title: Conformational studies of gas-phase ribose and 2-deoxyribose by density functional, second order PT and multi-level method calculations: the pyranoses, furanoses, and open-chain structures.
abstract: We present an extensive computational study of a complex conformational isomerism of two gas phase pentoses of biological and potential astrobiological importance, d-ribose and 2-deoxy-d-ribose. Both cyclic (α- and β-pyranoses, α- and β-furanoses) and open-chain isomers have been probed using second order Møller-Plesset perturbation theory (MP2), M06-2X density functional, and multi-level G4 methods. This study revealed a multitude of existing minima structures. Numerous furanose conformers found are described with the Altona and Sundaralingam pseudorotation parameters. In agreement with the recent gas-phase microwave (MW) investigation of Cocinero et al., the calculated free ribose isomers of lowest energy are the two β-pyranoses with the (1)C4 and (4)C1 ring chair conformations. Both β-pyranoses lie within 0.9kJ/mol in terms of ΔG(298K) (G4), thus challenge the computational methods used to predict the ribose global minimum. The calculated most favoured ribofuranose is the α-anomer having the twist (2)T1 ring conformation, put 10.4kJ/mol higher in ΔG than the global minimum. By contrast with d-ribose, the lowest energy 2-deoxy-d-ribose is the α-pyranose, with the most stable 2-deoxy-d-furanose (the α-anomer) being only 6.2kJ/mol higher in free energy. For both pentoses, the most favoured open-chain isomers are significantly higher in energy than the low-lying cyclic forms. A good overall agreement is observed between the M06-2X and MP2 results in terms of both the existing low-energy minima structures and intramolecular H-bonding geometrical parameters. The natural orbital analysis confirms the occuring of the endo- and exo-anomeric effects and maximization of intramolecular H-bonding in the lowest-lying pyranoses and furanoses of both sugars.
authors.affiliation: Faculty of Chemistry, Wroclaw University, F. Joliot-Curie 14, 50-383 Wroclaw, Poland. Electronic address: marek.szczepaniak@chem.uni.wroc.pl.
authors.forename: Marek
authors.initials: M
authors.lastname: Szczepaniak
authors.affiliation: Faculty of Chemistry, Wroclaw University, F. Joliot-Curie 14, 50-383 Wroclaw, Poland.
authors.forename: Jerzy
authors.initials: J
authors.lastname: Moc
mesh_terms: D002236:Carbohydrate Conformation; D003855:Deoxyribose / Q000737:chemistry*; D050113:Imino Furanoses / Q000737:chemistry*; D050112:Imino Pyranoses / Q000737:chemistry*; D008958:Models, Molecular; D011789:Quantum Theory; D012266:Ribose / Q000737:chemistry*
chemical_list: D050113:Imino Furanoses; D050112:Imino Pyranoses; D003855:Deoxyribose; D012266:Ribose
keywords: 2-Deoxy-d-ribose; G4; MP2; NBO; d-Ribose