Anisotropic Swelling Behavior Induced by Helix-Coil Transition in Liquid Crystalline Polypeptide Gels

Uniaxially oriented liquid crystalline poly(γ-benzyl l-glutamate) (PBLG) gel was prepared by cross-linking the lyotropic liquid crystalline PBLG solution in magnetic field, and PBLG was converted to poly[N⁵-(2-hydroxyethyl) l-glutamine] (PHEG) by side-chain aminolysis reaction. The prepared PHEG gel retained the liquid crystallinity after the conversion from PBLG to PHEG when immersed in α-helix-forming solvent such as ethylene glycol. By the addition of water into the immersion solvent, conformational transition of PHEG from α-helix to random coil was occurred, and the optical anisotropy of the gel was disappeared. With this helix-to-coil transition of PHEG, the gel was swollen in the direction perpendicular to the orientational axis of PHEG, and was shrunk in the parallel direction. As the result, the gel in cylindrical shape in ethylene glycol, in which α-helical PHEG is oriented along the cylindrical axis, changed to more isotropic shape in water. This anisotropic shape change of the gel was described by the conformational transition of the cross-linked PHEG from the anisotropic rodlike α-helix to the isotropic random coil.