Ribose found in the gas phase.

Sugars are clearly of biochemical interest as cellular energy fuels, metabolic intermediates, mediators of cellular interactions, or structural building blocks. Recently the interest in sugars has been boosted by the quest for organic molecules in interstellar space. The detection of prebiotic building blocks is relevant both to cosmochemistry and biology, since it could trace the construction of molecular complexity and provide a competing mechanism for the emergence of life on Earth. Simple C2 [2] and C3 [3] sugar building blocks have been found in molecular clouds and meteorites. However, the search for more complex sugars is hampered by a serious lack of accurate structural and spectroscopic information. Sugars are small molecules but structurally complex and elusive to experimental methods: the C5 d-aldose sugar, ribose, was only elucidated by X-ray crystal diffraction in 2010. At that time it was stated that ribose was “recalcitrant to crystallization”, requiring advanced zone-melting recrystallization techniques and complementary C MAS NMR data. Even when diffraction methods are feasible, their structural description is biased by crystal packing forces and it may not reflect the most stable structure for the isolated molecules. NMR spectroscopic methods conducted in condensed phases offer solvent-averaged indirect structures and do not represent an independent solution either. These difficulties are notorious for sugars since they are polymorphic systems. According to any standard textbook aldopentoses, such as ribose, can exhibit either linear forms or cyclic five-membered (furanose, fur) or six-membered (pyranose, pyr) rings, occurring either as a or b anomers depending on the orientation of the hydroxy group at C1 in Scheme 1 (anomeric carbon). Ribose is usually depicted as a b-furanose, predominant in ribonucleosides, RNA, ATP, and other biochemically relevant derivatives. Probably for this reason a computational study surprisingly considered only the b-furanose form. However, early NMR spectroscopy experiments suggested that the dominant form of ribose in aqueous solution was not b-furanose but b-pyranose, with the four ring species coexisting with ratios b-pyr:a-pyr:bfur:a-fur= 62:20.3:11.6:6.1. In the 2010 X-ray/NMR study the crystal was reported to be composed of pyranoses with a b :a ratio of 1.7–2:1, but stating that “it is difficult to estimate the reliability of these ratios” because of the different sample conditions. Furanose and pyranose rings exhibit additional conformational variations, making the system more difficult. Pyranoses have a relatively rigid chair skeleton with dominant conformations C4 or C1 (see Scheme 1) connected by ring inversion (C1-chair rings were reported in the X-Ray/ NMR study). Plausible boat structures might be also considered in ribopyranoses, although probably less favored. On the other hand furanoses are more flexible, because of the possibility of interconverting different envelope forms such as C2-endo or C3-endo through non-planar structures (“pseudorotation”). The multiple hydroxy groups finally add conformational complexity since each hydroxy group may produce many rotamers. The conflicting conformational ratios from the X-ray and NMR studies suggest that all these simultaneous effects are difficult to disentangle in condensed phases and give rise to several questions. Is the RNAanalogue b-fur representative of free ribose? What is the relative stability of a and b anomers? Can linear forms of sugars be observed in the isolated molecule? Finally, what are the preferred conformations of ribose: furanoses, pyranoses, or linear chains? To answer these questions we conducted a rotational study of ribose using microwave spectroscopy. A large volume of structural information on chromophore-tagged sugars has been collected by the Simons group using UV–UV and IR–UV laser spectroscopy techniques and theoretical calculations. However, laser spectroscopy provides only vibrational resolution and band assignment is not always unambiguous. Conversely, pure rotational spectroscopy requires no chromophore, offers very high (sub-Doppler) resolution, and leads to accurate moments of inertia for the isolated molecule. Rotamers and isotopologues can thus be dis[*] Dr. E. J. Cocinero, Dr. P. cija, Dr. F. J. Basterretxea, Dr. J. A. Fern ndez, Prof. F. CastaÇo Departamento de Qu mica F sica, Facultad de Ciencia y Tecnolog a Universidad del Pa s Vasco (UPV-EHU) Apartado 644, 48080 Bilbao (Spain) E-mail: [email protected] Homepage: http://www.grupodeespectroscopia.es/MW