piranoza

Piranoza, or pyranose, is a form in which a monosaccharide adopts a six-membered cyclic structure resembling the chemical ring pyran. In this form the ring comprises five carbon atoms and one oxygen atom. For aldoses, the ring typically forms when the carbonyl group at C1 reacts with the hydroxyl group at C5, creating a hemiacetal at C1 and resulting in a six-membered cyclic sugar.

Two anomeric forms, alpha and beta, can exist for a given pyranose, differing in the configuration at the anomeric carbon (C1). In solution these α and β anomers interconvert through mutarotation, a process influenced by solvent, temperature and the specific sugar. Generally, the beta form is more prevalent than the alpha form for many aldohexoses such as glucose.

Pyranose rings are common for many hexoses. Glucose, galactose and mannose, among others, exist predominantly as pyranose forms in their aqueous states. By contrast, some sugars prefer five-membered rings known as furanoses; for example, ribose often adopts a furanose form in RNA. The relative stability of pyranose versus furanose forms depends on the specific sugar and its environment.

In biology and chemistry, pyranose units appear in many polysaccharides. Starch and cellulose, for instance, are built from glucopyranose units linked by glycosidic bonds. Nomenclature commonly uses terms such as D-glucopyranose, α-D-glucopyranose and β-D-glucopyranose to specify both the sugar and its anomeric configuration. Interconversion between open-chain and cyclic (pyranose) forms underpins many sugar reactions, including glycosidic bond formation and mutarotation.