Physical properties of monosaccharides
Monosaccharides are colourless and crystalline compounds.
They are readily soluble in water.
They have sweet tase.
D-glucose and L-glucose are mirror images of each other.
The presence of asymmetric carbon atoms in a compound give rise to the formation of isomers of that compound. Such compound which are identical in composition and differs only in spatial configuration are called “stereo isomers’’. For example glucose can exist in two forms as shown below.
The orientation of the H and OH groups around the carbon atom just adjacent to the terminal primary alcohol carbon, eg. C5 in glucose determines the series. The D and L forms of glyceraldehyde are given below.
when the -OH group of this C2 is at the right, it belongs to D-series, when the -OH group is on the left it belongs to L-series.
A beam of ordinary light may be regarded as bundle of electromagnetic waves vibrating in all directions perpendicular to the axis of the beam. When such a beam of light is made to pass through a nicol prism, all vibrations except those in one plane are eliminated. This is called as plane polarised light. When such a beam of plane polarised light is passed through a solution of an optical isomer, and if the plane polarised light is found to rotate to the left, it is described as levorotation. If the plane polarised light rotates to an equal number of degrees to the right, it is described as dextrorotation. This phenomenon exhibited by asymmetric compounds, is called optical isomerism (Fig. 4.3 & 4.4).
Polarimeter is an instrument by which the specific rotations of optical isomers are detected.
Optical rotation to the left i.e levorotation is expressed with a sign of l- and rotation to the right i.e dextrorotation is expressed as d+.
When equal amounts of dextrorotatory and levorotatory isomers are present, the resulting mixture has no optical activity, since the activities of isomers cancel each other. Such a mixture is said to be a “racemic mixture”.
The separation of optically active isomers from a racemic mixture is called resolution.
When an aldohexose is first dissolved in water and the solution is kept in optical path and plane polarised light is passed, the initial optical rotation shown by the sugar gradually changes until a constant fixed rotation characteristic of the sugar is reached. This phenomenon of change of rotation is called as “Mutarotation”.
The mutarotation is due to the existence of two optical isomers of glucose, namely a, D glucose with a specific rotation +112.2° and b, D glucose with a specific rotation +18.7°
α and β isomers are called as anomers and the carbon atom responsible for this is the anomeric carbon atom. Anomers are isomers differing in configuration of a particular carbon atom alone.
A freshly prepared aqueous solution of α, D glucose has a specific rotation of +112.2°. When this solution is allowed to stand, the rotation falls to 52.7° and remains constant at this value. This gradual change in specific rotation is called mutarotation.
The value of mutarotation for α, D-glucose is +59.5°.
(+112.2°) - (52.7°) = +59.5°.
A freshly prepared solution of b, D glucose has a rotation value of 18.7°. It also gradually increases and reaches the same final value of + 52.7°.