Once all or most of the^1 H chemical shifts have been assigned, it simply remains to quantify NOESY
cross-peaks. These, together with coupling constants from PE-COSY–type spectra, may be used in a variety
of computational methods to generate 3D structures of nucleic acids. A simple method to quantify NOESY
cross-peaks is to categorise them as ‘small’, ‘medium’ and ‘large’ by assignment of distance ranges, for
example 3.7–5.8Å, 1.7–3.8Å and 0.7–2.8Å, respectively. Through the use of approximately 500nOe and
coupling constant restraints, a structure for the DNA/RNA hybrid was obtained (Figure 11.10).
There is much less certainty about atom positions in NMR structures than in those usually achieved with
X-ray crystallography. This is partly because in solution the molecule is flexible. But there may also be uncer-
tainty due to inappropriate or an insufficient number of structural constraints. The generation of reliable
Physical and Structural Techniques Applied to Nucleic Acids 437
Figure 11.10 One of the structures generated from nOe and coupling constant measurements made for the
DNA/RNA hybrid (Figure 11.6). The black strand is DNA and the red strand is RNA