Protein folding, anisotropic collapse and blue phases

Abstract

We study a homopolymer model of a protein chain, where each monomer carries a dipole moment. To mimic the geometry of the peptidic bond, these dipoles are constrained to be locally perpendicular to the chain. The tensorial character of the dipolar interaction leads naturally to a (tensorial) liquid crystal-like order parameter. For non chiral chains, a mean field study of this model shows that a classical $θ$ collapse transition occurs first; at lower temperature, nematic order sets in. For chiral chains, an anisotropic (tensorial) collapse transition may occur before the $θ$ temperature is reached: the ordered phase can be described as a ``compact phase of secondary structures'', and possesses great similarities with the liquid crystal blue phases.