with a standard right-handed helix, for which helix twist is 36° and base pair slide is 0.4 Å in B-DNA.
However, these extremes taken together can be accommodated by a left-handed, Z-type helix. A similar
analysis also explains the preference for a Z-helix in the polymer (dG–dT)n(dA–dC)n.
The net result of these changes is that the minor groove of Z-DNA is so deep that it actually contains
the helix axis whilst the major groove of Z-DNA has become a convex surface on which cytosine-C-5 and
guanine-N-7 and-C-8 are exposed (Figure 2.20 and Table 2.3).
Solution studies on poly(dG–dC) have shown a salt-dependent transition between conformers that can be
monitored by CD or by^31 P NMR (Section 11.2). In particular, there is a near inversion in the CD spectrum
above 4 M NaCl, which has been identified as a change from B- to Z-DNA. It appears that a high salt
32 Chapter 2
Figure 2.20 Van der Waals representation of 10 bp of Z-form DNA. The drawing illustrates the narrow minor
groove, visible in the centre of the top half of the duplex, and the lack of an effective major groove that
takes on the shape of a convex surface instead, visible on the left-hand side of the bottom half of the
duplex. The colour code is identical to that in Figure 2.16