3-De­oxy-1,2-di-O-isopropyl­idene-5-O-tosyl-d-threo-pentofuran­ose

In the crystal structure of the title compound, C15H20O6S, the two independent mol­ecules crystalllize in a chiral setting with two different conformations, twisted 4 T 3 and envelope 4 E, for the furan­ose rings. Weak C—H⋯O contacts strengthen the crystal structure

Key Science Goals for the Next-Generation Event Horizon Telescope

The Event Horizon Telescope (EHT) has led to the first images of a supermassive black hole, revealing the central compact objects in the elliptical galaxy M87 and the Milky Way. Proposed upgrades to this array through the next-generation EHT (ngEHT) program would sharply improve the angular resolution, dynamic range, and temporal coverage of the existing EHT observations. These improvements will uniquely enable a wealth of transformative new discoveries related to black hole science, extending from event-horizon-scale studies of strong gravity to studies of explosive transients to the cosmological growth and influence of supermassive black holes. Here, we present the key science goals for the ngEHT and their associated instrument requirements, both of which have been formulated through a multi-year international effort involving hundreds of scientists worldwide

Key Science Goals for the Next-Generation Event Horizon Telescope

The Event Horizon Telescope (EHT) has led to the first images of a supermassive black hole, revealing the central compact objects in the elliptical galaxy M87 and the Milky Way. Proposed upgrades to this array through the next-generation EHT (ngEHT) program would sharply improve the angular resolution, dynamic range, and temporal coverage of the existing EHT observations. These improvements will uniquely enable a wealth of transformative new discoveries related to black hole science, extending from event-horizon-scale studies of strong gravity to studies of explosive transients to the cosmological growth and influence of supermassive black holes. Here, we present the key science goals for the ngEHT and their associated instrument requirements, both of which have been formulated through a multi-year international effort involving hundreds of scientists worldwide

Conformational studies on 2',3'-unsaturated pentopyranosyl nucleosides by H-1 NMR spectroscopy. Impact of pi->sigma* interactions on the axial preference of the purine versus pyrimidine nucleobase

A variable-temperature H-1 NMR study on beta- and alpha-D-glycero-pent-2'-enopyranosyl nucleosides 1-10 has shown that the constituent pyranosyl moiety is involved in a rapid two-state H-6(5) reversible arrow E-5/S-5(4) conformational equilibrium. The results of the conformational analysis of (3)J(HH) coupling constants were compared with the trends of X-ray crystallography and ab initio calculations, which support the two-state dynamic equilibrium and suggest hat the equilibrium is affected by the nature of the heterocyclic aglycon. The beta nucleosides 1-5 adopt predominantly (approximately 95% in purine and 80% in pyrimidine nucleosides at 298 K) the conformation that is intermediate between the E-5 and S-5(4) canonical forms, where both the nucleobase and 4'-OH are in the axial orientation. The pyranosyl moiety of alpha-purine nucleosides 6 and 7 is involved in an unbiased two-state conformational equilibrium. On the other hand, the alpha-pyrimidine nucleosides 8-10 show a conformational bias of ca. 75% toward E-5 canonical form. The van't Hoff type analysis of the changes in the conformational equilibrium with temperature afforded thermodynamic data on H-6(5) reversible arrow E-5/S-5(4) conformational equilibrium in 1-10. The comparative analysis of Delta H degrees has shown that H-6(5) reversible arrow E-5/S-5(4) conformational equilibrium in 1-10 is driven by the fine tuning of O6'-Cl'-N1/9 anomeric effect, the gauche effect of the [O6'-C5'-C4'-O4'] fragment, the interaction between pi-system of the C2'-C3' bond with the heterocyclic aglycon and 4'-OH in the allylic position, and the intrinsic steric effect. Dissection of the individual contributions to the drive of conformational equilibrium of 1-10 has shown that the relative strength of pi(-->) sigma*(Cl'-N1/9) interactions in purine vs pyrimidine nucleobases increases in the following order: cytosine < uracil < thymine < adenine < ganine