Helicoidal bianisotropic mediums (HBMs) are periodically nonhomogeneous and possess structural chirality. We establish a variety of conditions concerning the equivalence of planewave propagation along the spirality axis of HBMs to propagation in isotropic chiral mediums (ICMs). These conditions illuminate the analogous roles of molecular chirality in ICMs and structural chirality in HBMs. In particular, structural chirality of even the simplest (i.e. dielectric) HBMs can be tailored to enhance optical activity in comparison to that of ICMs.