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The lack of organised secondary structure in protein molecules is also
important, for example secondary structures are more frequently linked by
`unstructured' polypeptide chain than by 
-turns. The terminology is
confusing however. Some literature refers to anything which is not helix or
strand as `coil' or `random coil' or `loop', thus ignoring the existence of
-turns, and the fact that loops do not have totally random
conformations. Indeed, antibody structures have well defined `canonical'
conformations in a number of loops[Chothia et al.,
1989].