3rd le

• When the word carbohydrate was coined, it originally referred to compounds of the general formula Cn(H2O)n. However, only the simple sugars, or monosaccharides, fit this formula exactly.
• The other types of carbohydrates, oligosaccharides and polysaccharides, are based on the monosaccharide units and have slightly different general formulas
• Oligosaccharides are formed when a few (Greek oligos) monosaccharides are linked; polysaccharides are formed when many (Greek polys) monosaccharides are bonded together.
• The reaction that adds monosaccharide units to a growing carbohydrate molecule involves the loss of one H2O for each new link formed, accounting for the difference in the general formula.
• Many commonly encountered carbohydrates are polysaccharides, including glycogen, which is found in animals, and starch and cellulose, which occur in plants.
• The building blocks of all carbohydrates are the simple sugars called monosaccharides.
• Monosaccharides can be classified as aldoses or ketoses depending on whether they contain an aldehyde group (-CHO) or a keto group (-C=O), respectively.
• A monosaccharide can be a polyhydroxy aldehyde (aldose) or a polyhydroxy ketone (ketose).
• The simplest monosaccharides contain three carbon atoms and are called trioses (tri meaning “three”).
• Glyceraldehyde is the aldose with three carbons (an aldotriose), and dihydroxyacetone is the ketose with three carbon atoms (a ketotriose).
• Aldoses with four, five, six, and seven carbon atoms are called aldotetroses, aldopentoses, aldohexoses, and aldoheptoses, respectively.
• Ketoses with four, five, six, and seven carbon atoms are called ketotetroses, ketopentoses, ketohexoses, and ketoheptoses, respectively.
• Six-carbon sugars are the most abundant in nature, but two five-carbon sugars, ribose and deoxyribose, occur in the structures of RNA and DNA, respectively. Four-carbon and seven-carbon sugars play roles in photosynthesis and other metabolic pathways.
• Some molecules are not superimposable on their mirror images and that these mirror images are optical isomers (stereoisomers) of each other.
• A chiral (asymmetric) carbon atom is the usual source of optical isomerism.
• The simplest carbohydrate that contains a chiral carbon is glyceraldehyde, which can exist in two isomeric forms that are mirror images of each other.

  The two forms of glyceraldehyde are designated D-glyceraldehyde and L-glyceraldehyde.
• Mirror-image stereoisomers are also called enantiomers, and D-glyceraldehyde and L-glyceraldehyde are enantiomers of each other.
• The configuration is the three-dimensional arrangement of groups around a chiral carbon atom, and stereoisomers differ from each other in configuration.
• The two enantiomers of glyceraldehyde are the only possible stereoisomers of three-carbon sugars.
• To show the structures of the resulting molecules, we need to say more about the convention for a two-dimensional perspective of the molecular structure, which is called the Fischer projection method, after the German chemist Emil Fischer, who established the structures of many sugars.
• In a Fischer projection, bonds written vertically on the two- dimensional paper represent bonds directed behind the paper in three dimensions, whereas bonds written horizontally represent bonds directed in front of the paper in three dimensions.
• The most highly oxidized carbon—in this case, the one involved in the aldehyde group—is written at the “top” and is designated carbon 1, or C-1.

  In the ketose, the ketone group becomes C-2, the carbon atom next to the “top.”
• Most common sugars are aldoses rather than ketoses, so our discussion will focus mainly on aldoses
• The designation of the configuration as L or D depends on the arrangement at the chiral carbon with the highest number.
• In the Fischer projection of the D configuration, the hydroxyl group is on the right of the highest-numbered chiral carbon, whereas the hydroxyl group is on the left of the highest-numbered chiral carbon in the L configuration.
• They are not superimposable on each other, but neither are they mirror images of each other. Such nonsuperimposable, non-mirror-image stereoisomers are called diastereomers.
• L-Erythrose is the enantiomer (mirror image) of —erythrose, and L-threose is the enantiomer of D-threose.

  L-Threose is a diastereomer of both D-and L-erythrose, and L-erythrose is a diastereomer of both D-and L-threose.
• Diastereomers that differ from each other in the configuration at only one chiral carbon are called epimers.

  D-erythrose and D-threose are epimers.
• The aldotetroses have two chiral carbons (C2 & C3), and there are 2^2, or four, possible stereoisomers.

  Two of the isomers have the D-configuration, and two have the L-configuration.

  The two D-isomers have the same configuration at C3, but they differ in configuration (arrangement of the —OH group) at the other chiral carbon, C2.

  Aldopentoses have three chiral carbons, and there are 2^3, or 8, possible stereoisomers—four D forms and four L forms.

  Aldohexoses have four chiral carbons and 2^4, or 16, stereoisomers—eight D forms and eight L forms
• Glucose is a ubiquitous energy source, and ribose plays an important role in the structure of nucleic acids.
• C1 and C5, form a cyclic hemiacetal (in aldohexoses).

  Another possibility is interaction between C2 and C5 to form a cyclic hemiketal (in ketohexoses).

  In either case, the carbonyl carbon becomes a new chiral center called the anomeric carbon.
• The cyclic sugars can take either of two different forms, designated α and β, and are called anomers of each other.
• The free carbonyl species can readily form either the α-or-β-anomer, and the anomers can be converted from one form to another through the free carbonyl species.
• Fischer projection formulas are useful for describing the stereochemistry of sugars, but their long bonds and right-angle bends do not give a realistic picture of the bonding situation in the cyclic forms, nor do they accurately represent the overall shape of the molecules.
• In Haworth projections, the cyclic structures of sugars are shown in perspective drawings as planar five-or-six membered rings viewed nearly edge on.

  A five-membered ring is called a furanose because of its resemblance to furan; a six-membered ring is called a pyranose because of its resemblance to pyran.
• The terminal —CH2OH group, which contains the carbon atom with the highest number in the numbering scheme, is shown in an upward direction.
• In the α-anomer, the hydroxyl on the anomeric carbon is on the opposite side of the ring from the terminal —CH2OH group (i.e., pointing down).

  In the β-anomer, it is on the same side of the ring (pointing up).

  The same convention holds for α-and-β-anomers of furanoses.