Biological Chemistry - Berezov, T. T., Korovkin, B. F. 1998

Carbohydrate Chemistry
Monosaccharides

Monosaccharides can be considered as derivatives of polyhydric alcohols containing a carbonyl (aldehyde or ketone) group. If the carbonyl group is located at the end of the carbon chain, the monosaccharide is an aldehyde and is called an aldose; if this group is in any other position, the monosaccharide is a ketone and is called a ketose.

The simplest monosaccharides are trioses: glyceraldehyde and dihydroxyacetone. The oxidation of the primary alcohol group of the trihydric alcohol glycerol yields glyceraldehyde (an aldose), whereas the oxidation of its secondary alcohol group leads to The formation of dihydroxyacetone (a ketose).

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Stereoisomerism of monosaccharides. All monosaccharides contain asymmetric carbon atoms: aldotrioses have one center of Asymmetry, aldotetroses have 2, aldopentoses have 3, aldohexoses have 4, and so on. Ketoses contain one fewer asymmetric carbon atom than aldoses with the same number of carbon atoms. Consequently, the ketotriose dihydroxyacetone contains no asymmetric carbon atoms. All other monosaccharides can exist as various stereoisomers.

The total number of stereoisomers for any monosaccharide is given by the formula N = 2ⁿ, where N is the number of stereoisomers and n is the number of asymmetric carbon atoms. As noted previously, glyceraldehyde contains only one asymmetric carbon atom and therefore can exist as two distinct stereoisomers.

The glyceraldehyde isomer in which the OH group at the asymmetric carbon atom is positioned to the right when the molecular model is projected onto a plane is designated as D-glyceraldehyde, while its mirror image is designated as L-glyceraldehyde:

Aldohexoses contain four asymmetric carbon atoms and can exist as 16 stereoisomers (2⁴), a representative example being glucose. For aldopentoses and aldotetroses, the number of stereoisomers is 2³ = 8 and 2² = 4, respectively.

All monosaccharide isomers are classified into D- and L-forms (D- and L-configuration) based on the similarity in the spatial arrangement of atomic groups at the terminal asymmetry center compared to that of D- and L-glyceraldehyde. Naturally occurring hexoses—glucose, fructose, mannose, and galactose—belong stereochemically to the D-series (Scheme 5.1).

It is well known that natural monosaccharides exhibit optical activity. The ability to rotate the plane of polarized light is one of the key characteristics of substances (including monosaccharides) whose molecules contain an asymmetric carbon atom or are asymmetric as a whole. The property of rotating the plane of polarized light to the right is denoted by a plus sign (+), and in the opposite direction by a minus sign (—). For instance, D-glyceraldehyde rotates the plane of polarized light to the right, meaning that D-glyceraldehyde is a D(+)-aldotriose, whereas L-glyceraldehyde is an L(—)-aldotriose. However, the direction of optical rotation, which is determined by the overall molecular asymmetry, cannot be predicted in advance. Monosaccharides belonging stereochemically to the D-series can be levorotatory. For example, the common naturally occurring form of glucose is dextrorotatory, while the common form of fructose is levorotatory.

Cyclic (hemiacetal) forms of monosaccharides. Any monosaccharide with specific physical properties (melting point, solubility, etc.) is characterized by a specific optical rotation value

Scheme 5.1 THE FAMILY OF D-KETOSES CONTAINING 3 TO 6 CARBON ATOMS

It has been established that the specific rotation of any monosaccharide solution changes gradually and reaches a definite, constant value only after prolonged standing. For example, for a freshly prepared glucose solution , this value reaches an equilibrium of after prolonged standing. The change in specific rotation over time upon standing of monosaccharide solutions is known as mutarotation. Obviously, mutarotation must be caused by A change in molecular asymmetry and, consequently, by the transformation of its Structure in solution.

The phenomenon of mutarotation has a clear explanation. It is known that aldehydes and ketones react readily and reversibly with an equimolar amount of alcohol to form hemiacetals:

Hemiacetal formation can also occur within a single molecule, provided there are no steric constraints. According to A. Baeyer's theory, intramolecular interaction between the alcohol and carbonyl groups is most favorable when it leads to the formation of five- or six-membered rings. The formation of hemiacetals gives rise to a new asymmetric center (C-1 in the case of D-glucose). Six-membered sugar rings are called pyranoses, and five-membered rings are called furanoses. The a-form is the form in which the orientation of the hemiacetal hydroxyl group is the same as that of the asymmetric carbon atom determining membership in the D- or L-series. In other words, in formulas representing the a-modification of D-series monosaccharides, the hemiacetal hydroxyl is written on the right, whereas for L-series representatives, it is written on the left. The reverse applies to the writing of the ß-form.

Thus, the phenomenon of mutarotation is due to the fact that each solid carbohydrate preparation exists as a single cyclic (hemiacetal) form, but upon dissolution and standing, this form converts via the aldehyde form into other tautomeric cyclic forms until an equilibrium state is reached. Consequently, the specific rotation characteristic of the initial cyclic form changes gradually. Eventually, a constant specific rotation is established, which is characteristic of the equilibrium mixture of tautomers. For instance, it is known that In aqueous solutions, glucose exists predominantly as a- and ß-glucopyranoses, to a lesser extent as a- and ß-glucofuranoses, and in very small amounts as the open-chain aldehyde form.

It should be emphasized that among the various tautomeric forms of glucose, only the a- and ß-pyranoses are known in the free state. Although the presence of small amounts of furanoses and the aldehyde form in solutions has been proven, they cannot be isolated in the free state due to their instability.

* Specific rotation is the angle of Rotation of the plane of polarized light passing through a 1 cm path length Cell containing a solution of the substance at a concentration of 1 mol/L. At a given Temperature, in a specific solvent, and at a specific wavelength of transmitted light, the value of specific rotation is determined solely by The Nature of the dissolved substance.

In the 1920s, W. Haworth proposed a more advanced method for writing structural formulas of CARBOHYDRATES. Haworth formulas are represented as perspective hexagons or pentagons, where the ring lies in a horizontal plane. Bonds closer to the reader are drawn with thicker lines (the carbon atoms of the ring are omitted). Substituents positioned to the right of the molecular backbone in its vertical projection are placed below the plane of the ring, while substituents on the left are placed above the plane of the ring. The reverse rule applies only to the single carbon atom whose hydroxyl group participates in the Formation of the cyclic hemiacetal. Thus, in D-sugars, the CH₂OH group is written above this carbon atom, and the hydrogen atom attached to it is written below.

Finally, it should be kept in mind that when writing structural formulas according to Haworth, the hydroxyl group at C-1 must be positioned below the ring plane in the a-form and above it in the ß-form:

Haworth projection formulas do not reflect the true conformation of monosaccharides. Much like cyclohexane, the pyranose ring can adopt two Conformations—the chair form and the boat form (conformational formulas). The chair form is generally more stable and appears to predominate in the majority of natural sugars (Fig. 5.1).