Immobilization of Retinoic Acid by Polyamino Acids:  Lamellar-Structured Nanoparticles

We discuss the preparation, the X-ray spectra and their theory, and electron microscopy of nanoparticles showing a lamellar internal structure. Retinoic acid was immobilized by complexation with poly(<SCP>l</SCP>-arginine), poly(<SCP>l</SCP>-histidine), and poly(<SCP>l</SCP>-lysine). The resulting solid-state complexes were prepared as nanoparticles with diameters of 320−380 nm, and with internal structures, which are smectic A-like. By small-angle X-ray scattering the unit cell was determined to be in the range of 3.10−3.62 nm, thus depending linearly on the monomer mass. The X-ray scattering of finite lamellar structures of “onion”- and “tart”-type spherical nanoparticles were determined theoretically. The uniform boundary of the onion-type particles results in secondary maxima near the Bragg reflections. Further, the highest degree of polydispersity to find these secondary maxima in scattering experiments was determined to be about 1% if the particle size is 300 nm. When electron microscopy was used, it was shown that the particles consist of a complex-containing core surrounded by a dispersing agent-containing shell. The core was revealed to be more of the tart- than the onion-type. When circular dichroism was used, the conformation of poly(<SCP>l</SCP>-arginine) and poly(<SCP>l</SCP>-lysine) within the complexes was determined to be an α-helix, whereas that of poly(<SCP>l</SCP>-histidine) was predominantly of the coil-type.