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

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 deg. 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 deg. 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 deg. 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