  Different aspects of protein folding are illustrated by simplified polymer
models. Stressing the diversity of side chains (residues) leads one to view
folding as the freezing transition of an heteropolymer. Technically, the most
common approach to diversity is randomness, which is usually implemented in two
body interactions (charges, polar character,..). On the other hand, the
(almost) universal character of the protein backbone suggests that folding may
also be viewed as the crystallization transition of an homopolymeric chain, the
main ingredients of which are the peptide bond and chirality (proline and
glycine notwithstanding). The model of a chiral dipolar chain leads to a
unified picture of secondary structures, and to a possible connection of
protein structures with ferroelectric domain theory.
