Monosaccharides, Biology tutorial


Monosaccharides are the simplest carbohydrates which are also known as simple sugars. They are first of three classes of carbohydrates classified by being products of hydrolysis of non simpler sugars (Oligosaccharides and Polysaccharides). Monosaccharides comprise single polyhydroxy aldehyde or ketone unit. Most plentiful monosaccharides in nature are 6 carbon sugar D-glucose, sometimes referred to as dextrose. Monosaccharides are of 2 families; those having aldehyde functional group, known as aldose and those with ketonic group are known as ketoses, each containing its own characteristic structure.

Structure of glucose:

The most plentiful, naturally-occurring monosaccharide is D-glucose. D-glucose is component of structures example glycogen, cellulose, starch and significant disaccharides like lactose, sucrose, and maltose. Structurally glucose can be denoted in straight chain and cyclic structure known as Fisher and Haworth projection formulae of D-glucose respectively.

562_Structure of glucose.jpg

Projection and perspective formulas:

Tetrahedral nature of carbon compounds presents the unique problem in writing three dimensional structure of the compound on the two dimensional surface like paper. This complexity persevered until Emil Fischer introduced projection formula in which four groups joined to carbon atom are projected onto plane. In Fischer's scheme, horizontal bonds are understood to be in front of the plane of paper (i.e nearer to reader or writer) and signified by solid lines while vertical bonds are behind plane of paper.

Fischer's projection:

Emil Fischer won Nobel Prize in chemistry for elucidating structure of glucose. From Fischer's work it has been achievable to write projection formula for glucose and ball and stick formula for D and L glucose. Fischer's projection formula for glucose can be represented.

1228_Fischer’s projection formula.jpg

Cyclisation of the fischer projection formula in monosaccharides:

It has been illustrated that two crystalline forms of glucose exist depending on method of crystallization. These two forms of glucose are α form and the β form. Observation of this observed behavior for glucose is attributed to fact that aldohexoses and other sugars react internally to create cyclic hemiacetals. Formation of cyclic hemiacetals is characteristics reaction between aldelyde and alcohol whereas hemiketals are created between ketones and alcohols. In glucose hemiacetal reaction takes place between alcoholic (-OH) group on carbon 5 and aldehyde group on carbon 1, therefore forming a 6 membrane ring (related structurally to pyran and thus referred to as pyranose). When the - OH group on carbon 4 partakes in hemiacetal formation, a 5 - membered ring structure is created (related structurally to furan, therefore known as furanose). Furanose form of glucose is less stable than pyranose form in solution therefore it is pyranose form which generally exists. Though, furanose forms of other monosaccharides like fructose are also stable and found in nature. Formation of pyranose rings confers some asymmetry on carbon atom 1 (Hemiacetal carbon) and therefore optical activity. The α and β forms of D-glucose vary only in configuration around hemiacetal carbon. These two forms of glucose are known as diastereo isomers or anomers. Term anomer is utilized to explain isomeric form of monosaccharides which vary each other only on their configuration about hemiacetal carbon atom like α - D - glucose and β - Dglucose. Hemiacetal or carbonyl carbon is known as anomeric carbon.

Optical activity in monosaccharides:

Optical activity is the concept shown by organic compounds like glucose containing anomeric carbon atom or chiral centre to rotate path of plane-polarized light in polarimeter. If path of plane polarized light is rotated clockwise it is known as (+) Dextrorotatory and if anticlockwise it is known as (-) or Laevorotatory signified by D and L respectively.

Measurement of optical activity:

Specific rotation is the quantitative measurement of optical activity of the stereoisomer. It is found form measurement of degree of rotation of the solution of pure steroisomer at the given concentration in tube of the given length in polarimeter. Specific rotation is computed as:

[α]D25degree = (observed rotation in degrees)/ (length of tube, (dm) x concentration(g/ml))

Where dm = decimeter (0-1m)

D = Line of sodium (indicating light at wavelength of 589nm)

25degree C = Temperature of measurement

(α) = Specific Rotation

Haworth's projection formula:

English chemist W.H. Haworth proposed that ring forms of monosaccharides must be represented by the hexagonal ring comprising of carbon atoms C-1 to C - 5 and oxygen atoms of glucopyranose in plane perpendicular to plane of paper. Side nearer reader must represented by thickened lines while substituents on carbon atoms in ring will extend above or below plane of hexagonal ring for example C-6, that is substituent on C-5 will be above plane of ring. Contrary to implication of Haworth projection formula hexagonal ring of pyranose is not planer. In most monosaccharides, it exists as chair conformation though in same may exist as boat confirmation.

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