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Monosaccharides

The word saccharide means, sugar. These are made up of only one monomeric unit and cannot be hydrolysed. They are commonly known as sugars and are sweet in taste and soluble in water. All monosaccharides and nearly all disaccharides are reducing sugars, as they can reduce a number of reagents, e.g.,

 

(a) Fehling’s solution (Cu++ →7 Cu20 red ppt.),

(b) Benedict’s reagent (CuSO4→7 Cu20 red ppt.),

(c) Tollen’s reagent (AgNO3 →7 Ag)

 

They have 3-7 C in backbone and are also found as building blocks of oligosaccharides and polysaccharides. Many are intermediates of respiratory and photosynthetic pathways.

 

All monosaccharides, except some, e.g., dihydroxyacetone that does not contain any C* (asmymmetric C atom, which is attached to 4 different groups), are optically active (bend the plane of polarised light) and exhibit stereoisomerism. Stereoisomers have similar chemical properties but, different physical and biological properties. The number of possible isomers for a sugar is 2n where, n is the number of C*.

 

Thus, due to the presence of asymmetric C* atoms, the sugar can occur in two absolute configuration, D and L, that are mirror images of each other:

 

1. D-conflguration , that can be:

 

(a) Dextrorotatory (d) or (+), which rotate the plane of polarized light to right                                                                                                                                        (clockwise) e.g., D-(+)-glucose or

(b) Levorotatory (f) or (-), which rotate- the plane of polarized light to left (anticlockwise)

e.g., D-(-)-fructose

 

2. L-configuration, that again can be:

(a) Dextrorotatory or

(b) Levorotatory

Most of the naturally occurring sugars are usually found in D-configuration.

 

 

 

Table 2.1

Sugar                         C*                            2”                                                           (d)               (1)

Aldoses                                                  

1. Trisoses               n = l                            2 ”                                                            1                   1

CHO                        CHO

|                               |

H-C2OH                   H-C2OH

|                                 |

CH2 OH             CH2 OH

D-glyceraldehyde          L-glycealdehyde

 

(Glyceraldehyde is  as standard and all the optically active compounds are compared with its D- or L-structure.)

 

2 Tetroses                 n = 2           22               (2 x 2 =4)                                 2                   2

3. Pentoses                n = 3           23              (2 x 2 x 2 = 8)                           4                  4

4. Hexoses                n = 4          24              (2 x 2 x 2 x 2 = 16)                    8                  8

(4 in living system)

  Ketoses:

 

l . Tetroses             n = 1               21             (2 x I = 2)                                 1                  1

2 Pentoses           n = 2               22             (2 x 2 = 4)                                 2                  2

3. Hexoses           n= 3               23              (2 x 2 x 2 = 8)                            4                 4

 

Classification of Monosaccarides

 

On the basis of number of C atoms in the backbone, the monosaccharides have been classified as:

1. Trioses

They have 3 C in backbone, e.g., C3H6O3.

e.g.,              (i) D-glyceraldehyde                (ii) Dihydroxy acetone

CHO                                      CH2OH

|                                            |

H-C-OH                                  C=O

|                                            |

H-CI –OH                               CH2OH

|

H

 

Since, it does not contain any asymmetric

C*, it is not optically active.

 

 

 

 

 

 

 

 

2. Tetroses

They have 4 C in backbone, e.g., C4H8O4.

e.g.,         (i) D-erythrose                                             (ii) D-threose

CHO                                                                 CHO

|                                                                           |

H-C-OH                                                             OH-C-OH

|                                                                           |

H-C-OH                                                              H-C-OH

|                                                                           |

CH2OH                                                               CH2OH

(i) D-erythrose               (ii) L-erythrose                          (iii)D-threose             (iv)l-threose

CHO                                     CHO                                       CHO                              CHO

|                                            |                                               |                                    |

H-C-OH                                H-C-OH                               OH-C-H                           H-COH

|                                            |                                              |                                       |

H-C-OH                           OH–C-H                                   H-COH                           OH-CH

|                                            |                                             |                                        |

CH20H                                  CH2OH                                 CH2OH                    CH2OH

Epimers  (diasteriomers) are pairs of stereoisomers that are not mirror images of other and they differ in the configuration of one of the several asymmetric C atoms (C*) in their molecules. There are 211 possible diastereomers for a sugar, where, n is the number asymmetric C atoms (C*) in that sugar. 4C aldoses have n = 2 (C*), thus, they can have 4 isomers  (22 = 2 X 2 = 4).

 

Enantiomers are stereoisomers that are mirror images of each other.

 

Anomers are stereoisomeric pairs that differ in the configuration of first asymmetric C atom

( I C*) that is involved in the ring fom1ation in ring forms. (C* = asymmetric C) e.g., a-D-erythrose and B-D -Erythrose

 

3.  Pentoses

 

They have 5 C in backbone (SC), e.g .. C5H10O5

e.g.,                          (i) D-ribose                                                (ii) D-xylose

CHO                                                           CHO

|                                                                     |

H–C-OH                                                        H-C-OH

|                                                                     |

H-C-OH                                                        OH-C-H

|                                                                       |

H-C-OH                                                      H-C-OH

|                                                                      |

CH2OH                                                     CH2OH

 

(Found in ATP, RNA, FAD, NAD)

(Found in wood)

Ring form of D-ribose

 

          (iii) D-ribulose                                                        (il) D-xylulose

CH2OH                                                                      CH2OH

|                                                                                 |

C = O                                                                        C=O

|                                                                                 |

H-C-OH                                                                    OH-C-H

|                                                                                 |

H-C—OH                                                                 H-C-0H

|                                                                                  |

CH2OH                                                                      CH2OH

[ Some sugars are found in L-form, e.g., L-arabinose that is a 5C aldose].

 

4. Hexoses

They have 6 C in backbone (6C), e.g., C6H12O6.

(i) D-glucose               (ii) D-mannose                                 (iii) D-galactose

CHO                         CHO                                                           CHO

|                                |                                                                   |

H-C-OH                OH-C-H                                                       OH-C-H

|                               |                                                                     |

OH-C-H                   OH-C-H                                                        OH-C-H

|                               |                                                                     |

H-C-OH                  H-C-OH                                                        H-C-OH

|                              |                                                                    |

H-C-OH                H-C-OH                                                         H-C-OH

|                             |                                                                      |

CH2OH                 CH2OH                                                          CH2OH

                                                Epimers ( diasteriomers)

Ring form of D-glucose

 

e.g., in 6 C aldoses with 4 C* , where 1 6 optically active forms are possible, (of which, 8

pairs of enantiomers are mirror images) some of the commonly found sugars are:

a pair of D- and L-glucose, a pair of D- and L-mannose and a pair of D- and L-galactose,

where, D-glucose and D-mannose differ in configuration of 2nd C* and D-glucose and D-galactoset differ in configuration of 3rd C*.

 

(iv) D-fructose

H

|

H – C – OH

|

C = O

|

HO – C – H

|

H – C – OH

|

H – C – OH

|

H – C – OH

|

H

(i) Glucose: It is commonly known as grape sugar or corn sugar or blood sugar, found in honey, urine of diabetics, etc. Since, it is dextrorotatory, it is also known as dextrose. It is most abundant in all hexoses, is a re spiratory substrate and its oxidation provides immediate energy. It is 3f41h as sweet as sucrose (sweetening power of various sugars: Fructose > Invert sugar > Sucrose > Glucose > Maltose > Lactose). Monomeric unit of starch, glycogen, and cellulose is glucose. Commercially it is obtained from starch by treating it with bot mineral acids. Glucose has 4 secondary and 1 primary – OH groups.

 

 

Conc. aqueous solution of Glucose

crystallization at 30°C

 

 

α-Glucose

(specific rotation + 113°)

+

Acetic acid solution of

 

Glucose

β-Glucose

(+ 19°)

mutarotation

(when allowed to stand)

mutarotation

Glucose

(+ 52.5°)

(constant value)

Glucose

(+ 52.5°)

 

 

 

 

α-D-Glucose and f3-D-Glucose are anomers.

 

 

H – C – OH

|

H – C- OH

|

OH – C – H

|

H- C -OH

|

H- C -OH

|

CH2OH

 

                      OH-C –H

|

H – C- OH

|

OH – C – H

|

H- C -OH

|

H- C -OH

|

CH2OH

 

 

Cl becomes asymmetric in cyclic forms:

D –Glucose (with 4c*)

Pyranose form of glucose is called glucopyranose in which Cl and C5 are linked  by an atom of O:

Fermentation of glucose in yeast yields alcohol:

 

glycolysis

Glucose                  Pyruvate                   Acetaldehyde                   C2H50H + C02

 

Oxidation of glucose yields:

Reaction with Fehling’s reagent: (or Benedict’s reagent)

Earlier, this reaction was used for the diagnosis of diabetes, but nowadays, diabetes is diagnose  using biosensor:

 

Glucose oxidase

Glucose + O2                                          Gluconolactone + H2O2

Peroxidase

Reduced compound + H2O2                                     Oxidised compound

(colourless)                                                                (coloured)

(measured spectrophotometrically)

 

 

Some important glucose derivatives are:

 

(a) Glucose phosphates that are trapped sugar (cannot pass through plasma membrane), e

(b) Deoxy sugars :

e.g., 2-deoxy-D-ribose (furanose) (derivative of ribose)

 

(c) Amino glucose derivatives (sugars):

e.g., found in bacterial cell wall (peptidoglycan) (Fig. 2.1)

(ii) Fructose: It is also known as levulose, as it is levorotatory. It is found in fruits (called fruit sugar), nectar, honey and is sweetest of all naturally occurring sugars. It is commercially obtained from inulin that is a polymer of fructose. It is also a respiratory substrate. Since, it has 3 C*, a total of 8 stereoisomers are possible [2n = 23 = (2 x 2 x 2) = 8 stereoisomers, of which 4 pairs are enantiomers].

 

Pyranose (5C + 1 0) form, in which -OH of C6 reacts with >C2 = 0, is found in free state.

 

At equilibrium optical rotation = -92.30

 

Whereas, furanose (4C+l0) form, in which -OH attached to C5 reacts with >C2 = 0, and is found in combined state.

Oxidation: It reduces Fehling’s solution, Tollen’s or Benedict’s reagent due to the presence of α-hydroxy ketonic group (it itself is oxidised by these reagents).

(iii) D-Galactose and D-mannose: Less commonly, they are found in free state and   generally, found in various polymers, as constituent of cell wall, etc., and their main function is structural. Galactose is a component of lactose sugar (a disaccharide).

 

5. Heptoses

They have 7 C in backbone (7C), e.g., C7H14O7

e.g., (i) D-sedoheptulose.

Ring Form Monosaccharides

 

Those aldehydes (monosaccharide) with 4C and more than 4C in their backbone and; those ketones (monosaccharide) with SC and more than SC in their backbone, predominantly are found in cyclic or ring forms in aqueous solution that generates an additional asymmetric C (C*). For example, Cl in Glucose becomes asymmetric in aqueous solution, which otherwise in chain form is found as symmetric C, thus, two more stereoisomers, a.-glucose and β-glucose (anomers) becomes possible. Two common types of ring forms are:

 

(i) 6-membered rings (5C + 1 0), called pyranoses as they resemble pyran. They are more stable and found predominantly in 6C aldoses.

 

(ii) 5-membered rings (4C + 10), called furanoses as they resemble furan.