handed rotation during The breakdown of sucrose, its hydrolysis is called the inversion of sugar, and hydrolyzed sucrose is known as invert sugar. An example of natural invert sugar is bee honey.

Sucrose consists of α-D-glucopyranose and β-D-fructofuranose linked together by their glycosidic hydroxyls:

The monosaccharide rings in the sucrose molecule are also brought into close proximity by two intramolecular Hydrogen Bonds:

Since both anomeric atoms of the monoses participate in The formation of the glycosidic bond, sucrose exhibits no reducing properties. Its molecule contains eight hydroxyl groups, of which the primary ones are capable of dissociation, which increases with higher solution alkalinity. In an alkaline environment (pH > 12.5), sucrose reacts with alkali and alkaline-earth metal hydroxides to form complex salts known as saccharates.

As a polyhydroxy compound, sucrose is capable of acylation and alkylation:

Sucrose octaacetate is used in the production of synthetic resins. Sucrose octapropionate, octabutyrate, and acetate-isobutyrate are also known. Derivatives of sucrose and Higher Fatty acids serve as effective eco-friendly detergents.

Trehalose (mycose, mushroom sugar) has been detected in Fungi, Algae, certain Bacteria, and insect hemolymph. It consists of two

molecules of α-D(+)-glucopyranose linked together by their glycosidic hydroxyls:

Aqueous solutions of trehalose do not undergo mutarotation, and its specific optical rotation is [α]D° = 178.3°. The absence of free glycosidic hydroxyl groups indicates that trehalose lacks reducing properties.

Lactose (milk sugar) is synthesized in the mammary gland Cells of mammals during Lactation and is found in animal milk (4.0–5.5% in cow's milk). It is 4 to 5 times less sweet than sucrose. Lactose consists of a β-D-galactose molecule and an a-D-glucose molecule linked together by an a-1,4-glycosidic bond:

Due to its free glycosidic hydroxyl group, it exhibits reducing properties, undergoes mutarotation in solution, and has a specific optical rotation of +52.6° for its aqueous solution. Unlike other disaccharides, lactose is relatively sparingly soluble in water, crystallizing as a crystal monohydrate that melts at 202 °C. It is non-hygroscopic, which is why it is used in pharmaceutical manufacturing as an excipient that does not degrade upon dehydration.

In reduction reactions, the lactose molecule is oxidized and converted into lactobionic acid:

Cellobiose is formed during the enzymatic breakdown of Cellulose by the enzyme cellulase, which is produced by microorganisms. The β-D-glucose residues in its molecule are linked by a β-1,4-glucosidic bond:

It is readily soluble in water. Crystalline cellobiose melts at 225 °C. The presence of a free hemiacetal hydroxyl group imparts reducing properties to cellobiose. It is capable of mutarotation. At tautomeric dynamic equilibrium, a cellobiose solution has [α]_D²⁰ = 34.6°. Oxidation of cellobiose yields cellobionic acid.

Maltose (malt sugar) is a product of the enzymatic (via malt) or acid Hydrolysis of Starch. It consists of two D-glucose molecules connected within the disaccharide molecule by an a-1,4-glucosidic bond. Maltose is capable of mutarotation, with the specific optical rotation of its aqueous solution at tautomeric dynamic equilibrium being [α]_D²⁰ = 13°. It crystallizes from solutions as a crystal monohydrate with a melting point of 102–103 °C. Maltose exhibits reducing properties, and its oxidation yields maltobionic acid:

Tri- and tetrasaccharides are derivatives of sucrose. Three isomeric trisaccharides—fructose polymer homologs (oligofructosides): 1-kestose, 6-kestose, and neokestose—have been found in the tissues of many plants.

1-Kestose (α-D-glucopyranosyl-1,2-β-D-fructofuranosyl-1,2-β-D-fructofuranoside) crystallizes as white flaky crystals, melts with decomposition at 200 °C, and has a specific optical rotation of its aqueous solution equal to +28.5°:

6-Kestose (α-D-glucopyranosyl-1,2-β-D-fructofuranosyl-6,2-β-D-fructofuranoside) crystallizes as white needle-like crystals with a melting point of 144 °C, [α]_D²⁰ = 27°:

Neokestose (β-D-fructofuranosyl-2,6-α-D-glucopyranosyl-1,2-β-D-fructofuranoside) is an amorphous substance with a specific optical rotation in aqueous solution of +29.0°:

A common structural feature of these trisaccharides is the substitution of hydrogen in one of the three primary alcoholic hydroxyl groups of the sucrose molecule with a fructosyl residue.

Sucrose also serves as a terminal group in many galactose oligosaccharides. Sucrose monogalactosides include raffinose, umbelliferose, planteose, etc.

Raffinose (melitose) — α-D-galactopyranosyl-1,6-α-D-glucopyranosyl-1,2-β-D-fructofuranoside:

Raffinose is found in large quantities in certain algae, fungi, tissues, and seeds of higher plants.

Sucrose digalactosides include tetrasaccharides such as stachyose, lychnose, isolychnose, and sesamose.

Stachyose (O-α-D-galactopyranosyl-1,6-α-D-galactopyranosyl-1,6-α-D-glucopyranosyl-1,2-β-D-fructofuranoside) is found in significant amounts in the roots, seeds, and bulbs of certain plants (particularly in Stachys tuberifera):

Most known higher oligosaccharides (ranging from penta- to octasaccharides) are sucrose polygalactosides. The involvement of sucrose in The structure of many other oligosaccharides illustrates its crucial role in plant METABOLISM.