Arabinose cyclic formula. Special sections of the course. a) Fisher projection formulas
The main form of existence of carbohydrates in solutions is, as it turned out unexpectedly, cyclic. The cyclic form of carbohydrates appears as a result of an intramolecular reaction of formation hemiacetal when a carbonyl group interacts with one of the hydroxyls of the same molecule (most often with the fifth). This results in a fairly stable six-membered ring structure, the conformations of which closely resemble those of cyclohexane. Since the six-membered rings containing oxygen are very similar in structure to pyran , they are called pyranose forms or simply pyranoses . In smaller amounts, solutions contain furanose forms of carbohydrates formed by the reaction of a carbonyl group with a hydroxyl at the fourth carbon atom. When crystalline D-glucose is dissolved in water, there is a more or less rapid (depending on the presence of catalysts for the formation of hemiacetal) change in the angle of rotation of the plane of polarized light from 112 deg to some equilibrium value (about 53.8 deg). This value is typical for a mixture of all five forms of D-glucose present in the solution (two pyranose, two furanose and linear). The share of the linear form accounts for less than 1 percent. As a result of cycling, additional center asymmetry, at carbon number 1. The conformation at this carbon atom now defines one of two new isomers, which are called anomers(a- and b-anomers). The interconversion of glucose forms into each other through the formation of a linear conformation is called mutarotation:
The most common form of representation of cyclic forms of carbohydrates are structures Haworth (Haworth). D-isomers in this image have a CH 2 OH group with a sixth (or fifth for ribose) carbon atom located above the plane rings. a-anomers are depicted as having an anomeric hydroxyl under the plane rings, and b-anomers- above the plane rings.
This is what cyclic shapes look like fructose and ribose(furanose):
All occurring cyclic forms of D-fructose:
The pyranose form of ribose is much less common:
The hydroxyl group at the new center of asymmetry is hemiacetal, which sharply distinguishes it in chemical properties from the rest of the hydroxyls in the molecule. Therefore, it is called anomeric (or glycosidic) hydroxyl. Under mild conditions of acid catalysis, the formation of full acetal (glycoside)
by the addition of a molecule of any alcohol or, generally speaking, as a result of interaction with any alcohol hydroxyl, including another anomeric hydroxyl. Typical glycosides of this kind are disaccharides.
Cyclic forms of galactose and mannose:
The rules for the transition from linear to cyclic forms are that the groupings on the right in linear forms are shown under the ring in cyclic forms, and those on the left are above the ring.
The most important monosaccharides
The most common monosaccharide is D-glucose. Its formula is very easy to remember: it is aldohexose, in the Fisher formula of which all hydroxyl groups, with the exception of one - the second from the top (at C-3) are located on the right.
D-mannose D-glucose D-galactose
Aldohexose, which differs from D-glucose in the location of the first hydroxyl group (at C-2), is called D-mannose, and the third (at C-4) is called D-galactose. D-fructose differs from D-glucose in that it is a ketose and not an aldose.
D-fructose D-arabinose L-arabinose
Except hexoses great importance also have pentoses. D-arabinose differs from D-glucose in the absence of C-1. In nature, L-arabinose is common, which is a mirror image of D-arabinose. It is contained in cherry glue. The xylose formula can be derived from the glucose formula by removing the last carbon atom. In the formula of D-ribose, which is part of nucleic acids, all hydroxyl groups are located on the right. The composition of nucleic acids also includes 2-deoxy-D-ribose, which differs from D-ribose in the absence of a hydroxyl group at the second carbon atom.
Ex. 2. Draw Fischer's projection formulas for the following D-series hexoses: glucose, mannose, galactose, and fructose.
D-xylose D-ribose 2-deoxy-D-ribose
Epimerization
Under the action of bases, for example on glucose, the hydrogen of the -carbon atom in relation to the carbonyl group passes to the oxygen of this group, as a result of which the enol form is formed. In this case, the chirality of the second carbon atom disappears. During the reverse transformation, the returning proton can come from either side of the plane, which will lead to both the formation of the original D-glucose and the new D-mannose carbohydrate, i.e. an isomer appears with a new position of the hydroxyl group. In addition, another carbohydrate arises with a new arrangement of the carbonyl group. Such a transformation is called epimerization.
D-glucose enol form (enediol) D-mannose
D-fructose
Two stereoisomers containing multiple chiral centers but differing in configuration only one of the centers called epimers. Two carbohydrates that differ in the position of the hydroxyl group are called epimers. The formation of an equilibrium mixture of three carbohydrates can take place by treatment with bases of any of these three carbohydrates.
Ex. 3. What monosaccharides are called epimeric? Write projection formulas for monoses of epimeric D-mannose.
Cyclic forms of monosis, mutarotation
A characteristic feature of hydroxyaldehydes and hydroxyketones, which include monoses, is their tendency to form cyclic hemiacetals and hemiketals; this happens especially easily if the resulting rings consist of 5 and 6 atoms, including oxygen.
5-hydroxypentanal (open form) (cyclic form)
Open and cyclic forms of carbohydrates are in balance with each other, achieved as a result of tautomeric transformation. This type of tautomerism is called ring-chain tautomerism.
The size of the cycle is indicated by replacing the generic suffix monoz - oz on the - pyranose- for six-membered cycles and - furanose for five-term cycles. The names of the cycles come from the names of the corresponding oxygen-containing heterocycles:
furan pyran
Unlike conventional aldehydes, aldoses do not react with sodium bisulfite and do not give a red color with fuchsinsulfuric acid. This is because aldoses exist predominantly in cyclic forms.
Glucose usually produces a six-membered hemiacetal and hence the hydroxyl group found at C-5 is used for this. When the cyclic form is formed, C-1 becomes a stereocenter: the hemiacetal hydroxyl group that appears in it (it is called glycosidic) can be placed either on the left or on the right:
D-glucopyranose D-glucose -D-glucopyranose
(Tollens formula) (Fischer formula) (Tollens formula)
The cyclic forms of aldoses are hemiacetals. They are formed by intramolecular interaction of hydroxyl and carbonyl groups. This reaction forms a new stereocenter at the C-1 atom. The cyclic forms of monoses are diastereomers. Such diastereomers are called anomers. The hemiacetal carbon atom is called the anomeric atom. Anomers are designated and -anomers, depending on the location of the hydroxyl group at the C-1 atom. At the -anomer, the glycosidic hydroxyl is located on the same side as that of the penultimate carbon atom (in the D-row on the right), and at the -anomer - on the other side (in the D-row - on the left). The full name of both anomers of D-glucose will be respectively - or -D-glucopyranose.
In the conformational formula of the α-anomer of D-glucopyranose, all hydroxyl groups and the -CH 2 OH group occupy the equatorial position. The α-anomer formula is distinguished by the axial arrangement of the anomeric hydroxyl. Both anomers
D-glucose in the crystalline state is quite stable and each of them can be isolated in pure form, both of them rotate the plane of polarized light.
To designate cyclic forms at present in the chemistry of carbohydrates, chair-shaped formulas are more often used, similar to those used to designate cyclohexane and its derivatives.
So pl. 146 about С T. pl. 150 o C
D-glucose -D-glucopyranose -D-glucopyranose
20D +112o +19O
D-(+)-Glucose crystallizes from water in the form of -D-glucopyranose, and from pyridine - in the form of -D-glucopyranose. In an aqueous solution, an equilibrium is established at which there is 36% -D-glucopyranose and 64% -D-glucopyranose, which gives an average value for the specific rotation of the solution 20 D = +52.5 O .
This phenomenon is called mutarotation. The angle of rotation of the plane of polarized light of the solution during the establishment of equilibrium between the isomers gradually changes. The phenomenon of mutarotation is explained by the fact that during decyclization, the stereocenter at C-1 disappears (transformation into a carbonyl group), and subsequent cyclization leads to the formation of both anomers. Only sugars with a free glycosidic hydroxyl, that is, capable of ring-chain tautomerism, undergo mutarotation.
If one of the anomers is transferred into solution, then each of them will turn into an equilibrium mixture of anomers with a specific optical rotation of +52.5 o, consisting of 36% of the -anomer and 64% of the -anomer. The concentration of the open form through which the anomers interconvert is only 0.024%.
Sometimes cyclic forms are depicted without specifying the orientation of the glycosidic hydroxyl:
D-glucopyranose
Since D-mannose differs from D-glucose in the location of the hydroxyl group only at C-2, and D-galactose - at C-4, the conformational formulas of these monoses are easily derived from the conformational formulas of the corresponding glucose anomers:
D-mannopyranose -D-galactopyranose
The preference for the axial position of the hydroxyl group is called anomeric effect.
Ex. four. Unlike glucose, D-mannose consists of 69% α-anomer and 31% α-anomer. Write the formulas for both anomers of mannopyranose.
Exercise 5. Hexose, in the Fischer formula of which all hydroxyl groups
located on the right, called D-allosis. Draw the open and cyclic formulas of D-allose.
In addition to conformational formulas for cyclic forms of carbohydrates, simplified cyclic formulas according to Hayworth (Haworth) are often used. The transition from conformational formulas to Hayworth formulas is very simple: the cycle is flattened, the bonds to the carbon atom of the substituents are depicted vertically.
conformational formula Haworth formula
D-glucopyranose -D-glucopyranose
Haworth's formulas for the remaining aldohexoses are easily derived from the formula
D-glucopyranose. Everything that is written on the right in the Fisher formula in cyclic formulas is written below and vice versa:
D-fructose -D-fructofuranose -D-glucopyranose
If we want to flip the cyclic formula with removal from the drawing plane, then all the substituents should be interchanged.
D-fructofuranose
Ex. 6. Name the following monoses:
Ex. 7. Write the conformational formulas for -D-glucopyranose, -D-glucopyranose, -D-fructofuranose, -D-galactopyranose and -D-mannopyranose.
Ex. eight. Hexose, which differs from glucose only in the location of the aldehyde group, is called gulose. Write the formula of this hexose and indicate in its name which series (D or L) it belongs to.
Ex. 9. Write the prospective Haworth formulas for -D-glucopyranose, -D-glucopyranose, -D-fructofuranose, -D-galactopyranose, and -D-mannopyranose.
Exercise 10. What phenomenon is called mutarotation? Explain using D-mannose as an example, given that both anomers in the equilibrium system are in the pyranose form. How can mutarotation be detected?
a) Fisher projection formulas
For faster and more convenient writing of the configuration, E. Fischer proposed to represent them with projection formulas. The carbon chain is represented by a vertical line, at the ends of which the first and last functional groups are written (the aldehyde group is always written at the top). The H and OH groups are written to the right or left of the chain, according to their spatial arrangement in the molecule. For example, glucose, according to Fischer, is written like this:
b) "Perspective" formulas (Haworth's formulas)
The formulas presented above are not capable of giving a comprehensive geometric representation of the hemiacetal structure of monose. In 1928, Haworth proposed "promising" formulas that more closely reflect the real structures of substances.
The oxygen atom is always placed in the upper right corner. For a more distinct image of the plane of the ring, the part of it facing the reader is indicated by thickened lines. The carbon atoms included in the cycle, as a rule, are not written, but only numbered. Vertical lines are drawn through them, at the ends of which hydrogen atoms and hydroxyl groups are written in accordance with their spatial arrangement in the molecule:
When writing the Haworth formula for any monosaccharide, the following rules should be followed:
1) all groups located to the right of the carbon core in the usual formulas (Fischer formulas) occupy a position under the ring plane in the Haworth formulas; and the groups on the left are above the plane of the ring, with the exception of the hydrogen atom, at C 4 in furanoses and C 5 in pyranoses;
2) the end group - CH 2 OH is also placed above the plane of the ring.
For cyclic forms of ketosis, Haworth's formulas are also used:
Haworth's projection formulas can create a misconception about the spatial structure of carbohydrate molecules - as if the pyranose and furanose rings are flat, which in reality is not. In fact, the pyranose ring can take two configurations - the shape of a chair and the shape of a boat:
From an energetic point of view, the shape of the chair is more stable; it is she who prevails in most of the natural monosaccharides.
However, Haworth projections have become widespread; they are simpler and better display Chemical properties monosaccharides.
2.4 Individual representatives of monosaccharides
Hexoses and pentoses are the most widely distributed in nature.
Among pentose The most important are arabinose, xylose, ribose and deoxyribose. Pentoses are found in nature mainly as constituents of polysaccharide molecules called pentosans, as well as vegetable gums.
L-arabinose
In nature, L (+)-arabinose is predominantly found. It is found as a monosaccharide in cherry glue, beets. L-arabinose is widely distributed in plants as a component of mucus, gums, pectins and hemicelluloses. Arabinose is obtained by hydrolysis of cherry glue or beet pulp. When arabinose is reduced, the polyhydric alcohol arabitol is obtained, and when oxidized, arabonic acid is obtained.
Monosaccharides open form can form cycles, i.e. close into rings.
Let's look at this with an example glucose.
Recall that glucose is six-atomic aldehyde alcohol(hexose). Its molecule simultaneously contains aldehyde group and a few hydroxyl groups OH(OH is the functional group of alcohols).
When interacting with each other aldehyde and one of hydroxyl groups belonging to the same molecule glucose, the latter forms cycle, ring.
The hydrogen atom from the hydroxyl group of the fifth carbon atom passes into the aldehyde group and combines with oxygen there. The newly formed hydroxyl group ( HE) is called glycosidic.
Its properties differ significantly from alcohol(glycose) hydroxyl groups monosaccharides.
The oxygen atom from the hydroxyl group of the fifth carbon atom combines with the carbon of the aldehyde group, resulting in the formation of a ring:
Alpha- and beta anomers of glucose differ in the position of the glycosidic group HE relative to the carbon chain of the molecule.
We have considered the origin of the six-membered cycle. But cycles can also be five-membered.
This will happen if the carbon from the aldehyde group combines with the oxygen of the hydroxyl group. at the fourth carbon atom, and not at the fifth, as discussed above. Get a smaller ring.
Six-membered cycles are called pyranose, five-term - furanose. The names of the cycles come from the names of related heterocyclic compounds - furan and pyrana.
In the names of cyclic forms, along with the name of the monosaccharide itself, the “end” is indicated - pyranose or furanose characterizing the size of the cycle. For example: alpha-D-glucofuranose, beta-D-glucopyranose, etc.
Cyclic forms of monosaccharides are thermodynamically more stable in comparison with open forms, therefore, in nature they are more common.
Glucose
Glucose(from other Greek γλυκύς - sweet) ( C6H12O6) or grape sugar - the most important of the monosaccharides; white crystals of sweet taste, easily soluble in water.
The glucose link is part of the series disaccharides(maltose, sucrose and lactose) and polysaccharides(cellulose, starch).
Glucose found in grape juice, in many fruits, as well as in the blood of animals and humans.
Muscular work is performed mainly due to the energy released during oxidation. glucose.
Glucose is a hexahydric aldehyde alcohol:
Glucose obtained when hydrolysis polysaccharides ( starch and cellulose) under the action of enzymes and mineral acids. In nature glucose produced by plants during photosynthesis.
Fructose
Fructose or fruit sugar С6Н12О6 – monosaccharide, a companion of glucose in many fruit and berry juices.
Fructrose as a monosaccharide link is part of sucrose and lactulose.
Fructose much sweeter than glucose. Mixtures with it are part of honey.
By structure fructose is a six-hydric keto alcohol:
Unlike glucose and other aldoses, fructose unstable in both alkaline and acidic solutions; decomposes under conditions of acid hydrolysis of polysaccharides or glycosides.
Galactose
Galactose - monosaccharide, one of the most commonly occurring six-hydric alcohols in nature is hexose.
Galactose exists in acyclic and cyclic forms.
Differs from glucose spatial arrangement of groups at the 4th carbon atom.
Galactose soluble in water, poorly in alcohol.
In plant tissues galactose is part of raffinose, melibiose, stachyose, as well as polysaccharides - galactans, pectin substances, saponins, various gums and mucus, gum arabic, etc.
In animals and humans galactose - component lactose (milk sugar), galactogen, group-specific polysaccharides, cerebrosides and mucoproteins.
Galactose included in many bacterial polysaccharides and can be fermented by the so-called lactose yeast. In animal and plant tissues galactose easily turns into glucose, which is better absorbed, can be converted into ascorbic and galacturonic acids.
Oligosaccharides. Sucrose.
Oligosaccharides is one of the types polysaccharides.
Oligosaccharides are carbohydrates consisting of several monosaccharide residues (from the Greek. ὀλίγος - few).
As a rule, their molecules contain from 2 before 10 monosaccharide residues and have a relatively small molecular weight.
The most common of oligosaccharides are disaccharides and trisaccharides.
disaccharides
Disaccharides are made up of two monosaccharides. The general formula for disaccharides is usually C 12 H 22 O 11 .
§ 2. MONOSACCHARIDES
Spatial isomerism
By their chemical nature, monosaccharides are aldehyde or keto alcohols. The simplest representative of monosaccharides, aldotriasis, is glyceraldehyde (2,3-dihydroxypropanal).
Considering the structure of glyceraldehyde, one can notice that the above formula corresponds to two isomers that differ in spatial structure and are a mirror image of each other:
Isomers that have the same molecular formula but differ in the arrangement of atoms in space are called spatial, or stereoisomers. Two stereoisomers that relate to each other as an object and a mirror image that does not coincide with it are called enantiomers. This type of spatial isomerism is also called optical isomerism.
The existence of enantiomers in glyceraldehyde is due to the presence in its molecule chiral carbon atom, i.e. an atom bonded to four different substituents. If there is more than one chiral center in the molecule, then the number of optical isomers will be determined by the formula 2 n , where n is the number of chiral centers. The stereoisomers that are not enantiomers are called diastereomers.
For the image of optical isomers on a plane, use Fisher projections. When constructing Fischer projections, it should be taken into account that atoms or groups of atoms lying on a horizontal line should be directed towards the observer, i.e. come out of the plane of the paper. Atoms or groups of atoms lying on a vertical line and constituting, as a rule, the main chain, are directed away from the observer, i.e. go beyond the surface of the paper. For the isomers of glyceraldehyde considered by us, the construction of Fischer projections will proceed as follows:
Glyceraldehyde is accepted as the standard for designating optical isomers. To do this, one of its isomers was designated by the letter D, and the second - by the letter L.
Pentoses and hexoses
As mentioned above, the most common in nature are aldopentoses and aldohexoses. Considering their structure, we can conclude that aldopentoses have 3 chiral centers (indicated by asterisks) and, therefore, they consist of 8 (2 3) optical isomers. Aldohexoses have 4 chiral centers and 16 isomers:
Comparing the structure of the last carbohydrate from the carbonyl group of the chiral center with the structure of D- and L-glycerol aldehydes, all monosaccharides are divided into two groups: D- and L-series. The most important representatives of aldopentoses are D-ribose, D-deoxyribose, D-xylose, L-arabinose, aldohexose - D-glucose and D-galactose, and ketohexose - D-fructose. Fischer projections of named monosaccharides and their natural sources are shown below.
Monosaccharides exist not only in the form of open (linear) forms, which are given above, but also in the form of cycles. These two forms (linear and cyclic) are capable of spontaneously passing one into the other in aqueous solutions. Dynamic equilibrium between structural isomers is called tautomerism. The formation of cyclic forms of monosaccharides occurs as a result of the reaction of intramolecular addition of one of the hydroxyl groups to the carbonyl group. The most stable are five- and six-membered cycles. Therefore, during the formation of cyclic forms of carbohydrates, furanose(five-membered) and pyranose(six-membered) cycles. Consider the formation of cyclic forms on the examples of glucose and ribose.
Glucose during cyclization forms a predominantly pyranose cycle. The pyranose ring consists of 5 carbon atoms and 1 oxygen atom. When it is formed, the hydroxyl group of the fifth (C 5) carbon atom participates in the addition.
The carbonyl group is replaced by a hydroxyl group called glycosidic, and derivatives of the glycosidic group of the carbohydrate - glycosides. Another spatial feature of cyclic forms is the formation of a new chiral center (C 1 atom). There are two optical isomers called anomers. The anomer, in which the glycosidic group is located in the same way as the hydroxyl group, which determines the ratio of the monosaccharide to the D- or L-series, is denoted by the letter , the other anomer by the letter . The structure of monosaccharides in cyclic form is often depicted in the form of Haworth's formulas. This image allows you to see mutual arrangement hydrogen atoms and hydroxyl groups relative to the plane of the ring.
Thus, in a solution, glucose exists in the form of three forms that are in mobile equilibrium, the ratio between which is approximately: 0.025% - linear form, 36% - - and 64% - form.
Ribose forms mainly five-membered furanose rings.
Chemical properties
The chemical properties of monosaccharides are determined by the presence of a carbonyl group and alcohol hydroxyls in their molecules. Consider some reactions of monosaccharides using the example of glucose.
As a polyhydric alcohol, glycol, glucose solution dissolves the precipitate of copper (II) hydroxide to form a complex compound.
The aldehyde group forms alcohols upon reduction. When glucose is reduced, a hexahydric alcohol is formed sorbitol:
Sorbitol has a sweet taste and is used as a sugar substitute. Xylitol, a product of xylose reduction, is also used for the same purpose.
In oxidation reactions, depending on the nature of the oxidizing agent, monobasic (aldonic) or dibasic (glucaric) acids can be formed.
Most monosaccharides are reducing sugars. They are characterized by: the reaction of the "silver mirror"
and interaction with Fehling's liquid (reduction of blue Cu(OH) 2 to yellow CuOH and then orange Cu 2 O).
The glycosidic group of cyclic forms of monosaccharides has an increased reactivity. So, when interacting with alcohols, ethers are formed - glycosides. Since glycosides lack a glycosidic hydroxyl, they are not capable of tautomerism, i.e. the formation of a linear form containing an aldehyde group. Glycosides do not react with ammonia solution of silver oxide and Fehling's liquid. However, in an acidic environment, glycosides are easily hydrolyzed to form the starting compounds:
Under the action of the enzyme systems of microorganisms, monosaccharides can be transformed into various other organic compounds. Such reactions are called fermentation. Alcoholic fermentation of glucose is widely known, as a result of which ethyl alcohol is formed. Other types of fermentation are also known, for example, lactic acid, butyric, citrate, glycerin.