Secret revealed: The crystal structure of ribose

In the journal Angewandte Chemie, the winners of this prize have just presented a first-hand report of their research. Also in Angewandte Chemie, a team of German and Swiss-based researchers has now presented another long-sought result: they have finally been able to solve the crystal structure of ribose.

Ribose belongs in the chemical class of sugars. Its backbone is a chain of five carbon atoms; four of them carry an OH group, the fifth an oxygen atom attached by a double bond. In most modern textbooks and handbooks, ribose is represented as a β-furanose: four of the carbon atoms and the oxygen atom form a five-membered ring. However, it has been known for over 40 years that in solution, ribose exists as a mixure of four different structures: α- and β-furanoses as well as α- and the dominant β-pyranose. Pyranoses are a form of sugar in which the five carbon atoms and an oxygen atom form a six-membered ring. The prefix α or β indicates whether a specific OH group lies above or below the plane of the ring.

But what form does crystalline ribose adopt? Whereas the structures of other important sugars have been known for a long time, ribose has been reluctant to reveal its secret; the compound is extremely difficult to crystallize. Despite such adverse conditions and countless failed attempts, the team led by Lynne B. McCusker, Beat H Meier, Roland Boese, and Jack D. Dunitz at the ETH Zurich (Switzerland) and the University of Duisburg-Essen have finally succeeded in cracking the structure. By using complex computer calculations, they were first able to obtain meaningful results from X-ray diffraction analyses of powder samples.

They were then also able to produce single crystals by zone-melting recrystallization. In this technique, only a small zone of the material is heated and this melt zone is moved. The cooling melt then solidifies with a uniform crystal structure to form the desired single crystal. This can then be examined in by X-ray crystal-structure analysis. Solid-state NMR spectroscopic studies yielded further, complementary information about ribose.

Overall, the researchers came to the realization that D-ribose molecules crystallize as pyranoses, which are six-membered rings. These are present in two crystalline forms that contain β- and α-pyranose in various proportions.

More information: Jack D. Dunitz, The Crystal Structure of D-Ribose—At Last! Angewandte Chemie International Edition 2000, 39, No. 26, 4503-4505, http://dx.doi.org/ … ie.201001266

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