Xh. Zhang et al., N-15 NMR OF RNA FRAGMENTS CONTAINING SPECIFICALLY LABELED GU AND GC PAIRS, Journal of the American Chemical Society, 120(4), 1998, pp. 615-618
N-15 and C-13 NMR have been used to study three specifically labeled R
NA fragments that include two tandem GU wobble pairs of different ther
mal stability and a tetraloop GU pair. The data are also compared to e
arlier N-15 NMR work on an isolated, intrahelical GU pair. The results
provide qualitative information on the relative contributions by stac
king and hydrogen bonding to chemical shift changes at specific sites
in a variety of GU pairs, Chemical shifts for the unpaired guanine ami
no groups in the isolated and both tandem wobble pairs are all signifi
cantly upfield of corresponding paired GC aminos. The guanine amino of
the tetraloop is further downfield, as expected for a base paired hyd
rogen bond donor, Thus, N-15 NMR can be diagnostic for the presence of
base-base pairing in RNA. Chemical shifts of the guanine N1 in all fo
ur GU pairs show significant shielding effects, particularly those of
the more stable tandem GU pair. Similar shielding is also seen in the
C-13 data for the C2 atom of the GU wobble pairs. Our results demonstr
ate that these adjacent ring atoms (N1 and C2) are in the shielding re
gions of neighboring bases, and that specific labeling can provide inf
ormation on base stacking. Thus, where global structure is known, spec
ific labeling can be invaluable as a complement to probe local interac
tions. It is likely, however, that the most important use of specific
labeling will be with complex systems in which global structural infor
mation is incomplete.