A-form DNA was first identified from fibre-diffraction
studies of DNA at ‘low’ (75%) relative humidity. More
recently, crystal studies have identified specific sequences
which can adopt A-DNA type of structures (Figure 1b). In
general, A-DNA for any sequence is favoured under
dehydrating conditions, and certain purine stretches will
favour an A-conformation, even in cases of higher
hydration levels. It appears that at least four purines (or
pyrimidines) in a row are enough to set up a local A-DNA
helix, although of course certain purine stretches are more
likely to form A-DNA than others. (For example, the
sequence AAAA crystallizes as B-DNA, not in the Ahelix.)
It is thus possible to have a DNA sequence that
contains some regions in the A-form within the context of a
mainly B-conformation