Anomeric carbon is the first carbon
14. Biomolecules: carbohydrates
Carbohydrates play a very important role in nature. They form the building blocks of all plant cell walls, act as a chemical energy storage system, and they are one of the most important sources of food. Cellulose, starch and table sugar are carbohydrates. Like glucose, many (but not all) of the simple building blocks of more complex carbohydrates have the general formula C.n(H2O)n.
In nature, carbohydrates are mainly produced via an as photosynthesis designated sequence of reactions:
The detailed mechanism of this implementation is complex and involves many stages.
14.1 The names and structures of carbohydrates
The simplest are carbohydrates Monosaccharidese.g. glucose and fructose, which cannot be hydrolyzed into smaller units. They contain an aldehyde or ketone group with at least two hydroxyl groups:
Complex carbohydrates are formed when two or more monosaccharide units are coupled. A Disaccharide is created, for example, from two monosaccharides by linking a (usually) acetal bridge (see Chapter 10.4.2 and Chapter 10.6). During hydrolysis, the two monosaccharide units regress:
A Trisaccharide arises from three monosaccharide units, a Polysaccharide from several.
Monosaccharides can go further than Aldoses or Ketosis be classified. Sugars with an aldehyde group are called aldoses, those with a keto group are called ketoses.
Because of their chain length, sugars are divided into Trios (3 carbon atoms), Tetroses (4 carbon atoms), Pentoses, hexoses etc.
14.1.1 The configuration of monosaccharides: Fischer projections
With the exception of 1,3-dihydroxypropanone, all sugars considered so far contain chiral carbon atoms. The simplest chiral sugar is glyceraldehyde with an asymmetric carbon. Fischer projections are used extremely often to represent sugars:
The horizontal lines represent bonds that are directed towards the viewer, vertical lines point away from him. By convention, the carbonyl group should always be on top:
Diastereomers (molecules with the same constitution but different relative configurations that do not behave as an image and mirror image (cf. enantiomers)) can easily be compared with the help of this system.
14.1.2 D and L sugars
Glyceraldehyde has an asymmetric carbon and therefore occurs in two enantiomeric forms, derived from the Cahn-Ingold-Prelog nomenclature (for rules see Chapter 7.3), as R. and S. must be designated:
But only one enantiomer occurs in nature, that R.Isomer, which is also the dextrorotatory enantiomer. (R.) - (+) - Glyceraldehyde is also referred to as D-glyceraldehyde for the reasons already mentioned (Section 7.4).
Because of their natural biosynthetic pathways, glucose, fructose, ribose and most of the other naturally occurring monosaccharides, the center of chirality furthest from the aldehyde or ketone group, have the same absolute configuration as D-glyceraldehyde:
In the Fischer projection of a monosaccharide one looks at the center of chirality furthest away from the carbonyl group. In the case of a D sugar, the OH group is always on the right-hand side of this center of chirality (see D-glyceraldehyde). Those with the reverse configuration at this point are considered L.-Monosaccharides denotes:
In D- and L-glucose (Enantiomers!) All centers of chirality are of course reversed, not just the lowest.
14.1.3 The configuration of the aldoses
As the number of centers of chirality increases, so does the number of stereoisomers. Aldotetrose has two centers of chirality and can therefore be found in 22 = 4 stereoisomers occur.
| Stereoisomerism |
Molecules with the same constitution, however
different geometries or topologies
| Enantiomers |
Stereoisomers related to each other
behave in mirror image and identical
Stereoisomers that do not look like picture
and mirror image behave that
have different structures,
and different physical
and must have chemical properties
Among these four there are two diasteroisomers, Erythrosis and Threose. Both erythrose and threose occur as a pair of enantiomers, one of which belongs to the D series, the other to the L series:
An aldopentosis has three centers of chirality and occurs in 8 stereoisomers. In the group of aldohexoses, 16 such isomers are already possible:
Almost all naturally occurring sugars are D-series. Obviously, when building up the structure of the sugar molecule, nature has only "chosen" one configuration for one end of the chain.
14.2 Monosaccharides form intramolecular hemiacetals
In Chapter 10.4.2 we saw that aldehydes and ketones react with alcohols and can first form half-acetals / half-ketals, then acetals / ketals:
Monosaccharides are hydroxycarbonyl compounds which, in principle, should form intramolecular hemiacetals. In fact, glucose and the other hexoses and pentoses are in equilibrium between the open-chain and the cyclic hemiacetal form, with the second form being strongly predominant:
Although in principle any of the five hydroxyl groups could add to the carbonyl group of the aldehyde, the formation of a 6-ring is preferred in this case, although 5-rings are sometimes also formed:
Monosaccharides, which are present as 6-membered rings, are called Pyranoses after the oxygen-containing 6-ring pyran. Monosaccharides that are present as a 5-ring are called Furanoses after the oxygen-containing 5-ring furan
Haworth developed a projection from which the actual 3D structure of the sugar molecule can be better recognized Haworth projection.
In contrast to glucose, which is almost exclusively in the pyranose form, with fructose there is an equilibrium mixture between fructopyranose and fructofuranose in a ratio of 70:30:
If one takes a closer look at the structures of the furanoses and pyranoses, one sees that the carbonyl-C is converted into a new center of chirality during the cyclization. When hemiacetal is formed, two new compounds are created, 2 diastereomers that differ in the configuration of the hemiacetal group:
If the anomeric hydroxyl group is at the bottom of this Haworth projection, the sugar is referred to as α anomer. If the hydroxyl group is on top, it is called a β anomer.
Since this type of diastereomer formation is only found in sugars, these isomers have been given a special name: Anomers. This is called the new center of chirality anomeric carbon atom.
14.3 Monosaccharide anomers: mutarotation
Glucose precipitates from concentrated solutions at RT and forms crystals, which at 146OMelt C. As has been proven by X-ray structure analyzes, only α-D - (+) - glucopyranose is present in the crystals.
If you dissolve crystalline α-D - (+) - glucopyranose in the water and immediately measure the optical rotation value, you get a value:[α]D.25 = + 112O
Strangely enough, this value increases over time to + 52.7O and then remains constant. This change can be accelerated by adding acid or base. Apparently some chemical change has taken place, due to which the original specific rotation of the sample has changed:
The specific rotation of the ß-shape is much smaller (= 18.7O) than that of the α-anomer: the observed rotation value of the solution thus decreases. The specific rotation of a solution of the pure β-anomer [mp. 150OC, obtainable by crystallization of glucose from acetic acid] steadily from +18.7O up to the final value +52.7O. The change in optical rotation when the equilibrium between a monosaccharide and its anomer is established is known as Mutarotation (mutare, Latin: change, transform).
14.4 Conformations of Monosaccharides
Haworth projections are still used extensively in sugar chemistry today, but they do not represent an exact picture of the 3D structures. We already know that five-membered rings tend to adopt an envelope conformation, while six-membered rings prefer a chair conformation. :
As in the Haworth projection, the ether oxygen is usually in the upper right corner and the anomeric carbon in the right corner of the envelope or chair. Substituents which are directed upwards in the Haworth projection remain "up" in the envelope or in the armchair:
Most aldohexoses adopt the chair conformation in which the relatively bulky hydroxymethyl group at C5 is equatorial. In the case of glucose, this means that four of the five substituents are equatorial in the α-form and are in the β-form all Equatorial substituents. This stable conformer is only possible with glucose; with the other seven D-aldohexoses, at least one substituent must inevitably be axially:
14.5 The chemistry of sugars
Simple sugars can occur in the form of a wide variety of isomers: as open-chain carbonyl compounds and as α- and β-anomers of ring compounds of different sizes. Since all isomers quickly equilibrate with one another, the relative speed of the reactions of the individual isomers with a reagent determines the product distribution resulting from a reaction. We can divide the reactions of sugars into two groups: those in which the sugar reacts from the open-chain form and those in which the sugar reacts in one of the ring forms.
14.5.1 Ester formation
Since they are polyhydroxy compounds, sugars can be converted into various alcohol derivatives. Esters can be represented using standard methods:
All hydroxyl groups including the anomeric hydroxyl group are converted.
14.5.2 Glycoside formation
In Chapter 10 we saw how the formation of an acetal, an aldehyde and alcohol proceeds via a hemiacetal:
By treating a monosaccharide hemiacetal with an alcohol and acid catalyst, a new acetal is created in a similar way, in which the anomeric OH group is replaced by a new alkoxy group:
Sugar acetals are known as GlycosidesSo, glucose forms Glucosides.Glycosides like other acetals are stable in water. There is no equilibrium with open-chain forms, and mutarotation does not occur with glycosides.
Only when an aqueous acid is present do acetals also undergo hydrolysis and give back the [hemiacetal = open-chain aldehyde] equilibrium + the alcohol:
see also Chapter 10.4.2
Glycosides are found very often in nature. The alcohol part can have different structures, e.g .:
14.5.3 Reduction of monosaccharides
Aldoses and ketoses can be reduced by the same reducing agents that convert aldehydes and ketones into alcohols. The resulting polyhydroxy compounds are referred to as Alditols. The hydride reagent (NaBH4 in alcohols) intercepts the amount of the open-chain form of the sugar present in equilibrium, whereby the equilibrium is shifted from the cyclic hemiacetal form to the product.
Many alditols occur in nature. D-glucitol can be found in red seaweed in concentrations of up to 14%, as well as in many berries, in cherries, plums, pears and apples.
14.5.4 Oxidation of the monosaccharides
The open-chain monosaccharides are polyfunctional compounds. If particularly mild reagents are used, at least selective oxidation reactions can be carried out. Aldoses contain the easily oxidizable aldehyde group and therefore give with the classic oxidation tests such as Fehling's solutiong or Tollens reagent a positive reaction:
In these reactions, the aldoses in Aldonic acids convicted. Sugars for which these tests are positive are known as reducing sugars. All aldoses can be called reducing sugars because they contain a free aldehyde group.
Glycosides are non-reducing sugars because they are no free aldehyde group contain.
When the anomeric hydroxyl group of a monosaccharide hemiacetal is replaced by a new alkoxy substituent, a glycoside is obtained. If the alcohol part is another sugar, the product is a Disaccharide:
Disaccharides contain two monosaccharides linked by an acetal bridge between an anomeric carbon atom and a hydroxyl group of the other sugar. A glycoside bond to the anomeric carbon atom can be α or β. A glycosidic bond between C1 of the first sugar and (for example) C4 of the second is referred to as a 1,4 'linkage. In Cellobiose two D-glucopyranoses are linked by a 1,4'-ß-glycoside bond.
Maltose contains two D-glucopyranoses linked by a 1,4'-α-glycoside bond.
Do maltose and cellobiose belong to the reducing sugars? ______. Why ?
Is mutarotation to be expected in cellobiose and maltose?
Despite their similar structures (diastereomers!), Cellobiose and maltose have dramatically different biological properties. Cellobiose cannot be digested in humans and yeast, although maltose is easily broken down in both organisms. With mineral acid, maltose and cellobiose are quickly hydrolyzed to glucose units.
Sucrose, the common table sugar, is also a disaccharide with the following properties;
- after acid hydrolysis, glucose and fructose are obtained.
- it is a non-reducing sugar
- no mutarotation is observed
The last two findings indicate the absence of hemiacetals, i.e. both monosaccharides must be glycosides. This can only be the case if glucose and fructose are simultaneously linked via their anomeric centers:
The specific rotation of sucrose is +66.5O. When treated with dilute acid, the rotation value increases continuously up to a final value of -20O from. This phenomenon is known as Raw sugar inversion and the hydrolyzate product as Invert sugar. The overall process consists of three individual reactions:
- the hydrolysis of the disaccharide into the two monosaccharides α-D-glucopyranose and β-D-fructofuransoe
- the mutarotation of α-D-glucopyranose up to equilibrium with the β form
- the mutarotation of β-D-fructofuranose to the somewhat more stable β-D-fructopyranose.
The same effect is observed if you mix a sucrose solution with the enzyme Invertase offset. Now the enzyme plays the role of the catalyst (instead of H.3O+) for splitting into glucose and fructose. The mutarotations then take place spontaneously. Since the specific rotation value of fructose (-92O) is more negative than the rotation value of glucose (+52.7O) positive, the rotation of the resulting mixture is negative overall.
Polysaccharides are the polymers of the monosaccharides. The three most common natural polysaccharides, cellulose, starch and glycogen are all derived from the same monomer, glucose.
Cellulose - a 1,4'-O- (β-D-glucopyranoside) polymer
Cellulose consists of around 3,000 monomer units and has a molar mass of around 500,000. A large proportion of all plant matter consists of cellulose. Cotton fibers, such as filter paper, are almost pure cellulose; wood and straw contain around 50% of it. When converting the free hydroxyl groups with HNO3 in nitric acid ester Nitrocellulose. If the nitrate content is high, the material is explosive. Lower nitrate content results in a polymer that was one of the earliest plastics - celluloid.
Starch, the reserve carbohydrate of plants, consists of two main components, Amylose (~ 20%) and Amylopectin (~ 80%).
This is another polysaccharide with a very similar structure to amylopectin, but with a higher degree of branching and a much larger molar mass Glycogen. This compound is of great biological importance because it is one of the most important polysaccharides for energy storage in humans and animals.
14.8 Modified sugars in nature
Many of the naturally occurring sugars have a modified structure or are bound to another organic molecule, e.g .:
In DNA, the sugar component consists of 2-deoxyribose, and instead of a hydroxyl group, there is a heterocyclic base on the anomeric carbon atom. 2-deoxyribose takes almost exclusively a furanose form.
There is a large class of sugars in which at least one of the hydroxyl groups has been replaced by an amine function. Your name is Glycosylamineswhen the nitrogen is bonded to the anomeric carbon atom, and Aminodeoxy sugar if it replaces an oxygen elsewhere in the molecule:
Glycosylamines, or more precisely derivatives thereof, are also found in nucleic acids. The Chitin molecule is a polymer made from β-D-glucosamine units. Chitin is together with CaCO3 the framework of the shell of crabs and lobsters.
If a sugar is bound to the hydroxyl group of another organic residue via its anomeric carbon, it is referred to as Glycon, the rest of the molecule (or the product that is formed after the hydrolytic cleavage of the sugar) as Aglycon.
Sugars also play an important role in signal transduction processes between cells. Oligosaccharides are often found on the cell surface, where they act as ligands for proteins from other cells.
E.g. with a blood transfusion it is important that you get the right blood type (A, B, AB or O). The different blood cells have different sugars on their cell surfaces, which then determine the blood groups:
Glycoproteins are macromolecules that consist of a protein and one or more covalently bound carbohydrate groups. The carbohydrate groups are usually covalently attached as a post-translational modification to asparagine (Asn), serine (Ser), threonine (Thr), or hydroxylysine residues. This process is called glycosylation.
Structural formulas of the sugars found in glycoproteins (only the β-anomers are shown):
A layer of glycopeptide (peptidoglycan) is found on the cell surface of bacteria, which is important to strengthen the cell membrane. The biosynthesis of peptidoglycan is blocked by many antibiotics (e.g. penicillin and vancomycin), which is fatal to bacteria.
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