  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 $\theta$ collapse 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 can be described as a ``compact phase of secondary
structures'', and possesses great similarities with the liquid crystal blue
phases.
