Biology Reference
In-Depth Information
178
Figure 7-1.
Configuration of a nucleotide unit.
PERIODIC STRUCTURES
Similar to polypeptides, Pauling and Corey (1953) also proposed that DNA
could exist in periodic structures. Many other periodic structures were
subsequently suggested, and several of these will be discussed in the next
Chapter. Some are right-handed, and others left-handed. However, unlike
polypeptides, the basic building block has many more single bonds free to
rotate. As a result, periodic structures of polynucleotides can not be
visualized easily in a two-dimensional Ramachandran plot. There is also no
simple paper model as that for the peptide backbone illustrated in Chapter 5.
Instead, a minimum of a five-dimensional space will be required for the
backbone, together with possible distortions of the ribose ring. Over the
years, virtual angles have been suggested. However, it is usually very
difficult to reduce any description in a five-dimensional space to a two-
dimensional space.
