How to Draw a Linear Monosaccharide Into a Chain

Monosaccharides

The recognition of simple sugars by plant lectins primarily depends on the interaction of some oxhydryls of the sugars with a few amino acid residues located in the carbohydrate-binding site.

From: Comprehensive Glycoscience , 2007

Bioreactor and Bioprocess Design for Biohydrogen Production

Kuan-Yeow Show , ... Duu-Jong Lee , in Biohydrogen (Second Edition), 2019

3.4 Types of Feedstock

Simple sugars like glucose, lactose, and sucrose are readily degraded and are favorite feedstocks for hydrogen production. Carbohydrates have been reported as the main in numerous studies of fermentative biohydrogen processes. The costs of pure carbohydrate feedstock, however, are often too high. Practical production can only be viable if substrates are derived from renewable or low-cost sources. Biomass and wastes of high sugars and/or complex carbohydrates contents appear to be the most favorable substrates [28,40,43,44].

Some feedstocks are not suitable for fermentative hydrogen processes because of their intricate structures. Nevertheless, they still can be readily converted by hydrogen-producing microbes after some forms of pretreatment [45]. Much higher hydrogen yield from cornstalk wastes after a pretreatment of acidification was reported [46]. Solid wastes like mixed wastes, food processing, organic residues, digester sludge, and municipal wastes have also been reported for fermentative production of biohydrogen [28,47,48]. Such complex wastes often contain high levels of fats and proteins, thus their conversion into hydrogen is relatively lower than that of carbohydrate-based effluents. Transformation of solid wastes and wastewater into biohydrogen is appealing from the environmental and the economical perspectives [28].

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Sugars

In Enological Chemistry, 2012

2 Structure of Carbohydrates

Simple sugars, recognizable by the suffix −ose, are polyalcohols with an aldehyde or ketone group. They are generally referred to as carbohydrates because their empirical formula is C_n(H₂O)_n ≡ (CH₂O)_n. They are divided into aldoses and ketoses and are commonly named according to their Fischer projection formula. In the case of aldoses, the carbons are numbered (from 1 upwards) starting from the aldehyde group, while in the case of ketoses, numbering starts at the carbon bearing the ketone group. Most sugars are chiral (meaning that they have asymmetric carbons) and are optically active. This activity is classified as (−) or (+) as follows:

(−) if they rotate plane polarized light to the left (levorotatory molecules)

(+) if they rotate plane polarized light to the right (dextrorotatory molecules)

FIGURE 6.1. Fischer projection of the main carbohydrates.

The prefixes d and l refer respectively to the (+) and (−) enantiomers of glyceraldehyde. Accordingly, monosaccharides in which the chiral center furthest from the aldehyde or ketone group has the same configuration as d-glyceraldehyde belong to the d series, while those having the opposite configuration belong to the l series. In the Fischer projection of d-glyceraldehyde, the −OH functional group lies to the right of its chiral center. In other words, it belongs to the d series. l-Glyceraldehyde, in contrast, belongs to the l series. Whether a monosaccharide belongs to the d or l series in terms of absolute configuration has no influence on its optical properties. In other words, it can be dextrorotatory or levorotatory.

Simple monosaccharides with increasing numbers of carbon atoms are considered derivatives of glyceraldehydes if they are aldoses or of dihydroxyacetone if they are ketoses. Both types of monosaccharides can have an absolute d or l configuration depending on whether the −OH group of the highest numbered asymmetric carbon is to the right or left of the carbon, respectively. Accordingly, the addition of carbons, one by one, to d-glyceraldehyde will give rise first to two d-tetroses, next to four d-pentoses, and finally to eight hexoses.

In the case of ketoses, the addition of a carbon atom to dihydroxyacetone produces just one tetrose (d series) and the addition of two atoms produces two pentoses. The lengthening of this carbon chain with a third atom gives rise to four ketohexoses. The same procedure applied to l-glyceraldehyde would produce l-aldoses.

2.1 Cyclization of Carbohydrates: Haworth Projection

The Fischer projection is only used to gain a better picture of the configuration of carbohydrates. In solution, the open-chain form of a monosaccharide is in equilibrium with the cyclic hemiacetal form (aldoses) or the hemiketal form (ketoses). This ring can be composed of five or six atoms (furanose and pyranose forms, respectively), and in such cases, the Haworth projection is more useful than the Fischer projection, as it shows the cyclic structure.

The cyclization of carbohydrates results in the appearance of a new chiral center at carbon 1 (anomeric carbon). This anomer is designated α when the −OH group is trans with respect to the −CH₂OH group which carries the sixth carbon atom (in other words, it is on the opposite side of the plane of the ring). Accordingly, it is designated β if the hemiacetal −OH group is cis with respect to the same group (on the same side of the plane).

FIGURE 6.2. d-Aldoses.

FIGURE 6.3. d-Ketosugars.

FIGURE 6.4. Haworth projection of glucose.

In solution, the α and β forms of all free monosaccharides are present in an equilibrium involving an open-chain form.

This phenomenon is known as mutarotation, as the two anomers do not generally have the same optical rotatory power and therefore an equimolecular solution of both anomers can have optical activity.

Equilibrium between the α and β forms in aqueous solution is reached when the mixture contains 63.6% of the β form. The optical rotatory power for this mixture is α_D = +52.7°.

Finally, to complete this brief overview of the structure of monosaccharides, it should be recalled that the actual conformation of a monosaccharide is not planar; accordingly, pyranose (six-membered) rings adopt the most stable carbohydrate form, the chair, while furanose (five-membered) rings adopt the conformation of an open envelope.

FIGURE 6.5. Mutarotation equilibria for glucose.

FIGURE 6.6. Chair diagram of α-d-glucose.

Monosaccharide Derivatives

Like simple pentoses and hexoses, oligosaccharides and polysaccharides in grapes and wine are also formed from a wide variety of monosaccharide derivatives such as:

•

Deoxy sugars

•

Methylated monosaccharides

•

Uronic acids

•

Aldonic acids

•

Aldaric acids

•

Amino sugars

A hydrogen atom can also be replaced by an alkyl group (−R) to give a branched monosaccharide.

The following symbols, consisting of three-letter abbreviations, tend to be used in complex formulas involving simple monosaccharides.

•

Glc→ glucose

•

Gal→ galactose

•

Fru→ fructose

•

Ara→ arabinose

•

Xyl→ xylose

•

Man→ mannose

•

Rha→ rhamnose (6-deoxy-l-mannose, which is a methyl-pentose)

•

Rib→ ribose

The symbols are accompanied by the suffixes p or f (in italics), depending on whether the rings are pyranose or furanose.

2.2 Monosaccharides of Interest in Winemaking

Of the aldoses that participate in the winemaking process, d-glucose (found in concentrations of several grams per liter in musts) and d-galactose (found in concentrations of 100 mg/L in wine) are of particular interest. In the case of C₆-ketoses, fructose is the only sugar that is considered essential. Together with glucose, this is the most important hexose found in grapes. Pentose sugar concentrations in must and wine vary between 0.3 and 2 g/L. The main sugars in this group are d-xylose and l-arabinose, which reach concentrations of several hundreds of milligrams per liter. d-Ribose and l-rhamnose (a methyl pentose), in contrast, do not exceed levels of 100 mg/L.

Glucose, fructose, and mannose, which all belong to the d series, are the most relevant simple hexoses in the study of grapes and wines and they are easily interconvertible thanks to their keto-enol tautomerism.

As mentioned in previous sections, the glucose found in grapes is dextrorotatory and is, accordingly, also referred to as dextrose. Likewise, fructose, which is levorotatory, used to be called levulose.

FIGURE 6.7. Interconversion between monosaccharides via keto-enol tautomerism.

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Carbohydrate Chemistry

Pratima Bajpai , in Biermann's Handbook of Pulp and Paper (Third Edition), 2018

17.2 Nomenclature

The monosaccharides , simple sugars that cannot be easily hydrolyzed into smaller units, are classified according to the number of carbon atoms in the molecule. This classification is used for carbohydrates with three to seven carbon atoms; that is, with trioses, tetroses, pentoses, hexoses, and heptoses. Aldoses are monosaccharides that have an aldehyde when in the acyclic form (in the absence of the hemiacetal form); ketoses are monosaccharides with a ketone when in the acyclic form (absence of the hemiketal bond). Glucose is an example of an aldohexose, and fructose is an example of a ketohexose or hexulose, a six-carbon ketose, as shown in Fig. 17.1.

Figure 17.1. Some examples of hexoses.

If the terminal RCH₂OH (at the C6 position) of an aldose is oxidized to a carboxylic acid, then the monosaccharide is known as an uronic acid; if the aldehyde is oxidized to a carboxylic acid, the compound is referred to as an aldonic acid; and if both terminal carbon atoms are oxidized to carboxylic acids, the compound is referred to as an aldaric acid. Monosaccharide constituents of particular importance in woody plant cell wall polysaccharides are the pentoses arabinose and xylose; the hexoses glucose, mannose, and galactose; and the uronic acid (4-O-methyl) glucuronic acid; these structures are shown with the hemicelluloses in Chapter 2, Volume 1.

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ORGANIC CHEMISTRY: FUNCTIONAL GROUPS AND THE MOLECULES OF BIOCHEMISTRY

Therald Moeller , ... Clyde Metz , in Chemistry: With Inorganic Qualitative Analysis, 1980

32.13 Carbohydrates

Carbohydrate molecules are either simple sugars, called monosaccharides ; or polymers of two to ten sugars, called oligosaccharides; or polymers of more than ten sugars, called polysaccharides. The monosaccharides are either polyhydroxy aldehydes or polyhydroxy ketones (Table 32.25). Most monosaccharides contain several asymmetrically substituted carbon atoms, and like amino acids, monosaccharides and their derivatives form optically active isomers.

TABLE 32.25. Some monosaccharides

Sugars with five or six carbon atoms are more stable in a ring structure than in the open structures of Table 32.25. The ring is formed by the reaction of a carbonyl group with a hydroxyl group to give what is called a hemiacetal. The general reaction is

For d-glucose, writing the structures as though the ring is perpendicular to the plane of the paper, the ring forms as follows

The α and β indicate the two different configurations possible at carbon atom 1.

Monosaccharides combine with each other by the loss of water between two OH groups to form a glycosidic linkage. For example, sucrose, or common table sugar, is a disaccharide of glucose and fructose.

Cellulose and starch are both polymers of d-glucose. In cellulose, which may contain from hundreds to thousands of glucose units, linear molecules are organized into bundles. Cotton is 98% cellulose. Starch, which is the major source of energy in plants, contains two different polymers of d-glucose—amylose and amylopectin. Amylose is a linear polymer and amylopectin is a highly branched, treelike polymer. The functions of carbohydrates are summarized in Table 32.26.

TABLE 32.26. Some functions of carbohydrates

Derivatives of monosaccharides	Metabolism intermediates
	Plant pigments
	Blood anticoagulant (heparin)
	Vitamin C
Structural polysaccharides	Plant cell walls (cellulose)
	Animal cell coating
	Skin (keratin)
Storage polysaccharides	Release monosaccharides (starch in plants; glyco gen in animals)
	Release energy when oxidized

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Natural Product Biomolecules

Gregory Roos , Cathryn Roos , in Organic Chemistry Concepts, 2015

Questions and Programs

Q 8.1.

Classify the following simple sugars (monosaccharides).

PROGRAM 25

Fischer/Haworth Diagrams

A It can be difficult at first to classify carbohydrates by their chirality centers. This is because there can be many chirality centers in the carbon backbone. This concept is simply an extension of stereoisomerism from Chapter 3. We now take a look at what this looks like for carbohydrate examples.

For single chirality centers you have seen perspective drawings. We now introduce the Fischer projection formula. This is best explained by an example.

Try to show what the Fischer projection rules are.

B It should be clear that all vertical bonds project into the plane of the paper, and all horizontal bonds project out of the plane of the paper. In the case of carbohydrates the arrangement of the ligands is also important. The most oxidized ligand, either aldehyde or ketone, is drawn at the top of the vertical axis and the most reduced –CH₂OH end is at the bottom.

In Fischer's original assignment of the glyceraldehyde enantiomers, he labeled the dextrorotatory (+) isomer as D and the levorotatory (−) isomer as L. These labels are still used in carbohydrate chemistry today. The labels connect all carbohydrates to either the D- or L-series. In a Fischer diagram, this shows whether the bottom chiral center of the vertical chain has the same arrangement as D- or L-glyceraldehyde. The following example shows this.

All three have the same arrangement as d-glyceraldehyde with the OH group on the right. Therefore all are labeled as D-series sugars.

C Because carbohydrates can have cyclic hemiacetal structures, the next problem is how to change from the Fischer projection to the cyclic Haworth projection. This projection is a view in which the ring is drawn as planar. The rings may be 5- or 6-membered, for furanose or pyranose, on which the substituents either point above or below the plane of the ring. The method for drawing a Haworth projection is shown below with the example of d-glucose.

Turn the Fischer projection on its side as shown. This causes the ligands on the right hand side to point below the proposed ring. The ligands from the left hand side point above the proposed ring.

For D-series sugars, you must rotate the C₄–C₅ bond as shown. This rotation places the C₅–OH in position for hemiacetal ring formation. As a result, the –CH₂OH group always points above the ring plane.

For L-series sugars, rotation in the opposite direction is needed. This rotation causes the –CH₂OH to point below the ring plane. Hemiacetal formation then gives the cyclic structure.

Clearly the hemiacetal formation can have either configuration at the new chirality center to give the α- (OH directed down) or β-anomer (OH directed up).

Try to draw these Haworth projections.

D You should have drawn the following:

Q 8.2.

Draw the possible pyranose and furanose structures for d-fructose.

Q 8.3.

Pick out any "reducing sugars" in Q 8.1.

Q 8.4.

How many isomers, including stereoisomers, are possible in a triglyceride which is made from one unit each of palmitic, stearic, and oleic acids?

Q 8.5.

The melting point of palmitoleic acid (cis-9-hexadecenoic acid) is −1   °C. This is very different from the melting point of palmitic acid which is 63   °C. Explain this.

Q 8.6.

Hydrolysis of an optically active triglyceride gives one equivalent each of glycerol and oleic acid along with two equivalents of stearic acid. Draw a possible structural formula for the triglyceride.

Q 8.7.

How many moles of hydrogen would be consumed during the hydrogenation of an oil which has one unit each of oleic, linoleic, and linolenic acids? Draw a structural representation of the product.

Q 8.8.

Draw the general fused hydrocarbon ring system which is characteristic of a steroid. Explain why steroids are classified as lipids, although they do not have any fatty acid component.

Q 8.9.

Draw the zwitterions, conjugate acids, and conjugate bases for the amino acids alanine and serine.

PROGRAM 26

Amino Acid Isoelectric Points

A The isoelectric point (pI) is a physical property of each amino acid. At the isoelectric point, the solubility of the amino acid is minimized. This is because, at the isoelectric point pH, the concentration of the zwitterion is at a maximum. This can be shown by a study of the species that is formed from an amino acid as the pH changes.

In the alanine example above, the zwitterion species dominates when the solution is between the pH values of 2.3 and 9.9.

Comment on the species present at each of the pH extremes.

B When the pH of a solution of an amino acid is equal to the pK _a of the acid, the concentration of the acid and its conjugate base are equal. At pH 2.3, the concentrations of the ammonium ion of alanine (conjugate acid) and the zwitterion are equal. At pH 9.9, the concentrations of the alanine carboxylate anion (conjugate base) and the zwitterion are equal.

The isoelectric point is the pH at which the concentration of the zwitterion is maximized. At higher pH values, the mixture of species moves toward an overall negative charge. At lower pH values, the move is toward an overall positive charge. For alanine, the isoelectric point is reached at pH 6.1.

Generally, each amino acid has a specific isoelectric point which is easily measured. The exact value of the isoelectric point is directly dependent on the structure of the amino acid. The effect of structure is greatest in amino acids which have side chain with acidic or basic groups. Proteins, polymers of amino acids, have isoelectric points which reflect the sum of their component amino acids.

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TEXTILES | Natural

M. Halbeisen , in Encyclopedia of Analytical Science (Second Edition), 2005

Chemical Reactivity

The chemistry of the cellulosic fibers is similar to that of the simple sugars, but more complex, as the stereochemistry of the alcohol and hemiacetal structures is affected by degree of crystallinity, e.g., the –OH groups on the carbon atoms 2, 3, and 6 have different reactivities. Reaction with mineral acids leads to cleavage of the glycosidic bond and formation of a reducing end (RO–CH–OH).

The number of these ends, and thus the extent of degradation, is determined by reacting a fiber sample with Fehling's solution to produce copper(I) oxide. The result is known as the 'copper number'. Cellulosic fibers degraded in this manner are known as hydrocelluloses.

The major environmental effects on cellulose are oxidation of the hydroxyl groups and hydrolysis of the glycosidic bonds. Both atmospheric acidity and light are the major catalysts for weathering. Oxidation of cellulose leads to the formation of carbonyl (C=O), carboxyl (COOH), and shorter-lived peroxo (COO) species. Carbonyl groups may be measured by titration with silver nitrate. Carboxyl species are determined by reaction with methylene blue dye. Peroxides, formed in greater quantity by light than by heat, are determined by measurement of the absorbance of a phenolphthalein–copper(II) sulfate solution that has reacted with a weighed amount of oxidized fiber.

Both carbonyl and carboxyl species are revealed, semiquantitatively, by the growth of a peak in the IR spectrum at ∼1740   cm⁻¹. In addition, carbonyl groups produce an increase in absorption in the ultraviolet region at ∼265   nm and are thought to be the chromophore that leads to the yellowing of cellulosics.

In the absence of oxygen, cellulose is highly resistant to alkali. Oxidized cellulose, however, is far less resistant to alkaline hydrolysis as formation of C=O groups adjacent to the glycosidic bond greatly weakens it. Celluloses that have been oxidized, either with oxygen or more specific oxidizers such as periodate, are known as oxycelluloses leading to the destruction of the material.

The chemistry of the protein fibers is more complex. Amino acids exhibit amphoteric behavior in that they contain both a Lewis base (the amine) and a Lewis acid (the COOH). In acid media the active hydrogen exists in the form of an ammonium ion (–NH₃ ⁺), while in basic media the negative ion is the carboxylate (–COO⁻). In addition to the peptides of the backbone chain, the pendant groups in protein fibers provide additional reactive sites.

Acids cause hydrolysis of the main-chain peptides, while alkalis are less selective in their action. They may react with the carboxylic acids found in the aspartic acid, glutamic acid, and proline components, with the amino groups in arginine and lysine residues, or with the S–S bond in cystine, as well as with the main-chain links.

Cystine is important in the chemistry of the wool fiber, since the disulfide (–S–S–) links form the intermolecular cross-links that give the fiber its coherence. Cystine is stable to acid, but readily attacked by dilute alkali. Mild oxidants such as hydrogen peroxide, the preferred bleach for protein fibers, will slowly convert cystine to cystic acid. Oxygen, in the presence of light also reacts with cystine, but its major effect is to attack the tryptophan residues and produce yellowing of the fibers.

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Computational Biochemistry

N.F. Brás , ... M.J. Ramos , in Reference Module in Chemistry, Molecular Sciences and Chemical Engineering, 2015

β-Galactosidase

β-Galactosidase is a homotetrameric enzyme that catalyzes the hydrolysis of lactose into simple sugars or the formation of galactooligosaccharides. These reactions have great industrial interest given their potential application in innovative nutrients and therapeutics. Each of the four monomers works independently and contains 1023 amino acid residues. The active site is located in a deep pocket, and the enzyme requires divalent or monovalent cations for its full catalytic activity. At the time, despite the large number of enzymatic studies available for glycosidases, several important mechanistic issues were unknown. The elucidation of the hydrolysis catalytic mechanism of β-galactosidase with atomistic detail was carried out using both a cluster model and QM/MM computational methods, as follows. ^45d Initially, a small quantum model was used, treated at the DFT level. Both vacuum and a polarized continuum model environment with dielectric constant of 4.24 (mimicking the protein environment) were considered. ¹¹⁰ Additionally, the effect caused by the inclusion of the enzyme was accounted for explicitly with the hybrid QM/MM ONIOM method. Within this scheme, a higher-level layer containing the substrate and the two catalytic residues and a lower-level layer extending up to 15   Å away from the substrate were used.

In both QM and QM/MM studies, it was found that β-galactosidase catalysis occurs by a double displacement mechanism involving a covalent galactosyl enzyme intermediate. A sugar ring change from a chair to a half-chair conformation along the reactions was also verified. As seen in Figure 4 , the cluster QM model gives a barrier of 31   kcal   mol^{−   1}, which is much larger than the experimental value of activation energy (ΔG _act). The inclusion of a dielectric contact only decreases ~   3   kcal   mol^{−   1}. Only when this reaction was performed at the ONIOM QM/MM level did we obtain the energetic profile for the hydrolysis of lactose by β-galactosidase in close agreement with the experimental data. Just to make sure, single-point ONIOM calculations with a reduced QM/MM model (227 atoms) were performed too. In agreement with the values in Figure 4 , we have verified that this intermediate size model is also unable to reproduce the experimental energetic values. Hence, in this particular case, the enzyme scaffolding is essential and consequently needs to be included. This means that, despite the great success that the cluster model had got so far in those days, its use is far from being universal, and it has to be applied with caution and care.

Figure 4. Schematic representation of the energetic profiles obtained for the hydrolysis reaction catalyzed by β-galactosidase, using different size models and different computational methods. ΔG _R, Gibbs reaction relative energy; ΔG _act, Gibbs activation relative energy.

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Analysis of carbohydrates (monosaccharides, polysaccharides)

Kamal Niaz , ... Muhammad Ajmal Shah , in Recent Advances in Natural Products Analysis, 2020

Abstract

Carbohydrates are a family of compounds formed of monosaccharides building blocks, ranging from the simple sugars such as mono- and disaccharides to the more complex and longer polysaccharides molecules. They are found in a wide variety of foods, like cereals (rice, wheat), potatoes, fruits and some vegetables, sugarcane, milk, among others. Carbohydrates are one of the most important energy providers in the human diet, with glucose as the key energy molecule. Sucrose, in the form of table sugar, and starch are the most commonly consumed forms as well as cellulose, as source of fiber. Apart from that, they have numerous industrial applications, mostly as sweeteners, thickeners, and gelling agents in food industry, but also in textile, pharmaceutical, and chemical industry. Due to the carbohydrates' importance in everyday human usage, there will be further efforts for development of newer and more optimized extraction, purification, identification, and quantification techniques.

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Aliphatic Alcohols and Ethers

P.W.G. SMITH , A.R. TATCHELL , in Fundamental Aliphatic Chemistry, 1965

4 Fermentation processes

Ethanol is produced by the action of enzymes, formed by growing yeast cells, on simple sugars, particularly glucose (C₆H₁₂O₆). This process is known as fermentation. Commercially the initial raw material is starch which is readily available as the principal carbohydrate constituent of cereals and potatoes. The starch in cereal grain is degraded into maltose during the process of malting which consists in allowing the grain to germinate in a warm moist atmosphere for a period of several days, after which it is dried and roasted; this material is known as malt. Starch is converted into maltose by the enzymes (α-and β-amylases) which become active during the germination. When potatoes are used as the raw material the starch is first gelatinized by steaming before being degraded to maltose by the action of added malt extract.

${\left({\text{C}}_{\text{6}}{\text{H}}_{\text{10}}{\text{O}}_{\text{5}}\right)}_n+\frac{n}{2}{\text{H}}_{\text{2}}\text{O}\to \frac{n}{2}{\text{C}}_{\text{12}}{\text{H}}_{\text{22}}{\text{O}}_{\text{11}}$

The aqueous solution of maltose from either of these sources is adjusted to pH 4–5, separated from insoluble matter and diluted so that after the subsequent fermentation the final concentration of ethanol is 7–10 per cent. Any attempt to achieve a higher concentration of alcohol may cause premature cessation of fermentation. Yeast is then added together with the appropriate inorganic nutrients (e.g. ammonium sulphate and sodium phosphate) and the fermentation allowed to proceed between 20 and 30°. During fermentation the maltose is converted by the enzyme maltase into glucose. The glucose is further degraded through several stages into ethanol and carbon dioxide by a variety of specific enzymes, collectively referred to as zymase, which are found together with maltase in the yeast.

${\text{C}}_{\text{12}}{\text{H}}_{\text{22}}{\text{O}}_{\text{11}}\stackrel{\text{maltose}}{\to }2{\text{C}}_{\text{6}}{\text{H}}_{\text{12}}{\text{O}}_{\text{6}}\stackrel{\text{zymase}}{\to }4{\text{C}}_{\text{4}}{\text{H}}_{\text{5}}{\text{OH+4CO}}_{\text{2}}$

Molasses, the non-crystallizable sugar residues from the industrial production of domestic sugar from sugar-beet and sugar-cane, provides an immediately available source of glucose, fructose and sucrose which may be fermented with zymase as above. Distillation of the fermented liquor gives a concentrated aqueous solution of ethanol together with smaller amounts of acetaldehyde and a higher boiling fraction ('fusel oil') which contains a mixture of propanol, isobutyl alcohol and the amyl alcohols. Further fractionation of the aqueous ethanol gives rectified spirit.

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De novo asymmetric synthesis of the pyranoses

Alhanouf Z. Aljahdali , ... George A. O'Doherty , in Advances in Carbohydrate Chemistry and Biochemistry, 2013

3 Reissig Approaches

Finally, Reissig's group has also explored the de novo synthesis of various hexoses from simpler sugars. ⁴⁷ Their approach begins with the chiral aldehyde 182, which is readily prepared from l-lactic acid in three steps. Addition of the lithiated methoxyallene 183 to aldehyde 182 gave the dihydrofuran derivative 184 (Scheme 28). Oxidation of the furan ring in 184 with DDQ gives keto-aldehyde 185, which upon exposure to an acidified solution of 2-propanol provides the pyranone 186 without loss of enantiomeric purity. Reduction under 1   bar of hydrogen pressure and 10   mol% rhodium on aluminum oxide led to the diastereomeric glycoside 187. Reduction of 187 with l-selectride gave the alcohol 188. Finally, acid hydrolysis of 188 yielded 2,6-dideoxy-3-O-methyl-l-ribo-hexose (l-cymarose) as the free sugar in 10 steps from lactic acid. ⁴⁸

Scheme 28. Synthesis of l-cymarose.

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How to Draw a Linear Monosaccharide Into a Chain

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