PNNL

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 amorphous state in an effort to reduce effects of crystal-packing. Between temperatures of -60 and -74 ?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, and (2) millisecond to microsecond motion of large amplitude with an apparent activation energy ≥4 kcal/mol. 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.