PEI-retinoate complex

Fig. 23 The correlation length of the lamellar mesophase of the PEI-retinoate complexes depending on the molecular weight of the PEI. Reprinted with permission from [179]. Copyright 2000 American Chemical Society...

Films of the complexes are stable in water at a pH of 7 while they dissolve at pH 5. This can be explained by the pKa value of retinoic acid, which is, for example, 6.05 in 150 mM NaCl and 6.49 in 5 mM NaCl [163]. Therefore, the anionic retinoic moieties within the complexes will be protonated at pH values lower than the pKa which lead to the cleavage of the ionic bonds in the complexes. The first experiments to evaluate the release properties of retinoic acid from thin films of the complexes were performed by using FTIR and surface tension measurements. Films were immersed in solutions of 0.15 m sodium chloride at pH 5 for both methods. The increase of the absorbance at 1255 cm-1 (C-0 stretch vibration) [186] in the FTIR spectra was used as a qualitative measure for the release of retinoic acid from the PEI-retinoate complexes. For comparison, the spectra of the complex and of the non-com-plexed retinoic acid are shown at wave numbers around 1255 cm 1 (Fig. 26, insert curves a and b). The time-dependency of the absorbance, which is a relative measure of the amount of released retinoic acid, is shown in Fig. 26. It can be seen that the increase of the absorbance, and therefore the release... 

Fig. 26 Time-dependency of the FTIR spectra of the PEI retinoate complexes at a wave number of 1255 cm-1. The samples are PEI-600 retinoate (diamonds), PEI-2000 retinoate (circles), PEI-25000 retinoate (squares) and PEI-750000 (triangles). The insert shows the transmission of a complex (a) and of non-complexed retinoic acid (b) around a wavelength of 1255 cm-1. Reprinted with permission from [179]. Copyright 2000 American Chemical Society...

The work presented in the next section concerns the immobilization of retinoic acid by three polyamino acids poly(-L-lysine), poly(-L-arginine) and poly(-L-histidine). The mesomorphous structure of these complexes, which are prepared as nano-particles, has been examined. Then we will report on the physicochemical characteristics of water-soluble complexes formed between PEO-PLL block copolymers and retinoic acid. The structure in the solid-state and solution are discussed. Further the dissociation of the complex when changing the pH is reported. Finally, the immobilization of retinoic acid by PEI with different molecular weights is presented. 

The immobilization of retinoic acid by branched PEI with different molecular weights (6 102 to 2xl06 g/mol) is described in the following section. The main interest is on the formation of mesomorphous structures, the release of retinoic acid from its complexes and the formation of nanoparticles. 

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