Poly(ethylene oxide)-b-poly( l -lysine) Complexes with Retinoic Acid

Properties of the complexes formed by poly(ethylene oxide)- b -poly( l -lysine)s (PEO−PLL) and all-trans -retinoic acid were investigated by differential scanning calorimetry, wide- and small-angle X-ray scattering, circular dichroism, dynamic light scattering, and atomic force microscopy. It was determined that for micellar solutions of complexed PEO−PLL the poly( l -lysine) blocks form an α-helix structure at pH values higher than 9 and that the coil−helix transition spans from pH 3.7 to 9.0. The high stability of the α-helix was attributed to a protective effect of the retinoate moieties and the PEO surrounding. Micelle structures were found to be of core−shell type. Each micelle has a compact core, which is formed by a smectic A-like poly( l -lysine) retinoate complex. The shell is formed by the PEO. When the pH value is lowered, the micelle sizes increases. The conformation of the noncomplexed PEO−PLL in the solid state was found to be a mixture of α-helix and β-sheet structures while that of the complexed polymers is a pure α-helix. The PEO blocks crystallize in the solid state of the pristine and the complexed polymers, but the degree of crystallinity is considerably lower in the complexed state. Such block copolymer complexes are considered to be useful for the development of new drug delivery formulations.