  We analyze a model statistical description of the polypeptide chain
helix-coil transition, where we take into account the specificity of its
primary sequence, as quantified by the phase space volume ratio of the number
of all accessible states to the number corresponding to a helical conformation.
The resulting transition phase diagram is then juxtaposed with the unusual
behavior of the secondary structures in Intrinsically Disordered Proteins
(IDPs) and a number of similarities are observed, even if the protein folding
is a more complex transition than the helix-coil transition. In fact, the
deficit in bulky and hydrophobic amino acids observed in IDPs, translated into
larger values of phase space volume, allows us to locate the region in
parameter space of the helix-coil transition that would correspond to the
secondary structure transformations that are intrinsic to conformational
transitions in IDPs and that is characterized by a modified phase diagram when
compared to globular proteins. Here, we argue how the nature of this modified
phase diagram, obtained from a model of the helix-coil transition in a solvent,
would illuminate the turned-out response of IDPs to the changes in the
environment conditions that follow straightforwardly from the re-entrant (cold
denaturation) branch in their folding phase diagram.
