2.2 Heteroglycans

2.3 Carbohydrate Biogenesis

Synonyms British Gum; Starch Gum; Leiocom; Pyrodextrin; Torrefaction dextrin; Canary dextrin; Yellow dextrin; White dextrin.

Preparation Dextrin is prepared by carrying out the incomplete hydrolysis of starch with dilute acid or by heating dry starch.

Various types of dextrin are prepared as detailed below namely:

(a) British Gum, Starch Gum: It is produced at high temperature in the absence of acid.

(i) Dark brown colour, odourous,

(ii) High viscosity, very soluble in cold water,

(iii) Does not reduce Fehling’s Solution, and

(iv) Gives reddish-brown colour with iodine.

(b) Canary Dextrin, Yellow Dextrin: It is prepared by hydrolyzing starch at high temperature for a longer duration but in the presence of small quantum of acid.

(i) Light brown to yellow colour, slight odour, and

(ii) Low viscosity, very soluble in cold water.

(c) White Dextrin: It is prepard by hydrolysis at low temperature for a shorter duration but in the presence of large quantum of acid.

(i) White colour, odourless,

(ii) Slightly soluble in cold water, and gives a red colour with iodine, and

(iii) Very soluble in hot water and gives a blue colour with iodine.

1. As an excipient for dry extracts and pills.

2. It is used for preparing emulsions and dry bandages.

3. It is employed for thickening of dye-pastes and mordants used in printing fabrics in fast colours.

4. It is used for sizing paper and fabrics.

5. It is employed for preparing felt and printing tapestries.

6. It is used for preparing printer’s inks, glues and mucilage.

7. It is employed for polishing cereals.

8. It is extensively used in making matches, fireworks and explosives.

Synonyms Purified cotton; Cotton wool; Surgical cotton.

Biological Source Cotton comprises of the epidermal trichomes (or hairs) from the seeds of different species of Gossypium, such as : G. herbaccum; G. hirsutum; G. barbedense; belonging to the family Malvaceae. In fact, absorbent cotton or purified cotton consists exclusively of the trichomes that are completely freed from adhering impurities, fat, properly bleached and finally sterilized.

Geographical Source Cotton is produced on large scale in USA, Egypt, China, India, South America and certain parts of Africa. Egyptian cotton–yarn enjoys a world-wide reputation. Both India and China are not only self-sufficient in the production of absorbent cotton but also exports a substantial quantity to various countries.

Preparation The cotton plant after flowering bears fruits which are also called ‘capsules’ or ‘balls’. These are usually 3-5 celled. Once the fruit ripens they open-up widely that contains a number of seeds per loculus. The brown coloured seeds are normally surrounded with a thick mass of white hairs. The long-lint hairs are known as ‘staple’ or ‘floss’; whereas the short-fuzz hairs are called ‘linters’. The cotton fibres (i.e., mass of white hairs) along with their seeds are collected manually by hand picking. The raw cotton is subjected to a mechanical process called ‘ginning’ whereby only the hairy substance is collected separately and the undesired substances, such as: dirt, leaf-fragments and other foreign materials are removed separately. ‘Delintering’ is the mechanical process which discards the short hairs that eventually passed along with the cotton fibres obtained from the ‘ginning’ process. The raw segregated long-sized cotton hairs are subsequently freed from colouring matters and traces of wax and oil coating the hairs which render them non-absorbent. The treated absorbent cotton obtained above is processed through the ‘carding machine’ so as to arrange the fibres in parallel direction and also to get rid of immature fibres completely. Short fibres are once again removed by ‘combing’ mechanically. Finally, the processed cotton fibres are defatted (with alkali) washed, bleached (with chlorinated soda) and then washed (with diluted mineral acid). It is again washed, dried, recarded and sterilized.

Description White, soft, fine, filament like hairs appearing under the microscope as hollow, flattened and twisted bands, striate and slightly thickened at the edges, practically odorless and tasteless. Cotton fibres are usually 2.5 to 4.5 cm in length and 25 to 35 µ in diameter.

Chemical Constituents Absorbent cotton is mostly cellulose 93-94% and moisture 6-7%

1. It is employed as surgical dressings.

2. It is mostly used in the textile industry to prepare a wide range of fibres.

3. It is invariably employed as its derivatives to be recognized as the most versatile adjunct in pharmaceutical formulations, for instance:

Microcrystalline cellulose – as Tablet Disitegrant

Carboxymethyl Cellulose (CMC) – as Binder and thickening agent;

Cellulose acetate phthalate – as an Enteric coating material;

Ethyl Cellulose – as Binder and Film

Methyl Cellulose coating substance;

Hydroxypropyl methyl Cellulose

Oxidised Cellulose – as local Haemostatic;

Purified ‘Rayon’ – as Surgical aid;

Pyroxylin – as an ingredient in the preparation of Collodian and nail polishes.

4. It is used as a filtering medium and also as an insulating material.

5. Pharmaceutical grade cotton seed oil is used as an emolient and in the preparation of Steroidal Hormone Injections.

6. It is used for making explosives.

Cellulose represents one of the most widely distributed and abundantly available organic matter on this planet. It is, in-fact, the most important structural element of higher-plant-cell walls. In nature, wood (40-50% cellulose) caters as the major source of cellulose for industrial utilities, whereas cotton (98% cellulose) provides the balance requirement globally.

Geographical Source It has been observed that nearly thirty billion MT of carbon is transformed annually into organic compounds by higher plants and out of this approximately 1/3rd is converted into cellulose. As cellulose is profusely utilized in the form of wood to build houses, paper industry and textile industry, a considerable amount of research has been duly conducted on this well-known polysaccharide.

Preparation The scientific and large-scale methods for preparing cellulose essentially involves the removal of excess of the non-cellulose substances e.g. Lignin. In fact, there are three well defined and established procedures whereby the undesired ‘lignin content’ present in wood shavings

are removed exhaustively, namely:

(a) Treatment with Sodium Bisulphite [Sulphonate Process]: The small wood chips are boiled with a solution of sodium bisulphite whereupon the lignin is removed as lignosulphonate,

(b) Treatment with Sodium Hydroxide [Soda Process]: The wood chips on being boiled with sodium hydroxide solution removes the lignin content as soluble products, and

(c) Treatment with NaOH and Na₂SO₄ [Sulphate Process]: The sodium sulphide (Na₂S) obtained by the interaction of NaOH and Na₂SO₄ will remove most of the lignin component from the wood shavings.

However, the traces of lignin may be removed by bleaching with chlorine. The remaining mixture of hemicellulose and cellulose are subsequently extracted by subjecting it to alkaline treatment. The readily soluble hemicellulose are removed by treatment with higher concentration of NaOH solution, whereas the cellusans (Xylans) may be removed by treatment with a 5% solution of NaOH.

Description Cellulose has molecular weights ranging from 250,000 to 1,000,000 or even more. It is assumed that at least 1500 glucose units may be present in each molecule. Based on the findings by X-ray analysis and electron microscopy it is revealed that these long chains lie side by-side in bundles, held together by H-bonds available between the huge number of adjoining –OH moieties.

Further, these bundles are twisted together to give rise to rope-like structures, that ultimately are clubbed together to yield the normal apparently visible fibers. Interestingly, in the case of wood

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these cellulose “ropes” are meticulously embedded in lignin to afford a structure that resembles to concrete reinforced structures used for making buildings.

Chemical Constituents Cellulose is comprised of chains of D-glucose units, whereby each unit is joined by a glycosidic linkage to C-4 of the next unit.

Cyclodextrins invariably consist of 6, 7 or 8 molecules (viz. α, β and γ cyclodextrin) in a 1, 4-configuration to result into the formation of rings having various diameters. In fact, based on the geometry of the chiral isomer, only one would possibly gain entry into the cavity in the ring while the other is excluded evidently.

Synonyms Cyloamyloses; Cycloglycans; Schardinger dextrins.

Biological Source Starch on being treated with the amylase of Bacillus macerans, a specific enzyme, gives rise to a mixture of cyclodextrins. They are naturally occuring carbohydrates.

Preparation It is obtained from the action of B. macerans amylase on starch to yield homogeneous cyclic α – (1→4) linked D-gluco-pyranose units.

Description The various rings constituting the cyclodextrins appear to be as doughnut shaped. However, α-cyclodextrin i.e., the smallest of the lot, has a diameter about two times that of 18-crown–6 (viz., as crown ethers) and its hole (4.5ºA across) is approximately two times as broad.

Chemical Constituents Cyclodextrins mainly are comprised of three different types, as detailed below:

The structure of α-cyclodextrin may be represented in Figure 3.2, in two different manners, namely:

1. As enzyme models based on the fact that , like enzymes, they first and foremost bind the substrate and then, through substituent groups, act on it.

2. As a complexing agent to explore the various types of enzyme action.

3. It may be employed as an additive to the mobile phase (in HPLC*), but it invariably gets bound to an innert support material.

Dextran is a carbohydrate substance made up predominantly of D-glucose units i.e. (C₆H₁₀O₅)n. It is α–1, 6 linked polyglucan.

Synonyms Dextraven; Expandex; Gentran;Hemodex; Intradex; Macrose; Onkotin; Plavolex; Polyglucin.

Biological Source A number of organisms produce dextrans; however, only two of them, namely; Leuconostoc mesenteroides and L. dextranicum, belonging to the family Lactobacteriaceae, have been used commercially.

Preparation Commercially, dextrans are manufactured by the process of fermentation of sucrose (a disaccharide) either by cell-free enzymatic fermentation technique or by whole culture technique. The enzymes that are responsible for producing dextrans from sucrose as a substrate are collectively known as ‘dextran-sucrases’.

Native dextran possesses a very high moleculer weight, whereas the clinical dextrans have lower moleculer weights, for instane; Dextran 40 [Gentran 40(R) (Baxter); Rheomacrodex(R) (Pharmacia)]; Dextran 70 [Hyskon(R) & Macrodex(R) (Kabi Pharmacia)]; Dextran 75 [Gentran 75(R) (Baxter)]; These may be accomplished by controlled depolymerization e.g., ultrasonic vibration, fungal dextranase and acid hydrolysis.

Description Dextrans obtained by the precipitation from methanol vary considerably with regard to their characteristic physical and chemical properties which solely depends upon the individual method of preparation.

Chemical Constituents The interaction between ‘n’ molecules of sucrose and ‘x’ number of glucose moieties yields dextran together with ‘n’ molecules of fructose as shown in the following equation:

nSucrose + (Glucose)x → (Glucose)x+n + nFructose

                     Primer             Dextran

1. Dextran 40 is employed as an isotonic solution either to prime pumps or to improve flow in surgery concerned with cardiopulmonary bypass. Thus, it exerts its effect by lowering the viscosity of blood and improving flow; and the latter is caused due to hemodilution.

2. Dextrans as a whole helps in minimising platelet adhesiveness, which property is gainfully exploited in their usage for prophylaxis of thrombosis and thromoembolism both during and after surgery.

3. Both Dextran 70 and Dextran 75 find their extensive use as plasma extender for the control and management of hypovolemic shock. Hypertonic solutions usually afford the dehydration of tissues, whereby the abstracted water being added to the plasma causing increase in its volume. For this reason they are quite useful in the prevention and treatment of toxemia of pregnancy and nephrosis.

4. Dextran 70 and Dextran 75 are used in 6% solutions to prevent pending shock caused by hemorrhage, trauma, and severe burns.

5. Dextran 40 (10% solution) is not only used to lower blood viscosity but also to improve microcirculation at low flow rates.

6. Dextrans are employed in the formulation of fat-soluble vitamins (viz. Vitamin A, D, E, K).

7. It is also used in preparing sustained released tablets.

8. Dextrans find their abundant applications in various types of confectionaries, for instance: icecreams, candies, jellies, syrups and cake–topings.

9. It is employed as an adjunct in cosmetic preparation exclusively meant for soothening wrinkles.

Synonym Moss starch; Lichen starch;

Biological Source Cetraria islandica (L.) Ach., Family: Parmeliaceae. It is known as Iceland Moss.

Description It is a cellulose like polysaccharide which occurs as a cell wall component in lichens. It is readily soluble in hot water to give rise to a colloidal solution. It is more rapidly hydrolyzed than cellulose. It produces cellobiose upon acetylation with acetic anhydride and sulphuric acid. It is a white powder.

On methylation followed by hydrolysis it yields 2,3,6-trimethylglucose as a major component and tetramethylglucose as minor one thereby suggesting that the chain present in lichenin is not branched at all as in cellulose.

Chemical Constituents The exact chemical structure of lichenin molecule is yet to be established; however, it has been indicated that it may contain both β-1, 4 and β-1, 6 linkages.

It is found to bear a close resemblance to starch except that it is a levulan rather than a dextran. The following characteristic features make it altogether different from starch, namely:

* Gives yellow colouration by iodine.

* Does not gelatinize with water.

* Not commonly found in plants in the form of granules having concentric layers, and

* Upon hydrolysis in acidic medium yields fructoses.

Synonyms Dahlin; Alantin; Alant starch.

Biological Source It occurs in certain plants of the Compositae family, such as:

Inula helenium Linn. : Roots contain inulin;

Eupatorium cannabinum Linn. : Plant contains inulin;

Cynara scolymus Linn. : Flower heads contain inulin;

Carpesium cernuum Linn. : Roots contain inulin;

Calendula officinalis Linn. : Roots contain inulin;

Aretium lappa Linn. : Roots contain 45% inulin.

A. lappa – Western Himalayas from Kashmir to Simla;

C. officinalis – India, Pakistan;

C. cernuum – Temperate Himalayas and Nilgiri Hills (India);

S. scolymus – Throughout India; E. canabinum—Temperate Himalayas;

I. helenium – Europe and Asia.

Preparation Isolated from dahlia tubers and from other members of the family Compositae.

Description The crystals are spherical in shape when prepared from water.

[Structure of INULIN showing arrangement of Fructofuranose residues in chain]

Inulin is quickly hydrolyzed by acids to D-fructose by the enzymes inulase but does not undergo hydrolysis by the amylases. However, methylated inulin upon hydrolysis gives rise to 3,4,6-trimethylfructose as a major product and 1,3,4,6- tertamethylfructose as a minor product, thereby suggesting that the fructose residues are present in the furanose form and the adjacent units are joined through C-1 and C-2 (i.e., the glycosidic hydroxyl moiety).

1. It is used in culture media as a fermentative identifying agent for certain bacterial species.

2. It is filtered exclusively by the glomeruli and is neither secreted nor reabsorbed by the tubules. Hence, it is employed as a diagnostic agent for evaluation of glomerular filteration i.e., renal–function test (or kidney function test).

3. It is considered to be valuable in the diet of the diabetic patients.

Hetastarch is a semisynthetic material that essentially comprises of more than 90% amylopectin, which has been treated with pyridine and ethylene chlorohydrin, so as to give rise to 7 to 8 hydroxyethyl substituents present for every 10 D-glucopyranose units of the starch polymer. The molecular weight is approximately 450, 000 daltons.

Where R or R′ = H or CH₂-CH₂-OH

Amylopectine Derivative It specifically differs from the amylose derivative in that the sequenceis frequently interrupted by a unit in which R is the residue of an additional o-hydroxyethylated α-D-glucopyranosyl moiety that essentially constitutes the first unit in a branch or sub-branch of the polymer.

1. It serves as a ‘Plasma Volume Expander’. A 6% solution is osmotically equivalent to a 5% albumin solution. But in the blood, it draws up certain quantum of water either from the intestinal or intracellular fluids, thus expanding the blood volume slightly in excess of the volume infused.This acquired expansion lasts for 24 to 36 hours.

2. It is employed in the management and treatment of hypovolemic shock*.

3. It is also used as a suspension medium for leukapheresis**.

4. It is employed as a cryoprotective*** agent for erythrocytes.

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Synonyms Madhu, Madh, Mel, Honey (English);

Biological Source Honey is a viscid and sweet secretion stored in the honey comb by various species of bees, such as: Apis dorsata, Apis florea, Apis indica, Apis mellifica, belonging the natural order Hymenotera (Family: Apideae).

Geographical Source Honey is available in abudance in Africa, India, Jamaica, Australia, California, Chili, Great Britain and New Zealand.

Preparation Generally, honey bees are matched with social insects that reside in colonies and produce honey and beeswax. Every colony esentially has one ‘queen’ or ‘mother bee’, under whose command a huge number of ‘employees’ exist which could be mostly sterile females and in certain seasons male bees. The ‘employees’ are entrusted to collect nector from sweet smelling flowers from far and near that mostly contains aqueous solution of sucrose (ie; approximately 25% sucrose and 75% water) and pollens. Invertase, an enzyme present in the saliva of bees converts the nector into the invert sugar, which is partly consumed by the bee for its survival and the balance is carefully stored into the honey comb. With the passage of time the water gets evaporated thereby producing honey(ie; approximately 80% invert sugar and 20% water). As soon as the cell is filled up completely, the bees seal it with wax to preserve it for off-season utility.

The honey is collected by removing the wax-seal by the help of a sterilized sharp knife. The pure honey is obtained by centrifugation and filtering through a moistened cheese-cloth. Invariably, the professional honey collectors smoke away the bees at night, drain-out honey, and warm the separated combs to recover the beeswax.

Appearances : Pale yellow to reddish brown viscid fluid,

Odour : Pleasant and characteristic,

Taste: : Sweet, Slightly acrid,

Specific gravity : 1.35-1.36

Specific rotation : +3^o to –15^o

Total Ash : 0.1-0.8%

However, the taste and odour of honey solely depends upon the availability of surrounding flowers from which nector is collected. On prolonged storage it usually turns opaque and granular due to the crystallisation of dextrose and is termed as ‘granular honey’.

Chemical Constituents The average composition of honey rangles as follows: Moisture 14-24%, Dextrose 23-36%, Levulose (Fructose) 30-47%, Sucrose 0.4-6%, Dextrin and Gums 0-7% and Ash 0.1-0.8%. Besides, it is found to contain small amounts of essential oil, beeswax, pollen grains, formic acid, acetic acid, succinic acid, maltose, dextrin, colouring pigments, vitamins and an admixture of enzymes eg; diastase, invertase and inulase. Interestingly, the sugar contents in honey varies widely from one country to another as it is exclusively governed by the source of the nector (availability of fragment flowers in the region) and also the enzymatic activity solely controlling the conversion

of nector into honey.

Substituents/Adulterants Due to the relatively high price of pure honey, it is invariably adulterated either with artificial invert sugar or simply with cane-sugar syrup. These adulterants or cheaper substituents not only alter the optical property of honey but also its natural aroma and fragrance.

1. It is used as a sweetening agent in confectionaries.

2. Being a demulsent, it helps to relieve dryness and is, therefore, recommended for coughs, colds, sore-throats and constipation.

3. Because of its natural content of easily assimilable simple sugars, it is globaly employed as a good source of nutrient for infants, elderly persons and convalescing patients.

A large number of plant products belonging to this particular category are namely: honey, starch, hetastarch, inulin, lichenin, dextran, cyclodextrins, cellulose, cotton, and dextrin.

In broader sense the polysaccharides or glycan may be classified into two major groups, namely:

(a) Homoglycans, and

(b) Heteroglycans

These two major groups would be described in details with the help of important representative members in the sections that follows:

The Germans first and foremost introduced the word ‘kohlenhydrates’ which was later on coined to carbohydrates. The name obviously suggests that these compounds are essentially the hydrates of carbon. In reality, all carbohydrates comprise of carbon, hydrogen and oxygen; whereas, the last two elements are found to exist in the same proportions as in water ( i.e., H₂O – 2:1). However, it has been observed that there are certain compounds that do conform to the said ‘hydrate rule’ i.e., maintain the ratio of H and O (2:1) but do not belong to the category of carbohydrates, for instance:

(i) Formaldehyde [HCHO] 2:1

(ii) Acetic Acid [CH₃COOH] 2:1

(iii) Lactic Acid [C₃H₆O₃] 2:1

Besides, there exist such compounds that evidently show the chemical properties of carbohydrates but do not necessarily abide by the above mentioned ‘hydrate rule’, for example:

In view of the above cited glaring examples with regard to various anomalies the terminology ‘Carbohydrates’ has still been retained to represent not only the sugars but also those substances that are related to them basically in structure and other characteristic features.

Invariably, the carbohydrates belong to the chemical class of the aldehydes, ketone alcohols, and also the condensation polymers of these partially oxidized polyalcohols collectively known as ‘Polysaccharides’ or ‘Oligosaccharides’.

Glycan is the generic term for polysaccharide and in the systematic nomenclature the latter is assigned a suffix “-an”. Generally, the polysaccharide may be classified into two broad heads, namely:

(a) Homoglycan: The polysaccharide is termed as homoglycan when it contains only one type of monosaccharide unit, and

(b) Heteroglycan: The polysaccharide is known as heteroglycan when it involves more than one kind of monosaccharide unit.

However, a more accurate and precise demarkation of polysaccharides essentially makes use of nomenclature that includes, first the type of monosaccharide building unit, and secondly, the exact position and configuration of the glycosidic linkage involved.

(i) Homoglycan: e.g., Cellulose. It may also be expressed as β-1, 4 –D-glycan by virtue of the following reasons, namely:

* Prevailing attached unit is D-glucose,

* D-glucose bears the β-configuration at the anomeric C-atom (i.e., C-1),

* C-1 is linked to C-4 of the next identical unit of D-glucose.

(ii) Heteroglycan: e.g., D-gluco-D-mannose. It is made up of D-glucose and D-mannose. The two altogether different monosaccharides usually show up in an orderly manner. In this particular instance, the diheteroglycan is composed of two different types of monosaccharides that has been arranged in an alternating and regular fashion.

It is worhwhile to mention at this juncture that plant kingdom provides a variety of complex polysaccharrides, such as: cellulose, starch, dextran, inulin and the like. These complex polysaccharides yield the respective sugar residues upon hydrolysis, for example:

Pentosans Hydrolysis-> Pentoses, Arabinose, Xylose, Ribose;

Hexosans Hydrolysis-> Hexoses, Glucose, Fructose;

Fructan Hydrolysis-> Inulin that results Fructose;

Glucan Hydrolysis-> Strach that gives Glucose.

Nevertheless, the starch and sugars find their abundant applications not only as food or food supplements, but also as indispensable adjuvants in the formulation of a wide range of pharmaceutical products all over the globe.