Like some other monosaccharides, pentoses exist in two forms, open-chain (linear) or closed-chain (cyclic), that easily convert into each other in water solutions.[3] The linear form of a pentose, which usually exists only in solutions, has an open-chain backbone of five carbons. Four of these carbons have one hydroxylfunctional group (–OH) each, connected by a single bond, and one has an oxygen atom connected by a double bond (=O), forming a carbonyl group (C=O). The remaining bonds of the carbon atoms are satisfied by six hydrogen atoms. Thus the structure of the linear form is H–(CHOH)x–C(=O)–(CHOH)4-x–H, where x is 0, 1, or 2.
The term "pentose" sometimes is assumed to include deoxypentoses, such as deoxyribose: compounds with general formula C 5H 10O 5-y that can be described as derived from pentoses by replacement of one or more hydroxyl groups with hydrogen atoms.
Classification
The aldopentoses are a subclass of the pentoses which, in the linear form, have the carbonyl at carbon 1, forming an aldehyde derivative with structure H–C(=O)–(CHOH)4–H. The most important example is ribose. The ketopentoses instead have the carbonyl at positions 2 or 3, forming a ketone derivative with structure H–CHOH–C(=O)–(CHOH)3–H (2-ketopentose) or H–(CHOH)2–C(=O)–(CHOH)2–H (3-ketopentose). The latter is not known to occur in nature and are difficult to synthesize.
In the open form, there are eight aldopentoses and four 2-ketopentoses, stereoisomers that differ in the spatial position of the hydroxyl groups. These forms occur in pairs of optical isomers, generally labelled "D" or "L" by conventional rules (independently of their optical activity).
The closed or cyclic form of a pentose is created when the carbonyl group interacts with a hydroxyl in another carbon, turning the carbonyl into a hydroxyl and creating an ether bridge –O– between the two carbons. This intramolecular reaction yields a cyclic molecule, with a ring consisting of one oxygen atom and usually four carbon atoms; the cyclic compounds are then called furanoses, for having the same rings as the cyclic ethertetrahydrofuran.[3]
The closure turns the carboxyl carbon into a chiral center, which may have any of two configurations, depending on the position of the new hydroxyl. Therefore, each linear form can produce two distinct closed forms, identified by prefixes "α" and "β".
