PROTEIN-FOLDING, ANISOTROPIC COLLAPSE AND BLUE PHASES

We study a homopolymer model of a protein chain, where each monomer ca rries a dipole moment. To mimic the geometry of the peptidic bond, the se dipoles are constrained to be locally perpendicular to the chain. T he tensorial character of the dipolar interaction leads naturally to a (tensorial) liquid crystal-like order parameter. For non chiral chain s, a mean field study of this model shows that a classical theta colla pse transition occurs first; at lower temperature, nematic order sets in. For chiral chains, an anisotropic (tensorial) collapse transition may occur before the theta temperature is reached: the ordered phase c an be described as a ''compact phase of secondary structures'', and po ssesses great similarities with the liquid crystal blue phases.