Monosaccharides (fromGreekmonos: single,sacchar: sugar), also calledsimple sugars, are a class oforganic compounds usually with the formula (CH2O)x.[1] By definition they have two or morecarbon-carbon bonds.[2] More specifically, they are classified as polyhydroxyaldehydes or polyhydroxyketones with the respective formulasH-[CHOH] n-CHO andH-[CHOH] m-CO-[CHOH] n-H. Monosaccharides can be classified by the numberx ofcarbon atoms they contain:triose (3),tetrose (4),pentose (5),hexose (6),heptose (7), and so on.
The monosaccharide glucose plays a pivotal role inmetabolism, where the chemical energy is extracted throughglycolysis and thecitric acid cycle to provide energy to living organisms. Maltose is thedehydration condensate of two glucose molecules.
Eachcarbon atom that supports ahydroxyl group ischiral, except those at the end of the chain. Saccharides of a given formula can exist as a number ofisomeric forms. Each form has distinct properties, especially in a biological context. In addition to the many possible isomers for a given formula, each saccharide can exist in at least one cyclic form. The combination of these factors - many "chiral centers" and the chain-ring equilibria - gives rise to particularly complicated chemistry.
With few exceptions (e.g.,deoxyribose), monosaccharides have thechemical formula (CH2O)x, where conventionallyx ≥ 3.[2] Glucose, used as an energy source and for the synthesis of starch, glycogen and cellulose, is ahexose. Ribose and deoxyribose (inRNA andDNA, respectively) are pentose sugars. Examples of heptoses include theketosesmannoheptulose andsedoheptulose. Monosaccharides with eight or more carbons are rarely observed as they are quite unstable. Inaqueous solutions monosaccharides exist as rings if they have more than four carbons.
Simple monosaccharides have a linear and unbranched carbon skeleton with onecarbonyl (C=O)functional group, and onehydroxyl (OH) group on each of the remaining carbonatoms. Therefore, the molecular structure of a simple monosaccharide can be written as H(CHOH)n(C=O)(CHOH)mH, wheren + 1 +m =x; so that its elemental formula is CxH2xOx.
By convention, the carbon atoms are numbered from 1 tox along the backbone, starting from the end that is closest to the C=O group. Monosaccharides are the simplest units of carbohydrates and the simplest form of sugar.
If the carbonyl is at position 1 (that is,n orm is zero), the molecule begins with aformyl group H(C=O)− and is technically analdehyde. In that case, the compound is termed analdose. Otherwise, the molecule has aketone group, a carbonyl −(C=O)− between two carbons; then it is formally a ketone, and is termed a ketose. Ketoses of biological interest usually have the carbonyl at position 2.
The various classifications above can be combined, resulting in names such as "aldohexose" and "ketotriose".
A more general nomenclature for open-chain monosaccharides combines a Greek prefix to indicate the number of carbons (tri-, tetr-, pent-, hex-, etc.) with the suffixes "-ose" for aldoses and "-ulose" for ketoses.[4] In the latter case, if the carbonyl is not at position 2, its position is then indicated by a numeric infix. So, for example, H(C=O)(CHOH)4H is pentose, H(CHOH)(C=O)(CHOH)3H is pentulose, and H(CHOH)2(C=O)(CHOH)2H is pent-3-ulose.
Two monosaccharides with equivalentmolecular graphs (same chain length and same carbonyl position) may still be distinctstereoisomers, whose molecules differ in spatial orientation. This happens only if the molecule contains astereogenic center, specifically a carbon atom that ischiral (connected to four distinct molecular sub-structures). Those four bonds can have any of two configurations in space distinguished by theirhandedness. In a simple open-chain monosaccharide, every carbon is chiral except the first and the last atoms of the chain, and (in ketoses) the carbon with the keto group.
For example, the triketose H(CHOH)(C=O)(CHOH)H (glycerone,dihydroxyacetone) has no stereogenic center, and therefore exists as a single stereoisomer. The other triose, the aldose H(C=O)(CHOH)2H (glyceraldehyde), has one chiral carbon—the central one, number 2—which is bonded to groups −H, −OH, −C(OH)H2, and −(C=O)H. Therefore, it exists as twoenantiomers, whose molecules are mirror images of each other (like a left and a right glove). Monosaccharides with four or more carbons may contain multiple chiral carbons, so they typically have more than two stereoisomers. The number of distinct stereoisomers with the same diagram is bounded by 2c, wherec is the total number of chiral carbons.
TheFischer projection is a systematic way of drawing theskeletal formula of an acyclic monosaccharide so that the handedness of each chiral carbon is well specified. Each stereoisomer of a simple open-chain monosaccharide can be identified by the positions (right or left) in the Fischer diagram of the chiral hydroxyls (the hydroxyls attached to the chiral carbons).
Most stereoisomers are themselves chiral (distinct from their mirror images). In the Fischer projection, two mirror-image isomers differ by having the positions of all chiral hydroxyls reversed right-to-left. Mirror-image isomers are chemically identical in non-chiral environments, but usually have very different biochemical properties and occurrences in nature.
While most stereoisomers can be arranged in pairs of mirror-image forms, there are some non-chiral stereoisomers that are identical to their mirror images, in spite of having chiral centers. This happens whenever the molecular graph is symmetrical, as in the 3-ketopentoses H(CHOH)2(CO)(CHOH)2H, and the two halves are mirror images of each other. In that case, mirroring is equivalent to a half-turn rotation. For this reason, there are only three distinct 3-ketopentose stereoisomers, even though the molecule has two chiral carbons.
Distinct stereoisomers that are not mirror-images of each other usually have different chemical properties, even in non-chiral environments. Therefore, each mirror pair and each non-chiral stereoisomer may be given a specificmonosaccharide name. For example, there are 16 distinct aldohexose stereoisomers, but the name "glucose" means a specific pair of mirror-image aldohexoses. In the Fischer projection, one of the two glucose isomers has the hydroxyl at left on C3, and at right on C4 and C5; while the other isomer has the reversed pattern. These specific monosaccharide names have conventional three-letter abbreviations, like "Glu" for glucose and "Thr" forthreose.
Generally, a monosaccharide withn asymmetrical carbons has 2n stereoisomers. The number of open chain stereoisomers for an aldose monosaccharide is larger by one than that of a ketose monosaccharide of the same length. Every ketose will have 2(n−3) stereoisomers wheren > 2 is the number of carbons. Every aldose will have 2(n−2) stereoisomers wheren > 2 is the number of carbons.These are also referred to as epimers which have the different arrangement of −OH and −H groups at the asymmetric or chiral carbon atoms (this does not apply to those carbons having the carbonyl functional group).
Like many chiral molecules, the two stereoisomers of glyceraldehyde will gradually rotate thepolarization direction of linearlypolarized light as it passes through it, even in solution. The two stereoisomers are identified with the prefixesD- andL-, according to the sense of rotation:D-glyceraldehyde isdextrorotatory (rotates the polarization axis clockwise), whileL-glyceraldehyde islevorotatory (rotates it counterclockwise).
D- andL-glucose
TheD- andL- prefixes are also used with other monosaccharides, to distinguish two particular stereoisomers that are mirror-images of each other. For this purpose, one considers the chiral carbon that is furthest removed from the C=O group. Its four bonds must connect to −H, −OH, −CH2(OH), and the rest of the molecule. If the molecule can be rotated in space so that the directions of those four groups match those of the analog groups inD-glyceraldehyde's C2, then the isomer receives theD- prefix. Otherwise, it receives theL- prefix.
In the Fischer projection, theD- andL- prefixes specifies the configuration at the carbon atom that is second from bottom:D- if the hydroxyl is on the right side, andL- if it is on the left side.
Note that theD- andL- prefixes do not indicate the direction of rotation of polarized light, which is a combined effect of the arrangement at all chiral centers. However, the two enantiomers will always rotate the light in opposite directions, by the same amount. See alsoD/L system.
Cyclisation of monosaccharides (hemiacetal formation)
A monosaccharide often switches from the acyclic (open-chain) form to acyclic form, through anucleophilic addition reaction between the carbonyl group and one of the hydroxyl groups of the same molecule. The reaction creates a ring of carbon atoms closed by one bridgingoxygen atom. The resulting molecule has ahemiacetal orhemiketal group, depending on whether the linear form was an aldose or a ketose. The reaction is easily reversed, yielding the original open-chain form.
In these cyclic forms, the ring usually has five or six atoms. These forms are calledfuranoses andpyranoses, respectively—by analogy withfuran andpyran, the simplest compounds with the same carbon-oxygen ring (although they lack the double bonds of these two molecules). For example, the aldohexoseglucose may form a hemiacetal linkage between the aldehyde group on carbon 1 and the hydroxyl on carbon 4, yielding a molecule with a 5-membered ring, calledglucofuranose. The same reaction can take place between carbons 1 and 5 to form a molecule with a6-membered ring, calledglucopyranose. Cyclic forms with a seven-atom ring (the same ofoxepane), rarely encountered, are calledheptoses.
Conversion between the furanose, acyclic, and pyranose forms ofD-glucosePyranose forms of some pentose sugarsPyranose forms of some hexose sugars
For many monosaccharides (including glucose), the cyclic forms predominate, in the solid state and in solutions, and therefore the same name commonly is used for the open- and closed-chain isomers. Thus, for example, the term "glucose" may signify glucofuranose, glucopyranose, the open-chain form, or a mixture of the three.
Cyclization creates a newstereogenic center at the carbonyl-bearing carbon. The −OH group that replaces the carbonyl's oxygen may end up in two distinct positions relative to the ring's midplane. Thus each open-chain monosaccharide yields two cyclic isomers (anomers), denoted by the prefixes α- and β-. The molecule can change between these two forms by a process calledmutarotation, that consists in a reversal of the ring-forming reaction followed by another ring formation.[5]
The stereochemical structure of a cyclic monosaccharide can be represented in aHaworth projection. In this diagram, the α-isomer for the pyranose form of aD-aldohexose has the −OH of theanomeric carbon below the plane of the carbon atoms, while the β-isomer has the −OH of the anomeric carbon above the plane. Pyranoses typically adopt a chair conformation, similar to that ofcyclohexane. In this conformation, the α-isomer has the −OH of the anomeric carbon in an axial position, whereas the β-isomer has the −OH of the anomeric carbon in equatorial position (consideringD-aldohexose sugars).[6]
^Collins, Peter M.; Ferrier, Robert J. (1995).Monosaccharides : their chemistry and their roles in natural products. Robert J. Ferrier. Chichester: Wiley & Sons. p. 4.ISBN0-471-95343-1.OCLC30894482.
^"Carbohydrates".Chemistry for Biologists. Royal Society of Chemistry. Retrieved10 March 2017.
^Pigman, William Ward; Anet, E. F. L. J. (1972). "Chapter 4: Mutarotations and Actions of Acids and Bases". In Pigman and Horton (ed.).The Carbohydrates: Chemistry and Biochemistry. Vol. 1A (2nd ed.). San Diego: Academic Press. pp. 165–194.
