Introduction:

Monosaccharides are the commonest carbohydrates in that they can't be hydrolyzed to smaller carbohydrates. They are aldehydes or ketones having two or more hydroxyl groups. The general chemical formula of an unmodified monosaccharide is (C•H₂O)_n, literally a 'carbon hydrate'. Monosaccharides are significant fuel molecules and also building blocks for the nucleic acids. The smallest monosaccharides, for which n = 3, are dihydroxyacetone and D- and L-glyceraldehyde.

Classification of monosaccharides:

Monosaccharides are categorized according to three different features: 

a) The placement of its carbonyl group. 

b) The number of carbon atoms it includes. 

c) Its Chiral handedness.

Whenever the carbonyl group is an aldehyde, the monosaccharide is the aldose; if the carbonyl group is a ketone, the monosaccharide is a ketose. Monosaccharides having three carbon atoms are known as Trioses, those having four are known as Tetroses, five are known as pentoses; six are hexoses and so forth. These two systems of categorization are often combined. For illustration, glucose is an aldohexose (that is, a six-carbon aldehyde), ribose is an aldopentose (that is, a five-carbon aldehyde), and fructose is a ketohexose (that is, a six-carbon ketone).

Each and every carbon atom bearing a hydroxyl group (-OH), by the exception of the first and last carbons, are asymmetric, making them stereo centers by two possible configurations each (R or S). Due to this asymmetry, a number of isomers might exist for any specified monosaccharide formula. The aldohexose D-glucose, for illustration, consists of the formula (C·H₂O)₆, of which all however two of its six carbons atoms are stereogenic, making D-glucose one of 2⁴ = 16 possible stereoisomers. In case of glyceraldehyde, an aldotriose, there is one pair of possible stereoisomers: enantiomers and epimers.  1, 3-dihydroxyacetone, the ketose corresponding to the aldose glyceraldehyde, is a symmetric molecule having no stereo centers. The assignment of D or L is formed according to the orientation of the asymmetric carbon furthest from the carbonyl group: in a standard Fischer projection whenever the hydroxyl group is on the right the molecule is a D sugar, or else it is an L sugar. The D- and L- prefixes must not be confused by d- or l-, which point out the direction that the sugar rotates plane polarized light. This usage of d- and l- is no longer obeyed in the carbohydrate chemistry.

Fig: The α and β anomers of glucose

Ring-straight chain isomerism:

The group of aldehyde or ketone of a straight-chain monosaccharide will react reversibly through a hydroxyl group on a different carbon atom to form a hemiacetal or hemiketal, making a heterocyclic ring having an oxygen bridge between the two carbon atoms. Rings having five and six atoms are known as furanose and pyranose forms, correspondingly, and exist in equilibrium by the straight-chain form. 

Throughout the conversion from straight-chain form to the cyclic form, the carbon atom having the carbonyl oxygen, known as the anomeric carbon, becomes the stereogenic center having two possible configurations: The oxygen atom might take a place either above or beneath the plane of the ring. The resultant possible pairs of stereoisomers are known as anomers. In α anomer, the -OH substituent on the anomeric carbon rests on the opposite side (that is, trans) of the ring from the CH₂OH side branch. The other form, in which CH₂OH substituent and the anomeric hydroxyl are on the similar side (that is, cis) of the plane of the ring, is known as the β anomer. As the ring and straight-chain forms readily interconvert, both the anomers exist in equilibrium. In a Fischer Projection, α anomer is showed by the anomeric hydroxyl group 'trans' to CH₂OH and cis in the β anomer.

Monosaccharide classifications based on the number of carbons:

Fig: Monosaccharide classifications

Most of the saccharide structures are different only in the orientation of the hydroxyl groups (-OH). This small structural difference forms a big difference in the biochemical properties, organoleptic properties (example: taste), and in the physical properties like melting point and Specific Rotation (that is, how polarized light is distorted). A chain-form monosaccharide which consists of a carbonyl group (C=O) on an end carbon making an aldehyde group (-CHO) is categorized as an aldose. Whenever the carbonyl group is on an inner atom making a ketone, it is categorized as ketose.

Tetroses and Pentoses:

Fig: Tetroses and Pentoses

Ring form of ribose is the component of ribonucleic acid (RNA). Deoxyribose, that is missing oxygen at position 2, is a component of deoxyribonucleic acid (DNA). In nucleic acids, the hydroxyl group joined to carbon number 1 is substituted by nucleotide bases.

Fig: ring form of ribose

Hexoses:

Hexoses, like the ones described here, encompass the molecular formula C₆H₁₂O₆. German chemist Emil Fischer (1852-1919) recognized the stereoisomers for such aldohexoses in the year 1894. He got the Nobel Prize for chemistry on the year 1902 for his work.

Fig: Hexoses

Which encompass opposite configurations of the hydroxyl group at merely one position, like glucose and mannose, are known as epimers. Glucose, as well termed as dextrose, is the most broadly distributed sugar in the animal and plant kingdoms and it is the sugar exists in blood as 'blood sugar'. The chain form of glucose is a polyhydric aldehyde, signifying that it consists of multiple hydroxyl groups and an aldehyde group. Fructose, as well termed as levulose or fruit sugar, is illustrated here in the chain and ring forms. Fructose and glucose are the major carbohydrate constituents of the honey.

Fig: Hexoses structure

Heptoses:

Sedoheptulose comprises the similar structure as fructose; however it consists of one additional carbon. Sedoheptulose is found in carrots. Mannoheptulose is the monosaccharide found in avocados.

Fig: Heptoses

Chain and Ring Structure:

Most of the simple sugars can exist in a chain form or a ring form, as described by the hexoses above. The ring form is preferred in aqueous solutions, and the method of ring formation is identical for most sugars. The glucose ring form is made when the oxygen on carbon number 5 links by the carbon including the carbonyl group (that is, carbon number 1) and transfers its hydrogen to the carbonyl oxygen to make a hydroxyl group. The rearrangement generates alpha glucose if the hydroxyl group is on the opposite side of the -CH₂OH group or beta glucose if the hydroxyl group is on the similar side as -CH₂OH group. Isomers, like these, which are different only in their configuration concerning their carbonyl carbon atom, are known as anomers. The little D in the name derives from the fact that the natural glucose is dextrorotary, that is, it rotates polarized light to the right, and however it now represents a particular configuration. Monosaccharides making a five-sided ring, similar to ribose, are known as furanoses. Such forming six-sided rings, such as glucose, are termed as pyranoses.

Stereochemistry:

Saccharides having similar functional groups however having different spatial configurations have various chemical and biological properties. Stereochemistry is the study of arrangement of atoms in three-dimensional (3-D) space. Stereoisomers are the compounds in which the atoms are connected in the similar order however differ in their spatial arrangement. The compounds which are mirror images of one other however are not similar, comparable to left and right shoes, are known as enantiomers. The following structures describe the difference between β-D-Glucose and β-L-Glucose. Similar molecules can be made to correspond to one other via flipping and rotating. Though, enantiomers can't be made to correspond to their mirror images via flipping and rotating. Glucose is at times described as a 'chair form' since it is a more correct representation of the bond angles of the molecule. The 'boat' form of glucose is not stable.

Fig: Stereochemistry-Monosaccharides

Monosaccharides in living organisms:

Monosaccharides are the main source of fuel for metabolism, being employed both as an energy source (that is, glucose being the most significant in nature) and in biosynthesis. Whenever monosaccharides are not instantly required by numerous cells they are often transformed to more space-efficient forms, often polysaccharides. In lots of animals, comprising humans, this storage form is glycogen, particularly in liver and muscle cells. In plants, starch is employed for the similar purpose.

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