Each
carbon atom that supports a
hydroxyl group is
chiral, except those at the end of the chain. Saccharides of a given formula can exist as a number of
isomeric 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 the
chemical formula (CH2O)
x, where conventionally
x ≥ 3. 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.
Open-chain stereoisomers Two monosaccharides with equivalent
molecular graphs (same chain length and same carbonyl position) may still be distinct
stereoisomers, whose molecules differ in spatial orientation. This happens only if the molecule contains a
stereogenic center, specifically a carbon atom that is
chiral (connected to four distinct molecular sub-structures). Those four bonds can have any of two configurations in space distinguished by their
handedness. 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 two
enantiomers, 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 2
c, where
c is the total number of chiral carbons. The
Fischer projection is a systematic way of drawing the
skeletal 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 specific
monosaccharide 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" for
threose. Generally, a monosaccharide with
n asymmetrical carbons has 2
n 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 where
n > 2 is the number of carbons. Every aldose will have 2(
n−2) stereoisomers where
n > 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).
Configuration of monosaccharides Like many chiral molecules, the two stereoisomers of glyceraldehyde will gradually rotate the
polarization direction of linearly
polarized light as it passes through it, even in solution. The two stereoisomers are identified with the prefixes - and -, according to the sense of rotation: -glyceraldehyde is
dextrorotatory (rotates the polarization axis clockwise), while -glyceraldehyde is
levorotatory (rotates it counterclockwise). The - and - 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 in -glyceraldehyde's C2, then the isomer receives the - prefix. Otherwise, it receives the - prefix. In the Fischer projection, the - and - prefixes specifies the configuration at the carbon atom that is second from bottom: - if the hydroxyl is on the right side, and - if it is on the left side. Note that the - and - 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 also
system.
of monosaccharides (hemiacetal formation) A monosaccharide often switches from the acyclic (open-chain) form to a
cyclic form, through a
nucleophilic 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 bridging
oxygen atom. The resulting molecule has a
hemiacetal or
hemiketal 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 called
furanoses and
pyranoses, respectively—by analogy with
furan and
pyran, the simplest compounds with the same carbon-oxygen ring (although they lack the double bonds of these two molecules). For example, the aldohexose
glucose 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, called
glucofuranose. The same reaction can take place between carbons 1 and 5 to form a molecule with a ring, called
glucopyranose. Cyclic forms with a seven-atom ring (the same of
oxepane), rarely encountered, are called
heptoses. 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 new
stereogenic 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 called
mutarotation, that consists in a reversal of the ring-forming reaction followed by another ring formation.
Haworth projection The stereochemical structure of a cyclic monosaccharide can be represented in a
Haworth projection. In this diagram, the α-isomer for the pyranose form of a -aldohexose has the −OH of the
anomeric 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 of
cyclohexane. 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 (considering -aldohexose sugars). Alpha-D-Glucopyranose.svg|α--Glucopyranose Beta-D-Glucopyranose.svg|β--Glucopyranose == Derivatives ==