α-helical-within-discotic columnar structures of a complex between poly(ethylene oxide)-block-poly(L-lysine) and a hexa-peri-hexabenzocoronene

Poly(ethylene oxide)-block-poly(L-lysine) (PEO - PLL) was complexed with an amphiphilic hexa-peri-hexabenzocoronene (HBC). This produced a thermotropic liquid crystalline material (PEO - PLL - HBC), which was investigated by FTIR spectroscopy and differential scanning calorimetry as well as by wide-and small-angle X-ray scattering. It was found that the poly(L-lysine) blocks form an α-helical secondary structure. Each helix is surrounded symmetrically by six discotic columns of HBC, which gives an α-helical-within-discotic column structural entity. The dense packing of these entities produces hexagonal sublattices (formed by the columns) in the frame of a two-dimensional hexagonal lattice (formed by the helices). An order - order transition from a columnar structure Col₁ to Col₂ was found at 54 °C. The unit cell constants are 5.75 nm (Col₁) and 6.60 nm (Col₂). The larger unit cell size of Col₂ was explained by a higher intracolumnar order of the latter in which the packing distance of the disklike HBC cores is well-defined (0.353 nm). PEO - PLL - HBC combines essential features of liquid crystals with a basic structural element of proteins into a single material.