We investigate a field theory for RNA-like polymers with periodic base sequence GCGCG..., where only single-strands aligned in the same direction form double strands. The field theory is derived from a lattice model that incorporates excluded volume effects, base sequence, and temperature dependent renaturation/denaturation. The artificial directionality leads to a novel universality class, not related to conventional branched polymers and Lee-Yang field theory. This universality class is unstable against natural pairing, where oppositely aligned single-strands form double strands. The denaturation/renaturation transition is a continuous crossover between two $\varphi_{ n=0}^{ 4}$ critical points and the critical point of the new universality class.
Categories: cond-mat.soft cond-mat.stat-mech physics.bio-ph