The Big Damn Post Of Carbohydrate-Related Chemistry Definitions
Last updated: July 23rd, 2019 |
Man, is there ever a lot of sugar nomenclature to know! This post aims to provide single place giving you the definitions of terms like anomeric carbon, alpha versus beta, pyranose versus furanose, and all the other most commonly encountered carbohydrate-related terms in a typical second-semester organic chemistry course.
[This reference document from IUPAC was extremely helpful in the preparation of this post.] Feedback on anything that is missing would be greatly appreciated.
α (Alpha) – the name given to the configuration of a cyclic sugar where the oxygen on the anomeric carbon is on the opposite face of the ring relative to the substituent on the other carbon flanking the ring oxygen. Contrasted with beta (β) which is where the two substituents are on the same faces of the ring.
Aldaric acid – a dicarboxylic acid derived from an aldose where both ends of the sugar have been oxidized, e.g. from the action of nitric acid:
Aldose – a monosaccharide with an aldehyde (or masked aldehyde) functional group. [“Masked aldehyde” here refers to a cyclic hemiacetal that is in equilibrium with an acyclic aldehyde through ring-chain tautomerism]. Contrast aldoses with ketoses, which are monosaccharides with a ketone (or masked ketone) group.
Alditol – an acyclic alcohol derived from the reduction of an aldose. The alditol of glyceraldehyde, for example, is glycerol. Sorbitol is the alditol of glucose, mannitol is the alditol of mannose, and so forth. If these names sound familiar, it is because alditols are commonly used as food additives and sweeteners.
β (beta) – the name given to the configuration of a cyclic sugar where the oxygen on the anomeric carbon is on the same face of the ring as the substituent on the other carbon flanking the ring oxygen. Contrast with alpha (α) where the two substituents are on the opposite faces of the ring.
Carbohydrate – originally just referred to monosaccharides (such as glucose) with the empirical formula Cn(H2O)n (i.e. “hydrates of carbon”). Now used as a more generic term referring to mono-, oligo-, and polysaccharides as well as many of their derivatives.
D- (and L-) A monosaccharide is assigned to D- or L- according to the configuration at the highest-numbered chiral center, i.e. the bottom-most chiral center in its Fischer projection. If the hydroxyl group of this carbon is on the right, the sugar is assigned the prefix D-; if it is on the left, it is assigned with the prefix L- . The enantiomer of a D-sugar is always an L-sugar. The simplest sugar is glyceraldehyde, which has one chiral center and exists in two enantiomeric forms, called D- and L- glyceraldehyde, respectively. [See this post on D- and L- sugars]
Disaccharide – a carbohydrate which can be hydrolyzed to give two monosaccharides. Some prominent examples of disaccharides are lactose, sucrose, and maltose. The sugars of a disaccharide are joined through a glycosidic bond.
Epimer – two sugars which differ in the configuration of a single chiral center. D-Glucose and D-Mannose are epimers, as are D-Mannose and D-Talose. [Note: generally speaking, when using the term “epimer”, one of the atoms on the chiral center is usually hydrogen]
Glycoside – fancy name for an acetal (or ketal) formed from a sugar. Formally, a glycoside is an acetal derived from condensation of the hemiacetal (or hemiketal) carbon of a sugar with the hydroxy group of another compound.
Glycosidic bond – the bond that links the anomeric carbon of a sugar to another compound. [For our purposes, this is through a C–O bond, but there are also such things as nitrogen, sulfur, and selenium glycosides (among others)]
Haworth Projection – a graphical depiction of monosaccharides that clearly shows the stereochemistry at each carbon (at the expense of accurately depicting its conformation). [See this post on Haworth projections]
Hexose – a sugar with six carbons, the most familiar example being glucose. A hexose that bears an aldehyde (or masked aldehyde) is called an aldohexose; a hexose bearing a ketone is called a ketohexose.
Ketose – a monosaccharide with a ketone (or masked ketone) functional group. Contrast with aldoses.
L- (and D- ) When drawn in a Fischer projection,the highest-numbered chiral center in all L- sugars will be pointing to the left. See D-
Monosaccharide – a sugar which cannot be hydrolyzed into simpler sugar units. Glucose, ribose, mannose, and many other sugars (including deoxyribose) are monosaccharides. Sucrose, by contrast, is a disaccharide which can be hydrolyzed into glucose and fructose.
Oligosaccharide – a carbohydrate with a moderate (~ 3 to ~10) number of monosaccharide units bound together by glycosidic bonds. There is no firmly defined boundary between “oligosaccharide” and “polysaccharide”, but carbohydrate with more than 10 saccharide units starts to get into polysaccharide territory.
Pentose – a five-carbon monosaccharide, such as ribose and deoxyribose. A pentose bearing an aldehyde is called an aldopentose; a pentose bearing a ketone is called a ketopentose.
Polysaccharide – a carbohydrate composed of many monosaccharides linked together. “Many” is somewhat arbitrary, but a carbohydrate with about ten or fewer monosaccharide units is often called an oligosaccharide.
Pyranose – a six-membered cyclic hemiacetal. Derived from the name pyran, a six-membered cyclic ether. Distinct from a furanose, which is a five-membered cyclic hemiacetal.[See post on furanoses and pyranoses ]
Reducing sugar – a mono- or disaccharide which gives a positive Tollens test. Practically, it refers to any mono- or di-saccharide with a hemiacetal that can undergo ring-chain tautomerism. Glycosides are not reducing sugars, since they are not in equilibrium with an aldehyde or ketone. [Post: Reducing sugars]
Ring-Chain Tautomerism – the equilibrium between linear and cyclic forms that occurs in aldoses and ketoses. [See this post on ring-chain tautomerism ] Ring chain tautomerism is possible when the cyclic form has a hemiacetal, but is not possible in glycosides (where the acetals and ketals are “locked”).
Saccharide – a sugar or sugar derivative. Synonym of carbohydrate.
Tetrose – a monosaccharide with four carbons. Erythrose and threose, both aldoses, are called aldotetroses, and tetroses bearing a ketone are called ketotetroses.
Triose – a sugar with three carbons. There are only three trioses: D- and L- glylceraldehyde (aldotrioses), and dihydroxyacetone (a ketotriose).
Hey, you reached the end! What’s missing? Comments welcome.