of the particular spin. The position or frequency of the spectral line is called the chemical shift, the fine
structure observed on the lines is referred to as their multiplicity, and the separations within a multiplet are
termed scalar coupling. A typical 1-(frequency) dimensional (1D)^1 H spectrum for a DNA/RNA hybrid
shows signals for over 300 different protons (Figure 11.6a), far too many to be readily distinguished and
assigned a chemical shift. The overcrowding (signal overlap) problem is alleviated by introduction of further
frequency dimensions.
A variety of experiments have been devised that enable scalar couplings (which are transmitted over
covalent bonds), nuclear Overhauser effects(nOes, which are transmitted through space up to a distance
of approximately 5Å) and dynamic features to be detected in another frequency axis. For example a section
of a 2D (^1 H–^1 H) TOCSYspectrum (revealing scalar coupling) and a NOESYspectrum (showing nOes) for
the same DNA/RNA hybrid may be shown as a contour plot (Figure 11.6b). Each set of contours connects
two protons (a cross-peak). Each cross-peak has a volume that relates to the size of the nOe (i.e.a large
cross-peak volume means the protons are close in space) or the size of the coupling.
434 Chapter 11
Figure 11.6 (a) A section of the 1D^1 H NMR spectrum for a 16 base-pair DNA/RNA hybrid 5 -d(ACTCGATTTCAT-
AGCC)3 /5 -r(GGCUAUGAAAUCGAGA)3. (b) The H5/H1 to H6/H8 region of the (A) TOCSY and
(B) NOESY spectrum. The black lines are for the DNA residues, the red lines for the RNA residues. This
illustrates that nuclei that are close through bonds (shown in TOCSY spectrum) are generally also close
through space (shown in NOESY spectrum)