Knowledge Bank

Sugars are aldoses or ketoses with multiple hydroxy groups which have been elusive to spectroscopic studies. Here we report a rotational study of the aldopentose ribose. According to any standard textbook aldopentoses can exhibit either linear forms, cyclic five-membered (furanose) structures or six-membered (pyranose) rings, occurring either as $\alpha$- or $\beta$- anomers depending on the orientation of the hydroxy group at C-1 (anomeric carbon). $\beta$-Furanose is predominant in ribonucleosides, RNA, ATP and other biochemically relevant derivatives, but is $\beta$-furanose the native form also of free ribose? Recent condensed-phase X-ray \textbf{49}, 4503, 2010. ii) W. Saenger \textit{Angew. Chem. Int. Ed.} \textbf{49}, 6487, 2010.} and older NMR \textbf{52}, 1965. ii) R. U. Lemieux and J. D. Stevens \textit{Can. J. Chem.} \textbf{44}, 249, 1966. iii) E. Breitmaier and U. Hollstein \textit{Org. Magn. Reson.} \textbf{8}, 573, 1976.} studies delivered conflicting results. In order to solve this question we conducted a microwave study on D-ribose that, owing to ultrafast UV laser vaporizationo \textit{Angew. Chem. Int. Ed.} in press: DOI: 10.1002/anie.201107973, 2012.}, has become the first C-5 sugar observed with rotational resolution. The spectrum revealed six conformations of free ribose, preferentially adopting $\beta$-pyranose chairs as well as higher-energy $\alpha$-pyranose forms. The method also allowed for unambiguous distinction between different orientations of the hydroxy groups, which stabilize the structures by cooperative hydrogen-bond networks. No evidence was observed of the $\alpha$-/$\beta$-furanoses or linear forms found in the biochemical derivatives.