Observation of a distinct transition in the mode of interconversion of ring pucker conformers in non-crystalline d-ribose-2′-d from ²H NMR spin-alignment

Abstract

Internal motions of d-ribose selectively ²H-labeled at the 2^′ position were measured using solid state ²H NMR experiments. A sample of d-ribose-2 ^′-d was prepared in a hydrated, non-crystalline state to eliminate effects of crystal-packing. Between temperatures of −74 and −60 °C the C2^′–H2^′ bond was observed to undergo two kinds of motions which were similar to those of C2^′–H2^′/H2^′′ found previously in crystalline deoxythymidine (Hiyama et al. (1989) J. Am. Chem. Soc., 111, 8609–8613): (1) Nanosecond motion of small angular displacement with an apparent activation energy of 3.6 ± 0.7 kcal mol⁻¹, and (2) millisecond to microsecond motion of large amplitude with an apparent activation energy ≥4 kcal mol⁻¹. At −74 °C, the slow, large-amplitude motion was best characterized as a two-site jump with a correlation time on the millisecond time scale, whereas at −60 °C it was diffusive on the microsecond time scale. The slow, large-amplitude motions of the C2^′–H2^′ bond are most likely from interconversions between C2^′-endo and C3^′-endo by way of the O4^′-endo conformation, whereas the fast, small-amplitude motions are probably librations of the C2^′–H2^′ bond within the C2^′-endo and C3^′-endo potential energy minima.