pentofuranose

Pentofuranose refers to the five-membered ring form (furanose) of a pentose sugar. It arises when an aldopentose or ketopentose cyclizes: the carbonyl group reacts with a hydroxyl on C-4 to form a hemiacetal or hemiketal, yielding a ring that contains four carbon atoms and one ring oxygen. The anomeric carbon is C-1, and the ring can adopt alpha or beta configurations at C-1, in addition to the overall D or L stereochemistry of the parent sugar.

In aldopentoses, cyclization yields ribofuranose with variants such as beta-D-ribofuranose; in ketopentoses, forms such as xylofuranose or xylulose-furanose are possible. Commonly discussed pentofuranoses include beta-D-ribofuranose (the sugar in RNA) and beta-D-deoxyribofuranose (the sugar in DNA). Other pentofuranose forms, such as arabinofuranose and xylofuranose, are recognized as components in various glycoconjugates and polysaccharides in different organisms. The term pentofuranose emphasizes the five-membered ring geometry rather than a single specific sugar.

Biologically, pentofuranose forms are part of nucleotide sugars and many glycoconjugates. In solution, pentoses can interconvert between different ring forms (furanose and pyranose) and between anomeric configurations, a process related to mutarotation. The furanose form is especially important for the structural context of nucleic acids, while other ring forms can predominate in different chemical and biological environments.