N-15 NMR OF RNA FRAGMENTS CONTAINING SPECIFICALLY LABELED GU AND GC PAIRS

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.