, Volume 26, Issue 3, pp 249-257

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

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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.