Ans. is 'a' i.e., Mannose Isomerism of sugarso Sugars Exhibit Various Forms of Isomerismo Glucose, with four asymmetric carbon atoms, can form 16 isomers.(Formula given as 2n,where n is number of asymmetric carbon atoms).o The more important types of isomerism found with glucose are:D and L isomerism :The designation of a sugar isomer as the D form or of its mirror image as the L form is determined by its spatial relationship to the parent compound of the carbohydrates, the three-carbon sugar glycerose (glyceraldehyde).The L and D forms of this sugar, and of glucose, are shown in Figure.The presence of asymmetric carbon atoms also confers optical activity on the compound.When a beam of plane-polarized light is passed through a solution of an optical isomer, it rotates either to the right, dextrarotatoiy (+), or to the left, levorotatory (-).The direction of rotation of polarized light is independent of the stereochemistry of the sugar, so it may be designated D(-), D(+), L(-), or L(+).Pyranose and furanose ring structures:The ring structures of monosaccharides are similar to the ring structures of either pyran (a six-membered ring) or furan (a five-membered ring). For glucose in solution, more than 99% is in the pyranose form.Alpha and beta anomers:The ring structure of an aldose is a hemiacetal, since it is formed by combination of an aldehyde and an alcohol group.Similarly, the ring structure of a ketose is a hemiketal.Crystalline glucose is a-D-glucopyranose.Epimers:Isomers differing as a result of variations in configuration of the -OH and -H on carbon atoms 2, 3, and 4 of glucose are known as epimers. Biologically, the most important epimers of glucose are mannose and galactose, formed by epimerization at carbons 2 and 4, respectively.Aldose-ketose isomerism:# Fructose has the same molecular formula as glucose but differs in its structural formula, since there is a potential keto group in position 2, the anomeric carbon of fructose , whereas there is a potential aldehyde group in position 1, the anomeric carbon of glucose.